Proposal lead: Prof Steve Milan, University of Leicester ([email protected])
Ravens • Auroral and magnetospheric imaging mission • 1
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Ravens
Proposing team
Steve Milan, Nigel Bannister
University of Leicester, UK
Eric Donovan, Emma Spanswick
University of Calgary, Canada
Nikolai Østgaard, Kjellmar Oksavik
University of Bergen, Norway
Kirsti Kauristie, Minna Palmroth
Finnish Meteorological Institute, Finland
Benoit Hubert
University of Liége, Belgium
Hermann Opgenoorth, Stas Barabash,
Hans Nilsson
Swedish Institute of Space Physics (IRF), Sweden
Andrew Fazakerley
University College London, UK
Malcolm Dunlop
Rutherford Appleton Laboratory, UK
Proposal lead and contact
Steve Milan
Radio and Space Plasma Physics Group
Department of Physics and Astronomy
University of Leicester
Leicester LE1 7RH, UK
[email protected]
Tel: +44 116 223 1896
Mission website: www.ion.le.ac.uk/ravens.html
Pontus Brandt, Joseph Westlake
The Johns Hopkins University Applied Physics
Laboratory, USA
James Wild
University of Lancaster, UK
Iannis Dandouras, Benoit Lavraud
IRAP, France
Mervyn Freeman
British Antarctic Survey, UK
Susan McKenna-Lawlor
National University of Ireland (Maynooth),
Space Technology Ireland, Ltd., Ireland.
The Norse god Odin on his horse Sleipnier with his
Ravens, Huginn (thought) and Muninn (memory), at
his side. The ravens were Odin’s eyes and ears, continuously circling the world to gather news for their
master. Woodcut by Gerhard Munthe (1849-1929).
Ravens • Auroral and magnetospheric imaging mission • 2
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Ravens – A proposal to the ESA M-Class mission opportunity call 2015
ESA Cosmic Visions 2015-2025
Contents
Title page
1
4. Proposed Mission Configuration
32
Proposing team
2
5. Management Scheme
38
0. Executive Summary
3
1. Science Objectives
5
2. Science Requirements
16
3. Proposed Science Instruments
21
Annex A.
Bibliography
43
Lead proposer contact details
46
0. Executive Summary
Science objectives. The Ravens magnetospheric and auroral imaging mission will determine the interaction of the magnetosphere with the solar wind, and the transport of plasma and the dissipation
of energy within it. Ravens will provide for the first time global, continuous, 3D, and conjugate observations of key magnetospheric regions: the plasma sheet, plasmasphere, ring current, and ionosphere, measurements that are critical to understanding the globally-coupled magnetosphere.
These Ravens observations are required not only to understand solar wind-magnetosphere coupling,
but also the control of all processes within the magnetosphere, such as magnetic field perturbations,
radiation belt variability, large-scale current flow, and ionospheric disturbances. Ravens is the first
mission that recognizes that the Earth is coupled to space through two polar regions, which do not
respond similarly to solar wind forcing, and both regions have to be measured simultaneously to obtain information about the complete system. In addition, Ravens observations will supply a continuous stream of data optimized for Space Weather now- and forecasting, and the validation and development of European global physics-based magnetospheric models, which are key to providing a
full understanding the geospace environment and its impact on society.
The Earth’s magnetosphere is not a passive recipient of solar wind input, but displays emergent
structures and behaviours that reveal its nature as a highly non-linear system driven far from equilibrium by a continuous but variable energy source. Moreover, energy is not channelled smoothly
from the outer boundary of the magnetosphere to the inner boundary of the atmosphere, but complex positive and negative feedback processes between intersecting and interacting hot and cold
plasma regions – the plasma sheet, the plasmasphere, the ring current, the ionosphere, the atmosphere – modulate the energy flow within and the coupling with the solar wind on the outside. In this
regard, the magnetosphere is a microcosm of processes active in astrophysical systems throughout
the Universe, processes responsible for creating diversity, structure, and complex behaviour wherever plasmas of different origins and characters interact.
The Ravens mission will provide a step-change in our understanding of our immediate space environment and answer fundamental problems in magnetospheric science, relevant also to other magnetospheric systems, including the following key questions, which are directly aligned with international research priorities as laid out by the Committee on Space Research (COSPAR Roadmap “Understanding Space Weather to Shield Society” [Schrijver et al., 2015]):
•
•
•
•
How does a magnetosphere assemble and organize itself?
What are the feed-back loops in solar wind-magnetosphere coupling?
What creates geomagnetic storms?
Why are the northern and southern auroras asymmetric?
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Mission Strategy. The Ravens mission will monitor the global response of the magnetosphere to
incoming solar wind disturbances using a suite of remote-sensing instrumentation including Far Ultraviolet (FUV) and X-ray auroral imagers, Extreme Ultraviolet (EUV) plasmasphere imagers, and energetic neutral atom (ENA) cameras. Ravens will provide for the first time (a) continuous measurements of the auroras, (b) frequent and systematic measurements of the auroras from both hemispheres (true “global auroral imaging”), and (c) continuous and stereoscopic remote-sensing of the
plasmasphere and ring current.
Ravens will comprise two identically-instrumented spacecraft, payload mass ~130 kg, in highlyelliptical polar orbits, with apogee close to 8 RE over each pole. The orbits of the two spacecraft will
be phased such that one spacecraft is at perigee while the other is at apogee. Hence, one of the
spacecraft will always be in a position to monitor auroral activity, and for long periods the two
spacecraft will be ideally located to view both northern and southern hemisphere auroras simultaneously. From multi-wavelength imaging the entire energy range of the incident electrons important for ionospheric electrodynamics will be derived. One spacecraft will always be in a position
to monitor the plasmasphere and ring current, and for long periods stereoscopic views will enable
reconstruction of 3D plasma structures. Ravens will be supported by a suite of ground-based observatories, both north and south, and computational physics-based modelling of the magnetosphere, providing a systems level approach to magnetospheric sensing and understanding.
Programmatics and Costs. Ravens is built on existing technology, much of which has flown on previous missions. Little technology needs to be developed, making a launch in the 2025 time-frame a
certainty. Hence, Ravens represents a low-risk investment for high science return. The overall mission cost to ESA is estimated to be 359 M€, which is lower than the cost envelope of an M-class mission.
Conclusions. Ravens will be ESA’s first magnetospheric imaging mission. Ravens is a cost-effective,
next-generation mission delivering science that will transform our understanding of magnetospheric
dynamics and, by extension, plasma physics throughout the cosmos. It builds upon principles developed for previous imaging missions, but with an innovative two-spacecraft mission design that will
provide the observations necessary to answer fundamental questions regarding the behaviour of the
coupled solar wind-magnetosphere system.
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1. Science Objectives
1.1. Introduction
The magnetosphere represents the outer boundary of our immediate environment, shielding the
Earth and its atmosphere from the solar wind. Although the magnetosphere is central to our survival as a species, we still have a poor understanding of how it forms, how it is coupled to the solar environment on the outside, how it is coupled to the atmosphere on the inside, and how it responds to
extreme solar disturbances. Ravens will for the first time give a System Level understanding of magnetospheric structure, magnetospheric dynamics, and geomagnetic storms. Ravens will determine
the interaction of the magnetosphere with the solar wind, and the transport of plasma and the dissipation of energy within it. This is key to understanding external influences on the Earth, the radiation environment in near-Earth space, and the fundamental physical processes occurring in magnetized plasmas, such as magnetic reconnection and charged particle acceleration.
Figure 1.1. (Left) A schematic showing the main features of the magnetic topology and plasma regions of the
magnetosphere, including the plasma sheet (orange), ring current (red), plasmasphere (dark blue), and magnetotail lobes (light blue). The magnetospheric regions are interconnected with the polar atmosphere and ionosphere along magnetic field lines; precipitation of charged particles from these regions produces auroral features that track the evolution and interconnection of these regions. (Right) A sequence of auroral images (Polar-UVI) from 28 August 1998, preceding and during the impact of an interplanetary shock on the magnetopause, showing features associated with specific processes in the overlying magnetosphere.
Astrophysical systems comprise plasma, usually a mixture of plasmas from a variety of sources and
of different character. The Earth’s magnetosphere is an example of the emergent structures and
phenomena which arise from the non-equilibrium dynamics driven by the interaction of mixtures of
hot and cold magnetized plasmas of different origins, in the Earth’s case the solar wind and the
planetary ionosphere. Structure arises as a consequence of the interaction, leading to the formation
of different plasma regions – the hot plasma sheet, the cold ionosphere and plasmasphere, the ring
current region, the radiation belts, and the evacuated magnetotail lobes – with complex overlaps,
interactions, and feedbacks in the energy exchanges between them, the solar wind, and the Earth’s
atmosphere (Fig. 1.1). The interest in Ravens arises from the recognition that geospace must be understood as a complex coupled system, in which fundamental plasma processes, magnetospheric
morphology, and global dynamics interact. For instance, the magnetosphere breaths with a natural
rhythm, the substorm cycle, that produces vivid auroral displays every few hours. The solar wind is
gusty, but this does not explain the inherent characteristic quasi-periodicity of magnetospheric dynamics; even in the event of constant solar wind driving, time-dependent behaviour arises. Rather,
this is symptomatic of a non-linear storage-and-release system that accumulates energy tapped
from the solar wind before explosively dissipating it in the atmosphere in intense, short-duration
bursts. This non-linear behaviour arises because of the structure that emerges from the magnetic
flux and plasma transport processes that are driven by the solar wind interaction itself (Fig. 1.2).
Ravens • Auroral and magnetospheric imaging mission • 5
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Figure 1.2. Ravens will explore the coupling that produces magnetospheric structure and time-dependent dynamics.
Ravens will study the magnetospheric engine that drives dynamics, it will study the magnetospheric structure that emerges as a consequence of the dynamics, and it will study the feedback
mechanisms which cause time-dependent behaviour to arise. The NASA Time History of Events and
Macroscale Interactions during Substorms (THEMIS) multi-satellite mission was predicated on the
dichotomy of the inside-out vs. outside-in scenario of substorm triggering. Ravens will answer the
question: why does spontaneous time-dependent behaviour (substorms and geomagnetic storms)
occur at all?
The Earth’s magnetosphere acts as a laboratory for exploring the large-scale dynamics of the environments of other magnetized astrophysical systems. In particular, the magnetosphere presents an
unparalleled opportunity to study the emergent structures and behaviours that arise when a nonlinear plasma system is driven into a non-equilibrium state through the continuous and variable external forcing of a stellar wind (Fig. 1.3).
Figure 1.3. The interaction between many astrophysical bodies and
their environments is thought to be mediated by magnetic fields.
The red giant Mira (above) reveals in UV a bow shock and long tail
with structure similar to that of a magnetosphere [Wareing, 2008].
Comet Encke (right) had its magnetotail docked by the impact of a coronal mass ejection and the resulting solar wind-magnetosphere coupling [Jia et al., 2009]. In this sequence of images, the Sun is to the left and the
sheath and magnetic cloud of the CME can be seen as the labelled light and dark regions labelled “Sheath” and
“Flux rope”.
The Earth’s magnetosphere is small in comparison with most other astrophysical systems, and is certainly more accessible, and yet its sheer volume presents a formidable challenge to characterizing its
structure and dynamics in either an average sense or instantaneously through in situ observations.
However, remote-sensing, and in particular imaging, of the plasma and magnetic environment of the
Earth is a tractable approach to studying the systems level structure and behaviour of the magnetosphere. The NASA Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite mission
made great strides towards true systems-level science (see the review “Magnetospheric Imaging:
Promise to Reality” by Burch [2005]), providing global images of the auroras, the plasmasphere, and
the ring current. However, the images of the different regions of the magnetosphere provided by
IMAGE were not continuous, and they were not necessarily contemporaneous. In contrast, the most
significant disturbances of the magnetospheric system, geomagnetic storms, evolve rapidly, on timescales less than an hour, but last from 10s of hours to days and involve all regions of the magnetosphere. Lack of suitable, continuous, high temporal resolution global observations has been the major stumbling block to understanding how of the coupled Sun-Earth system works.
Ravens • Auroral and magnetospheric imaging mission • 6
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The Ravens magnetospheric imaging mission will study this highly-interacting system, directly addressing the ESA Cosmic Vision themes 2.1. From the Sun to the edge of the Solar System and 1.3.
Life and habitability in the Solar System. To achieve this, Ravens must continuously measure the
time-dependent rate of magnetic flux transport within the magnetosphere and it must measure the
evolving morphology of geospace, the reservoirs of magnetic and hot and cold plasmas in the tail
and inner magnetosphere. Ravens will for the first time provide this vital continuous coverage by
utilizing two spacecraft. It will also allow observations of the auroras in the northern and southern
hemispheres simultaneously, which will provide information on the magnetic mapping between
magnetospheric regions and the ground in a way that has only very rarely been possible previously
(see Fig. 1.8) and never with identical cameras. These two innovations will provide a tremendous
advance in our understanding of magnetospheric dynamics.
In addition, a focus on coordinated ground-based observations and modelling will greatly enhance
the science return of the mission. For instance, large collaborative ground-based observatories such
as SuperDARN and SuperMAG, and large networks of all-sky imagers will be important complements
for Ravens (see Sect. 2.5). Ravens will harness existing and developing ground-based observatories
to provide measurements supportive of the science goals that cannot be obtained from spacecraft,
including detailed measurements of ionospheric parameters such as conductances and horizontal
plasma motions. It will also incorporate state-of-the-art physics-based modelling to provide a 3D,
time-dependent framework within which to assimilate and synthesise the space- and ground-based
observations.
In summary, the Ravens Science Questions are:
•
•
•
•
How does a magnetosphere assemble and organize itself?
What are the feed-back loops in solar wind-magnetosphere coupling?
What creates geomagnetic storms?
Why are the northern and southern auroras asymmetric?
To address these questions, Ravens will investigate the Magnetospheric Engine, Magnetospheric
Structure, and Magnetospheric Feedback.
1.2. The Magnetospheric Engine
1.2.1. Magnetic reconnection and magnetic flux transport. The dynamics of the magnetosphere
are driven by the fundamental plasma process of magnetic reconnection occurring at the magnetopause and in the magnetotail, the Dungey cycle [Dungey, 1961]. To first order, magnetized plasmas
of different origins cannot mix, leading to cellular regions of space demarcated by thin current
sheets. The boundary between interplanetary space, dominated by the solar wind and it’s embedded solar or interplanetary magnetic field (IMF), and the magnetosphere is such a current sheet, the
magnetopause. Magnetic reconnection occurs at such boundaries to create a topological interconnection of the magnetic fields, allowing ingress and egress of plasma and imparting tangential stress
between regions (Fig. 1.4). The rates and time-dependence at which reconnection occur at the
magnetopause and in the magnetotail are central to creating the dynamics and structure of the
magnetosphere. The factors which control magnetic reconnection, and hence magnetospheric dynamics and energy deposition in the atmosphere, are not understood. Ravens will for the first
time make continuous global observations of the auroras and the polar caps, allowing unbroken
determination of the rates of magnetic reconnection and the factors that influence these. The auroral oval maps to the plasma sheet, the region of closed magnetic flux sandwiched between the
open lobes. The dim polar cap inside the northern and southern auroral ovals map to the lobes and
the size of these indicates the proportion of flux that is open [e.g. Laundal and Østgaard, 2010]. The
rate at which the polar caps expand and contract is a measure of the reconnection rates (Figs. 1.5
and 1.6): as magnetic reconnection occurs at the magnetopause the polar caps expand; when reconnection occurs in the magnetotail the polar caps contract, accompanied by an explosive enRavens • Auroral and magnetospheric imaging mission • 7
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hancement of precipitation into the ionosphere, causing increased electrical conductance and energy deposition [Cowley and Lockwood, 1992; Milan et al., 2003]. The expansions and contractions of
the polar caps are linked to ionospheric motions which in turn lead to frictional heating of the atmosphere. The rates of energy deposition in the atmosphere by precipitation and Joule heating
are not well known; Ravens will address this and determine the factors that influence it. By imaging both polar hemispheres in the entire energy range important for ionospheric electrodynamics,
Ravens will provide an unprecedented picture of how the Earth is coupled to space.
Auroral images and combined ionospheric plasma flow measurements can provide localised measurements of reconnection rates along the open/closed boundary [Hubert et al., 2006, 2008; Chisham
et al., 2008], identifying meso-scale structure within reconnection regions. This is important for
probing reconnection spatial and temporal sub-structure within the magnetotail and at the magnetopause, either reconnection transients within substorms (e.g. poleward boundary intensifications,
Lyons et al. [1999]), quiet-time magnetotail reconnection [e.g. Grocott et al., 2005], subsolar reconnection (flux transfer events or FTEs, e.g. Milan et al. [2000a]), or high latitude cusp reconnection
[Chisham et al., 2004; Milan et al., 2000b; Imber et al., 2006].
Figure 1.4. The magnetic topology of the
magnetosphere. Magnetic reconnection occurring at the dayside magnetopause at a rate
ΦD has interconnected with closed magnetic
field lines (red) to create open magnetic field
lines (blue). Tangential stress stretches the
open field lines to produce an extended magnetotail in which magnetic energy accumulates. Reconnection occurs sporadically in the
nightside magnetosphere to reclose magnetic
flux at a rate ΦN. The open magnetic flux content of the magnetosphere, FPC, dictates the
size of the polar caps, the dim regions encircled by the auroral ovals. The combined action of dayside and nightside reconnection
creates a time-dependent transport of magnetic flux across the polar caps, at a rate ΦPC,
which also leads to antisunwards drift of the
ionosphere across the poles.
Although there is still great debate about the factors that control the dayside reconnection rate [e.g.
Borovksy, 2014], there has been some success in relating this to upstream solar wind conditions [e.g.
Newell et al., 2007; Milan et al., 2007, 2012]: the magnetosphere seems to accumulate open flux at
the rate that magnetic flux is transported towards it in the solar wind, that is linear driving. However, the magnetospheric response to solar wind driving is varied. As first speculated by Lockwood and
Cowley [1992] and later confirmed [e.g. Milan et al., 2003, 2007] the episodic disconnection of open
flux is related to the occurrence of substorms; that is, substorms represent the closure of the Dungey cycle and are a fundamental aspect of the solar wind’s interaction with the magnetosphere.
Sometimes the response is inherently time-dependent, releasing magnetic flux and depositing energy in discrete bursts: substorms. At other times, the magnetotail reconnection rate adjusts itself to
the dayside rate, the magnetosphere does not accumulate energy and substorms do not occur: the
result is a laminar convection state known as steady magnetospheric convection (SMC) events. The
proportions of time that the magnetosphere displays SMC or substorm behaviours, the factors
that govern the response, and the ramifications for energy deposition in the atmosphere are not
known. Previous imaging has been discontinuous, prohibiting measurements over all phases of all
behaviours. Ravens will provide for the first time continuous observations of the coupling to address these issues. Over a three-year mission, assuming 3 substorms a day, Ravens will observe
3500 substorms, with continuous monitoring of changes in open flux.
Ravens • Auroral and magnetospheric imaging mission • 8
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When the magnetosphere undergoes substorm cycles, it is thought that the accumulation of open
flux causes the magnetosphere to inflate and the tail magnetopause flares outwards. This increases
the cross-sectional area that the magnetosphere presents to the solar wind and leads to the build-up
of pressure in the tail. This in turn increases the chance of the onset of magnetotail reconnection in
the central tail. It is not known what controls the open flux threshold for substorm onset, nor indeed what causes the delay in onset of magnetotail reconnection to give substorm behaviour.
Continuous observations of open flux content using high-cadence imaging will allow this to be explored.
Figure 1.5. (Left) observations of the expanding/contracting polar cap (a and b) over a period of
11 days (c) from IMAGE-SI. Large enhancements in
dayside reconnection (e) occur during solar wind
disturbances. These produce large enhancements
in polar cap size, which in turn drive enhanced
magnetotail reconnection, injecting plasma from
the plasma sheet into the inner magnetosphere,
enhancing the ring current, here sensed by the
magnetic perturbation it produces, measured as
depressions in the Sym-H index. Enhanced polar
cap sizes and ring current intensity are thought to
be linked, though the link is not understood.
Figure 1.6. (Right) Observations of the auroras on
timescales of a few hours show the characteristric
“breathing” of the magnetosphere, known as the substorm cycle, the open flux of the polar cap (FPC) increasing and decreasing as the polar cap expands and contracts (a). Expansions occur during periods of magnetopause reconnection associated southwards-directed
IMF, BZ < 0 (d). Contractions are associated with magnetotail reconnection, that is the occurrence of substorms (two minor, two major in this example), leading
to enhanced auroral emissions (b) and activations of the
auroral electrojets (c). The observations of FPC allow the
dayside and nightside reconnection rates (e) and the
magnetic flux transport in the Dungey cycle (f) to be
quantified. Ravens will provide such observations continuously for the duration of the mission; previously this
was only possible for periods of 10 hours (IMAGE).
1.2.2. Temporal and spatial variability. While expansions and contractions of the polar caps reveal
the cycles of the magnetospheric engine, specific features within the auroras reveal the working
parts of that engine, allowing the spatial and temporal variability of reconnection to be investigated.
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Reconnection at the magnetopause and in the magnetotail is episodic due to the natural variability
of the solar wind, but also by an apparent inherent variability within the reconnection process itself.
At the magnetopause during southward IMF this manifests itself as quasi-periodic bursts of reconnection, known as flux transfer events (FTEs), with repetition periods thought to range from 10s of
seconds to several minutes [e.g. Lockwood and Wild, 1993]. FTEs can have characteristic auroral
signatures which were first identified in global auroral images by Milan et al. [2000a]. When the IMF
is directed northwards, “lobe” magnetic reconnection occurs at high latitudes, tailwards of the openings of the cusps. Under sufficiently dense solar wind conditions, the footprint of the reconnection
site can be visible as an auroral “cusp spot” within the noon-sector polar cap (see Fig. 1.7), its location in local time being controlled by the east-west orientation of the IMF [Milan et al., 2000b; Frey
et al., 2002]. Unlike southward-IMF reconnection, lobe reconnection is not constrained to occur
equally in the northern and southern hemispheres, and indeed the interrelation of lobe reconnection in the two hemispheres is unknown. A special case occurs when the IMF is oriented directly
northwards: in this situation lobe reconnection can occur in both hemispheres on the same IMF field
line, known as “dual-lobe reconnection”, closing previously open flux [Imber et al., 2006]. This process has profound implications for magnetospheric dynamics as it is postulated to be the most efficient magnetospheric mass-loading process known (see also Sect. 1.3.5). The cadence of previous
imagers has not been sufficient to properly analyse the dynamics of flux transfer events nor cusp
spots. More importantly, the conjugate nature of these cusp and substorm features is entirely
unknown as interhemispheric observations are not available. Ravens will provide high cadence,
interhemispheric observations, with identical instruments, of dayside and nightside reconnection
signatures.
Figure 1.7. IMAGE-WIC images of the global FUV auroral distribution during a particularly active period. Note the unusually large cusp features in the first three images.
Figure 1.8. Simultaneous views of the auroras in the northern and
southern hemispheres by the Polar and IMAGE spacecraft show
that while some features are symmetrical, many are not, challenging current understanding of the mapping of the magnetic field
between hemispheres and the fundamental plasma-physical processes that result in auroral emission. Observations of this nature
are exceedingly rare due to the lack of coordination between previous missions.
This study was selected for the cover
of Nature (v. 460, no. 7254, 2009),
reflecting the importance attached
to the acquisition of conjugate auroral images for improving our understanding of magnetospheric structure and dynamics. Taken from
Laundal and Østgaard [2009].
Ravens • Auroral and magnetospheric imaging mission • 10
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Based on simultaneous imaging by IMAGE and Polar it is now well established that the substorm onset locations in the conjugate hemispheres are usually asymmetric, controlled by the orientation of
the IMF [Østgaard et al., 2004; Wang et al., 2007], though the reasons for this asymmetry are poorly
understood. Laundal and Østgaard [2009] reported that the auroras can be completely asymmetric
(see Fig. 1.8), which may be the first clear observations of interhemispheric currents due to seasonal
differences [Richmond and Roble, 1987; Benkevich et al., 2000]. Simultaneous interhemispheric
imaging provided by Ravens will for the first time allow study of this important but poorlyunderstood asymmetric interaction between solar wind, magnetosphere and ionosphere.
1.3. Magnetospheric Structure and Feedback
1.3.1. Relationship between dynamics and structure. The observed structure of the magnetosphere is created by the dynamics associated with solar wind-magnetosphere coupling (Fig. 1.1).
The opening of magnetic field lines allows ingress of solar wind plasma into near-Earth space; the
stretching and increase in volume of these flux tubes as they form the geomagnetic tail creates two
regions of space containing a very dilute plasma, the magnetotail lobes. Magnetic reconnection in
the central plane of the tail causes the magnetic field lines of the lobes to converge, sandwiching
plasma in a plasma sheet. This dense plasma is heated by the reconnection process, creating a reservoir of hot plasma. Reconnection injects this plasma into the inner magnetosphere where it becomes trapped; the resulting plasma pressure (dominated by ions) is to first-order in balance with
the J×B force, where J are the required electrical currents and B is the perturbed field. This electrical
current is what is referred to as the ring current, in the case of an azimuthally symmetric pressure
distribution [Parker, 1957]. Particles from the plasma sheet and ring current precipitate into the atmosphere enhancing the ionospheric density and modifying its conductivity. The atmosphere feeds
ionospheric plasma up into the magnetosphere creating an inner region which does not participate
in the Dungey cycle flow, but corotates with the Earth: the plasmasphere acts as a reservoir of
dense, cold plasma deep in the magnetosphere. The ionosphere is mostly produced by solar illumination and photochemistry, but is also modified by processes occurring due to transport in the Dungey cycle and precipitation from the overlying plasma regions. As the polar caps expand and contract, the size and relative locations of the different plasma regions change, such that they overlap
and interact (Fig. 1.9). These regions are created by the dynamics, but once formed they also
feedback on the dynamics, altering the behaviour of the system, leading to highly non-linear behaviour. This behaviour is not understood and it is a central aim of Ravens to investigate this behaviour and understand the influence of each region on the system.
Figure 1.9. FAST ion energy spectra
(left panel) for 135 hours bracketing
two geomagnetic storms (depressions
in Sym-H, top left). The data is from
successive passes of the evening sector auroral oval (see footprints bottom right). During these storms, the
equatorward (inner) boundary of the
ion plasma sheet undergoes significant movement in latitude, carrying it
deep into the inner magnetosphere,
across the outer radiation belts.
Studying the two-dimensional spatiotemporal interaction of different
plasma regions is a primary objective
of Ravens.
1.3.2. Plasma sheet. The plasma sheet is supplied with solar wind plasma through the convective
transport associated with the Dungey cycle. This cycle keeps the plasma sheet sandwiched between
the overlying magnetotail lobes, though sometimes the nightside auroras show a double-oval conRavens • Auroral and magnetospheric imaging mission • 11
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figuration, which is not currently understood. When the convection cycle slows, due to low dayside
reconnection, the magnetosphere can become topologically complex and transpolar arcs (TPAs) or
theta aurora bisect the polar cap (Fig. 1.10). There is controversy regarding whether TPAs occur as a
consequence of field-aligned currents associated with convection flow shears on open magnetic field
lines or whether they represent a protrusion of the plasma sheet to unusually high latitudes, closed
magnetic flux bisecting the open lobes [e.g. Zhu et al., 1997]. Recently proposed formation mechanisms [Milan et al., 2005; Fear et al., 2014] suggest that TPAs may be magnetically conjugate, but
not symmetrically located in the two hemispheres; the conjugacy or lack thereof of TPAs is crucial to
understanding this phenomenon [Østgaard et al., 2003]. To investigate these complex magnetospheric topologies and connect their origins to solar wind-magnetosphere interactions and magnetospheric dynamics, Ravens will observe transpolar arcs and other auroral phenomena in both
hemispheres under a wide range of solar wind and IMF conditions, including events where those
conditions are changing rapidly.
During enhanced geomagnetic activity, intense particle precipitation into the atmosphere and Joule
heating lead to the upwelling and outflow of heavy ionospheric ions into the plasmasheet, impacting
subsequent magnetospheric behaviour [e.g. Gazey, 1996]. Currently, the rate of mass-loading of
the plasmasheet and its relationship to specific outflow-mechanisms is poorly understood. Ravens
will investigate ionospheric outflow rates and their relation to geomagnetic activity.
Fig. 1.10. DE1 observations of a transpolar arc or theta aurora. Interhemispheric
observations of such arcs are essential to further understanding of their complicated magnetic topology.
1.3.3. Ring current. The plasma acceleration and transport processes that form ring currents are
universal phenomena that are still surrounded by mystery. Recent observations and modelling
strongly imply that “fronts” of plasma are not simply convected in and heated adiabatically [Yang et
al., 2008]. Instead, plasma instabilities seems to play a critical role in overcoming the “pressure catastrophe”, by transporting plasma in finger-like structures into the inner magnetosphere, not unlike
interchange instabilities. In-situ measurements have only been able to speculate on their nature,
and no imaging mission has been able resolve these yet in the ring current, but IMAGE-WIC images
have revealed patterns in the auroras that are consistent with modelled interchange instabilities in
the ring current [Yang et al., 2008]. Ravens will investigate interchange instabilities, charge exchange losses, adiabatic drift mechanisms, and ring current-plasmasphere interactions within the
inner magnetosphere.
Figure 1.10. Example of the
dynamic ring current. IMAGE/HENA images showing a
transition from a highly asymmetric main phase ring current
to a symmetric recovery phase
ring current.
The acceleration of plasma sheet plasma and its transport inwards is critical for controlling dynamics
of the inner magnetosphere. The resulting plasma pressure is highly dynamic (Fig. 1.10) and severely distorts the magnetic field of the inner magnetosphere [Tsyganenko et al., 2003], which is the
governing framework for the outer electron radiation belt. Ravens will investigate the inward
transport of plasma to form the ring current and the magnetospheric behaviours, e.g. substorms,
that cause this.
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The pressure also drives a 3D current system, which is the so-called partial ring current (PRC) during
storm main phases and the symmetric ring current during recovery phases. Its ionospheric fieldaligned current counterpart is the Region-2 FAC system, which connects the magnetosphere and
ionosphere, producing fast, localized flows known as Sub-Auroral Polarization Streams (SAPS)
[Brandt et al., 2008], modifying the convective flows driven by the Dungey cycle (Fig. 1.11). Ravens
will investigate the role of the ring current in magnetosphere-ionosphere coupling and geomagnetic storm phenomena.
Figure 1.11. A typical plasma pressure distribution
depicted here in the equatorial plane, retrieved
from IMAGE/HENA images; simultaneous TEC and
ionospheric flow measurements from Millstone
Hill and SuperDARN provide the ionospheric response such as SAPS that are a core part of disrupting navigation systems.
1.3.4. Plasmasphere. The plasmasphere is cold plasma of ionospheric origin that is trapped within
the corotating portion of the inner magnetosphere. The cold plasma distribution has a significant
modifying effect on a number of important particle-particle and wave-particle interactions within
the inner magnetosphere. The most important, but poorly understood, loss process for trapped
energetic electrons in the radiation belts is pitch-angle diffusion driven by self-generated whistler
mode waves. This interaction between the cold and very hot plasma regions is a key area that Ravens will address.
The extent of the plasmasphere (Fig. 1.12) depends on the balance between corotation and convection, which in turn depends on the strength of the solar wind interaction and reconnection within
the magnetotail, such that the plasmasphere expands during quiet magnetospheric conditions and
contracts when the convection is strongly driven, for instance during geomagnetic storms. During
contractions the outer edges of the plasmasphere are stripped off and “drainage plumes” become
entrained within sunwards convection on the dusk-side magnetosphere [Spasojević et al., 2003] and
intersect the dayside solar wind-magnetosphere coupling region (see Sect. 1.3.5). On the other
hand, following a storm the emptied flux tubes are replenished with ionospheric plasma over a period of days. The convection process is variable on time scales ranging from minutes to days, and the
contrasting time scales of source and loss, and the variable position of the plasmapause produce a
wide variety of plasmaspheric structures, including a steady “drainage plume”, variable plasma blobs
detached from the plasmasphere, shoulders and channels. The processes by which plasmaspheric
refilling occurs and the resulting distribution of plasma along the field lines are not presently wellunderstood, nor are the time-dependent processes that produce plasmaspheric loss and the
transport of cold plasma within the magnetosphere. Ravens will be able to resolve the importance of these source and loss processes for plasmasphere, and study the interaction of plasmaspheric plasma with other magnetospheric processes.
Figure 1.12. The plasmasphere as observed by the EUV instrument on IMAGE. The Earth, dayside airglow, and auroras are apparent in the centre of the image. The diffuse background surrounding this is EUV sunlight scattered from the plasmasphere.
The outer edge of the plasmasphere represents the boundary
between corotation and convective magnetospheric flows; timedependent source and loss produces complex structure.
Storm-time drainage plumes can
intersect the dayside magnetopause where they hinder solar
wind-magnetosphere coupling.
Ravens • Auroral and magnetospheric imaging mission • 13
13
1.3.5. Feedback and Geomagnetic Storms. The preceding discussion indicates that solar windmagnetosphere-ionosphere coupling is a complicated process, involving an interplay of the plasma
regions that will be imaged by Ravens. Fig. 1.13 presents a schematic of some of the interactions
that will be studied by Ravens. It is possible that geomagnetic storms, which occur when the magnetosphere is subjected to extreme solar wind conditions such as a solar wind shock or greatly enhanced dayside coupling, represent a pathological interaction and feedback between these systems.
The following feedback paths associated with intense strong solar wind-magnetosphere coupling
have been identified, but have yet to be fully studied:
•
•
•
•
•
Enhanced dayside reconnection leads to tail reconnection, injecting plasma into the inner magnetosphere which ultimately precipitates into the ionosphere; enhanced ionospheric conductance leads to frictional coupling between the ionosphere and atmosphere braking convection
[Grocott et al., 2009].
Enhanced atmospheric heating associated with the auroral precipitation can lead to the outflow
of heavy ionospheric ions which mass-load the plasma sheet and modify its subsequent behaviour [e.g. Gazey, 1996].
The formation of a “cold, dense plasma sheet” by solar wind capture through inward diffusion
at the magnetospheric flanks or lobe reconnection [e.g. Imber et al., 2006] can also mass-load
the plasma sheet and modify its subsequent behaviour .
Precipitation into the inner magnetosphere enhances the ring current; outside the ring current
region this produces a magnetic perturbation that dipolarizes the magnetotail chocking
nightside reconnection. More open flux accumulates in the magnetospheric lobes, causing the
magnetotail to flare outwards until the internal pressure rises sufficiently to restart reconnection [Milan et al., 2009] (cf. relationship between open flux and ring current intensity in Fig.
1.5).
Enhanced dayside reconnection leads to strong convection which strips the outer plasmasphere
into a drainage plume; the drainage plume mass-loads the dayside magnetopause with heavy
plasma and chokes the dayside reconnection process [Borovsky and Denton, 2006; Walsh et al.,
2014].
It is probable that the time-dependent substorm behaviour of the magnetosphere during more quiescent periods is also produced by similar feedback mechanisms. Another key area of uncertainty is
the influence of asymmetrical (summer/winter) ionospheric conductance on magnetospheric dynamics.
Figure 1.13. Flow chart of mass end energy flow between various regions in geospace.
Ravens • Auroral and magnetospheric imaging mission • 14
14
The complex interplay of these processes can only be properly studied with the coordinated observations of different magnetospheric regions and processes that will be provided by Ravens. It
is essential that these observations be continuous in nature, but also of sufficiently high time resolution, as storms can evolve over many 10s of hours, with many of the processes occurring at different stages in the storm and with different time-scales. A full understanding of storms has not
previously been achieved because of a lack of continuity in the observations. Ravens is specifically
designed to overcome this lack of continuity.
1.4. Summary of Science Objectives
Here, we summarize the Science Objectives of Ravens, which are directly aligned with international
research priorities (see Sect. 2.6). Ravens will study:
•
the engine that drives magnetospheric dynamics, the magnetospheric structure that arises
as a consequence of the dynamics, and the feedback mechanisms which lead to timedependent behaviour;
•
the factors which control magnetic reconnection at the magnetopause and in the magnetotail; to determine the onset thresholds for storms and substorms;
•
the time-dependence and conjugacy of reconnection signatures at the magnetopause and in
the magnetotail;
•
the rate of energy deposition in the atmosphere by precipitation and Joule heating and the
factors that influence it;
•
the proportions of time that the magnetosphere displays steady or time-dependent behaviour, and the factors that govern this response;
•
the conjugacy of auroral features in both hemispheres to understand complex magnetospheric topologies;
•
mass-loading of the plasma sheet by ionospheric heavy-ion outflow, and the impact on subsequent magnetospheric behaviour;
•
the relationship between substorms and ring current injections, and the role of the ring current in geomagnetic storm phenomena;
•
the interaction of the ring current and plasmasphere, including interchange instabilities,
charge exchange losses, and adiabatic drift mechanisms;
•
the interaction between the plasmasphere and radiation belts, including the loss processes
of trapped energetic electrons;
•
plasmaspheric filling and loss mechanisms, including drainage plumes, and their interaction
with other magnetospheric processes.
Ravens • Auroral and magnetospheric imaging mission • 15
15
2. Science Requirements
2.1. Overarching Science Requirements
To achieve the science objectives summarized in Sect. 1.4, Ravens is required to make
•
•
•
•
•
•
continuous observations of the northern and/or southern auroras
regular views of both northern and southern auroras
continuous imaging of the plasmasphere
regular stereoscopic views of the plasmasphere
continuous imaging of the ring current
regular stereoscopic views of the ring current
In addition, the Ravens mission will be much enhanced by coordination with ground-based observatories, to provide ionospheric convection measurements and magnetic perturbations associated
with the ring current and ionospheric electrojets. Data exploitation will be further enhanced by coordinated physics-based modelling of the magnetosphere.
The observing requirements and instruments proposed to satisfy them (see Sect. 3) are summarized
in Table 2.1. Instrument performances are well-understood in terms of previous imaging missions,
and indeed many of the instruments proposed are heritage designs from Polar, IMAGE, or other previous imaging missions.
Supporting facilities
Ravens spacecraft / instruments
Table 2.1. Ravens science requirements (including cadence and spatial resolution where appropriate), related
observables, the instruments that will provide the required measurements
Requirement
Observable(s)
Electron auroras, energy discrimination, continuous observation, 10 s, 50
km (500 km for X-rays)
LBH-l and LBH-s emissions
from N2, bremsstrahlung Xrays
UVAMC-0, -1
XIR
Proton auroras, continuous observation, 10 s, 50 km
Doppler shifted Lyman-α
from charge-exchanging H
Ravens-SI
Plasmasphere density,
continuous observation, 3D reconstruction, 30 min
30.4 nm sunlight scattered by
+
He
EPI
Ring current H and O ion density in
the 10-200/nuc keV range, continuous observation,3D reconstruction,
heavy ion outflow <3 keV, 10 min
Energetic Neutral Atoms from
charge-exchanging ring current ions and outflowing
ionospheric heavy ions
NAIR-Hi, -Lo
Ionospheric parameters, frequent or
continuous observation, auroral observations, 1 min
Electron density profile,
Ion outflow,
Current density
Ionospheric convection, northern and
southern hemisphere, continuous
observation, 1 min, 50 km
Horizontal F region plasma
drift
SuperDARN
Physics-based modelling and assimilation
Plasma parameters, magnetic
fields, processes, feedbacks
GUMICS-4+
+
+
Instrument(s)
EISCAT,MIRACLE
SuperMAG, ALIS
NORSTAR
2.2. Auroral imaging
Auroral imaging provides information on the structure and dynamics of the plasma sheet and ring
current, energy deposition in the atmosphere by precipitation, the open magnetic flux content of
the magnetosphere, and the conductivity distribution of the ionosphere. Auroral emission is produced by the precipitation of energetic electrons and protons. The imaging requirements of Ravens
Ravens • Auroral and magnetospheric imaging mission • 16
16
are similar to previous missions (e.g. Polar and IMAGE); new science arises from the requirement
that there are two spacecraft, allowing continuous and conjugate imaging, and that the spacecraft
are three-axis stabilized, allowing continuous pointing and hence high cadence imaging and/or long
exposure times to suppress dayglow contamination.
2.2.1. Spatial resolution and field-of-view. The spatial resolution of imaging of electron and proton
auroras needs to be of order 50 km, similar to the spatial resolution of previous imaging missions
and of the spatial resolution of ionospheric radars such as SuperDARN. To provide continuous imaging from the proposed orbits (see Sect. 4), targeting latitudes poleward of 50° geomagnetic latitude,
the angular (edge-to-edge) field-of-view of the cameras is required to be 20°.
2.2.2. Electron auroras. Most auroral emission is produced by impact-excitation of atmospheric
constituents by high energy precipitating electrons. These electrons carry upward field-aligned currents and so are a key signature of magnetosphere-ionosphere coupling. High energy electrons
penetrate deeper into the atmosphere and produce ionization at lower altitudes. Deposition in the
E region (below 150 km) increases the conductance of the ionosphere, and quantifying the conductance is a key science requirement of Ravens. The energy spectrum of the precipitation can be deduced by simultaneously imaging at a range of wavelengths that respond differently to different
electron energies.
It is proposed to measure electron auroral emissions in four wavelength bands: the long- and shortwavelength bands of the FUV Lyman-Birge-Hopfield emissions of N2 (LBH-l and LBH-s), OI-3s 5S0 - 2p4
3
P emission at 135.6 nm, and X-ray emissions. LBH-l emissions are proportional to the energy flux of
electrons above 1 keV. LBH-s emissions are absorbed by O, so emissions from deep in the atmosphere are attenuated at a spacecraft at high altitude. Hence, the ratio of intensities of LBH-l to LBH-s
are indicative of electron energies between 1 and 20 keV [Germany et al., 1994]. X-ray emission is
produced by Bremsstrahlung of electrons with energies in excess of 30 keV, and quantify the highenergy tail (up to 150 keV) of the precipitating spectrum. OI-135.6 nm emission is proportional to
total electron flux. Cameras capable of imaging LHB-s, LBH-l and X-rays were flown on Polar (UVI
and PIXIE, respectively). Imaging at 135.6 nm was achieved with the Spectrographic Imager flown on
IMAGE (IMAGE-SI).
2.2.3. Proton auroras. Proton auroras are seen at the footprint of the magnetospheric cusp, where
they are injected from the magnetosheath by lobe reconnection [Frey et al., 2002; Phan et al.,
2003], though their relationship to similar electron auroras [Milan et al., 2000b] is still unknown.
They also precipitate on the nightside due to pitch angle-scattering in the non-dipolar field geometry, providing a tracer of the magnetotail magnetic topology [Donovan et al., 2003]. Due to their
much larger mass, protons are not so much affected by electric fields as the electrons are and their
interaction with waves and scattering processes are more dominant.
Proton precipitation produces Lyman-α emission as protons charge-exchange with atmospheric constituents to produce excited H atoms. To distinguish emission by precipitating (down-going) protons
from the background Lyman-α emission of the atmosphere, the red-shifted wing of the Lyman-α line
is imaged. This requires a system that efficiently rejects both the geocoronal Lyman-α emitted near
the 121.6 nm rest wavelength and the NI multiplet a 120 nm. IMAGE-SI was capable of imaging proton auroral emission.
2.2.4. Dayglow suppression. The ability to suppress non-auroral emissions is vital for Ravens science. This is necessary for conjugate imaging because the dayside aurora will be sunlit in at least
one of the hemispheres most of the time. It matters for global dynamics, because quantifying for
example the amount of open flux requires being able to identify the polar cap boundary at all local
times. Proton auroral imaging spectrographs suffer less dayglow than do electron auroral imagers
because suppression of non-Doppler shifted Lyman-α emissions does not reduce the in-band signal.
For the electron auroral imagers, filters will be used to suppress out-of-band contamination due to
dayglow, but this does decrease the in-band signal. For the electron auroral observation the requirement to suppress the effects of dayglow place limits on cadence, resolution, and sensitivity.
Ravens • Auroral and magnetospheric imaging mission • 17
17
2.2.5. Cadence. Ravens requires auroral imaging at cadences of order 10 s to image wave processes
and rapid substorm and dayside auroral phenomena.
2.2.6. Conjugacy. Ravens requires imaging of both northern and southern auroras to study the
magnetic topology of the magnetosphere, the mapping of dayside and nightside auroral phenomena, and differing auroral acceleration between hemispheres.
2.3. Plasmaspheric imaging
Ravens will obtain global images of the plasmasphere by observing EUV sunlight at 30.4 nm resonantly scattered from singly-ionised Helium, a minor magnetospheric constituent which allows extrapolation of overall magnetospheric density [Sandel et al., 2003]. He+ 30.4 nm is the brightest ion
emission from the plasmasphere, it is spectrally isolated, and the background at that wavelength is
negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically
thin, so its brightness is directly proportional to the He+ column abundance.
The imaging requirements of Ravens are similar to those of the EUV instrument flown on IMAGE:
~0.6° or ~0.1 RE in the equatorial plane seen from apogee, with a 90° (edge-to-edge) angular field-ofview. A sensitivity of 0.2 count sec-1 Rayleigh-1 is sufficient to map position of the plasmapause with
a time resolution of 10 minutes or better.
With Ravens, simultaneous views of the plasmasphere emissions from differing viewpoints will allow
an unprecedented 3D reconstruction of the structure and dynamics of cold plasma in the magnetosphere.
2.4. Ring current, plasma sheet, and ion outflow imaging
Energetic neutral atoms (ENA) are produced when singly positively charged energetic ions undergo
charge exchange collisions with cold neutral atoms or molecules. The ions will become neutral and
propagate unaffected by electromagnetic fields. If the initial energy is much greater than the planetary escape energy (0.6 eV/nucleon), then the ENAs are unaffected by gravitational fields and will
maintain their energy and momentum. In addition to carrying with it spectral and directional information of the energetic ions, the ENA also is a direct measurement of the composition of those ions.
In the terrestrial magnetosphere the ring current will charge exchange with the geocorona at high
altitudes and emit ENAs, allowing the ring current and plasma sheet ion populations to be imaged,
including the inner magnetosphere ion pressure distribution and resulting currents [Roelof et al.,
2004]. At low altitudes, ENAs are emitted by charge-exchanging outflowing ionospheric ions, allowing outflow processes to be studied.
The closest any cameras have come to imaging the plasma pressure of the magnetosphere was the
High Energy Neutral Atom (HENA) camera on board the IMAGE mission, which measured the velocity, trajectory, energy, and mass of ENAs in the 10-500 keV energy range. This camera covered about
65% of the plasma pressure of the inner magnetosphere, which is dominated by protons and O+ in
the ~10-300 keV range. The TWINS mission flies MENA-heritage cameras (lower energy range than
HENA and no clear mass resolution) on two spacecraft which allows retrieval of the spatially dependent pitch-angle distributions of the ring current, and thus provides a better 3D representation.
The requirements of the Ravens ENA cameras is similar to IMAGE/HENA: sensitive to H, He, O, and
heavier atoms, with an angular (edge-to-edge) field-of-view of 120° × 90°, angular resolution of 5° ×
5°, energy range of 20-500 keV nucleon-1, energy resolution ∆E/E of 0.2, and a velocity resolution of
50 km s-1 (time-of-flight of 1 ns).
Ravens • Auroral and magnetospheric imaging mission • 18
18
2.5. Coordination with ground-based observatories and modelling
The Ravens mission will be augmented by two crucial additions: ground-based observations of the
ionosphere which provide information on the lower boundary of the magnetosphere and its coupling with the atmosphere, and physics-based simulation of the magnetosphere which will allow the
physical coupling between the regions imaged by Ravens to be investigated.
2.5.1 Ground-based instrumentation. Systematic recordings of auroral phenomena with groundbased (GB) instrument networks (magnetometers, auroral cameras and ionospheric radars) were
started in the northern Fennoscandia and Svalbard in the late 1950s during the International Geophysical Year. Some 30 years later the first comparison studies of GB data with satellite data were
made [Opgenoorth et al., 1980; Pellinen et al., 1984], and since then joint analysis of magnetically
conjugated GB and space-based observations has been fundamental to many advances in understanding the coupled solar wind-magnetosphere-ionosphere system. The following GB networks are
examples of the instrumentation that will support and augment Ravens science.
MIRACLE. The Magnetometers/Ionospheric Radars/All-sky Cameras Large Experiment (http://www.
ava.fmi.fi/MIRACLE/) is a meso-scale network of magnetometers and auroral cameras in the Fennoscandian sector, covering an area from sub-auroral to polar cap latitudes over a longitude range of
about two hours of local time. MIRACLE data are used to deduce value added data products, like
maps of equivalent currents, auroral precipitation fluxes, and Joule heating rates [Amm and Viljanen,
1999; Janhunen, 2001; Vanhamäki et al., 2009], parameters key to quantifying magnetosphereionosphere coupling in conjunction with global auroral images.
SuperDARN. The Super Dual Auroral Radar
Network (http://superdarn.jhuapl.edu/) continues to develop as the pre-eminent means
of measuring the ionospheric convection pattern at mid and high latitudes. A recent decadal review of SuperDARN science [Chisham
et al., 2007] reveals the profound connection
between auroral and convection measurements for studies of magnetospheric dynamics; an example of combining auroral imagery
and convection maps is presented in Fig. 2.1.
Figure 2.1. Combined auroral imagery and SuperDARN
measurements of global ionospheric convection.
ALIS. Auroral Large Imaging System (http://alis.irf.se) is a network of steerable auroral imagers in
northern Scandinavia designed for 3D measurements.
EISCAT. The existing European Incoherent Scatter radar system (http://www.eiscat.se/) is planned
to be augmented by a new EISCAT3D system, with planned operations for at least for 30 years. EISCAT3D will be unique in its capability to do volumetric imaging of ionospheric conditions, including
ion and electron temperatures, electron density and the ion velocity vector.
SuperMAG is a worldwide collaboration of ground based magnetometers. It currently includes data
from more than 300 stations. SuperMAG provides measurements of ground magnetic field perturbations in the same coordinate system, identical time resolution and with a common baseline removal
approach.
Geospace Observatory Canada and THEMIS ground-based auroral imaging. Across Canada and Alaska there are now more than 40all-sky imagers and soon a network of 10 imaging riometers. These
instruments provide contiguous imaging with high space and time resolution across multiple hours
of MLT and spanning the auroral oval in latitude. This network will provide small-scale quantitative
observations of the auroras in white-light, the Oxygen redline, and full-colour, as well as maps of
riometer absorption. These observations will be nested within the Ravens global images, providing
an exciting bridge between scales observed by Ravens and scales that matter for auroral electrodynamics.
Ravens • Auroral and magnetospheric imaging mission • 19
19
2.5.2. Physics-based magnetospheric modelling. In connection to the global imaging, modelling is a
key component of modern space research. Earlier space missions have provided mainly in situ
measurements from a spatially limited volume, making global models (e.g. the European GUMICS-4
model (Grand Unified Magnetosphere-Ionosphere Coupling Simulation), Fig. 2.2 [Palmroth et al.,
2001]) important for understanding the observed variations, and extrapolating the global circulation
of energy within the magnetosphere [e.g. Palmroth et al., 2006, 2010]. Such simulations are difficult
to calibrate as the conditions within the ionosphere are not quantitatively known [Palmroth et al.,
2006], however the global imaging data from Ravens provides this ground truth. The models are
currently being updated to treat the non-MHD, multi-temperature, multi-component plasmas in the
inner magnetosphere, the ring current and plasmasphere. Such new simulation tools will be of vital
importance in interpreting the Ravens imaging data, as they provide the self-consistent feedbacks
between the different domains, such as how the inner magnetospheric source population is energized and precipitated to the ionosphere, and what type of auroral phenomena they produce.
Figure 2.2. Simulation output from GUMICS, including
magnetospheric mass density and ionospheric flows and
conductances.
2.6. Alignment of Ravens with International Science Priorities
The Science Objectives and Science Requirements of Ravens are directly aligned with international
research priorities. The Committee on Space Research (COSPAR) has recently published its Roadmap
“Understanding Space Weather to Shield Society” (http://tinyurl.com/swxrm [Schrijver et al., 2015]),
the objective of which is to identify key areas of research leading to a demonstrable improvement in
service provision capability in the short, medium and longer term. Here we quote from Chapter 7
“Concepts for highest-priority research and instrumentation”:
The response of magnetosphere to solar wind driving depends on the previous state of magnetosphere. Similar sequences in energy, momentum and mass transfer from the solar wind to magnetosphere can lead in
some cases to events of sudden explosive energy release while in other cases the dissipation takes place as a
slow semi-steady process. Comprehensive understanding on the factors that control the appearance of the
different dissipation modes is still lacking, but obviously global monitoring of the magnetospheric state and
system level approach in the data analysis would be essential to solve this puzzle. Continuous space-based
imaging of the auroral oval would contribute to this kind of research in several ways. The size of polar cap
gives valuable information about the amount of energy stored in the magnetic field of magnetotail lobes.
Comparison of the brightness of oval at different UV wavelengths yields an estimate about the energy flux and
average energy of the particles that precipitate from magnetosphere to ionosphere. […]
The shape and size of the oval and intensity variations in its different sectors enable simultaneous monitoring
of the magnetosphere’s recovery from previous activity while new energy comes into the system from a new
event of dayside reconnection. There is consequently a need to achieve continuously global UV or X-ray images to follow the morphology and dynamics of the auroral oval […]
Imager data combined with ground-based networks […] allows solving the ionospheric Ohm’s law globally
which yields a picture of electric field, auroral currents and conductances with good accuracy and spatial resolution. This would mean a leap forward in our attempts to understand magnetosphere-ionosphere coupling,
particularly the ways how ionospheric conditions control the linkage, e.g., by field-aligned currents.
Ravens • Auroral and magnetospheric imaging mission • 20
20
3. Proposed Science Instruments
3.1 Overview
The Ravens science payload will comprise auroral imagers to observe the auroras from apogee, a
plasmasphere imager, and a ring current imager. Each Ravens spacecraft will be three-axis stabilized, with the instruments nadir-pointing, providing continuous imaging. The strawman instrument
payload (and potential providers) comprises:
Section
Acronym
Instrument package name
Potential provider
3.2
UVAMC
Ultra-violet Auroral Monitoring Cameras
University of Calgary
3.3
Ravens-SI
Far Ultraviolet Spectrographic Imager
University of Liège
3.4
XIR
X-ray Imager for Ravens
Universities of Bergen, Leicester
3.5
EPI
EUV Plasmasphere Imager
Mullard Space Science Laboratory
3.6
NAIR
Neutral Atom Imager for Ravens
JHU/APL, IRF, University of Ireland
UVAMC and Ravens-SI will image FUV auroral emissions from near apogee; XIR will image the X-ray
auroras from apogee; EPI and NAIR will image the plasmasphere and ring current, respectively, at all
points of the orbit.
Three auroral imaging packages are necessary to provide images of auroras produced by electron
precipitation and the energy spectrum of the incident electrons, and images of auroras produced by
proton precipitation. UVAMC will image the electron auroral emissions in the Lyman-Birge-Hopfield
Nitrogen waveband, discriminating between the LBH-long and LBH-short bands, which provide spectral information about precipitating electrons in the range 1-20 keV. XIR will image bremsstrahlung
radiation from highly energetic incident particles from which spectral information about the precipitating electrons from 20-150 keV can be obtained. Ravens-SI will measure total electron precipitation from the OI-135.6 nm line. Together, UVAMC, XIR, and Ravens-SI will provide comprehensive
information regarding the energy spectrum of the incident electrons producing the auroras, covering
all energies important for the ionospheric electrodynamics. In addition, Ravens-SI will image Doppler-shifted Lyman-α emission from charge-exchanging precipitating protons.
Shared Data Processing Unit. All instruments will have similar processing needs, so the possibility of
a shared DPU will be investigated during the Definition phase, to reduce mass and power requirements.
Operating modes. The auroral imaging instruments are expected to operate in a mode which captures an image every 10 s. Other modes will be available for specific situations including “windowed” and downsampled modes, in-flight calibration modes (e.g. observation of Earth’s limb or
selected stars), and one or more safe modes, e.g. powering down of charged particle-sensitive instrumentation during passage through the South Atlantic Anomaly.
Table 3.1. Mass and power usage estimates for the Ravens scientific payload. The estimates for each instrument include DPU requirements; a shared DPU between instruments could significantly reduce the overall
mass and energy requirements.
Instrument package
Mass (kg)
Power (W)
UVAMC (UVAMC-0 and -1)
35
38
Ravens-SI
20
3
XIR
29
16
EPI
32
31
NAIR (NAIR-Hi and -Lo)
7
15
5
128
5
108
Shared DPU
Total
Ravens • Auroral and magnetospheric imaging mission • 21
21
Baseline data storage and telemetry requirements.
Each auroral camera channel (2 × UVMAC, 2 × Ravens-SI, XIR) will capture one image every 10 s,
8640 images per day. For reference, a 256×256 pixel image with each pixel encoded in 16 bits requires 128 kB. The other instruments produce considerably less data due to lower imaging cadence.
Including housekeeping data, the baseline scenario for data storage is 6 GB day-1 per spacecraft, or
545 kb s-1 per spacecraft. These figures assume no compression. Compression ratios of ~1.4-1.8 are
possible using lossless algorithms (e.g. Rice), and considerably higher ratios are available via lossy
image compression techniques; this could significantly reduce the telemetry requirements. SD-RAM
and flash memory COTS solutions are available in radiation-tolerant space-qualified packages which
would meet these requirements, subject to the use of a compatible DPU architecture.
Cleanliness. For all instruments, a high level of cleanliness is required during handling, installation
and testing. Optical components must not be exposed to environments exceeding 4 particles cm-3,
greater than 0.5 microns in size. All parts must be protected from contamination during shipping
and handling. In general, sealed aperture doors will provide some measure of protection of the filters and coatings. UV optics are particularly sensitive to contamination and require special attention
during assembly, integration and verification; reference documents are provided in Section 5.2.
Proposed procurement approach. Instrument providers will approach their respective funding
agencies from which they typically obtain support for space missions, or, where necessary, funds will
be sought through ESA’s PRODEX.
3.2. Ultra-Violet Auroral Monitoring Cameras (UVAMC)
Description of measurement technique. UVAMC will consist of two cameras designed to image the
long- and short-wavelength portions of the Lyman-Birge-Hopfield N2 wavelength bands (UVAMC-0
and UVAMC-1, respectively). The baseline design of the UVAMC imagers has previously been developed through a Canadian Space Agency Phase-A study. The optical design of UVAMC is motivated by
a desire to record snapshots of auroral dynamics with high temporal resolution under night-time as
well as fully sunlit conditions.
Figure 3.1. UVAMC twin camera mechanical design (left). Single camera optical design (right).
UVAMC utilizes a four mirror on-axis system with an intensified FUV CMOS-based camera (see Figure
3.1). The design calls for thin film filter coatings to be deposited on each of the imaging mirrors to
provide the vast majority of the signal filtering. Further filtering is accomplished via the detector
(photocathode, window, MCP, and CMOS sensor). The twin imagers will be nearly identical, with the
exception of differences resulting from optimization for the two wavelength bands (different mirror
coatings and detector distances). Preliminary system modelling shows that the combination of the
Ravens • Auroral and magnetospheric imaging mission • 22
22
four filtering surfaces, image intensifier and a CMOS sensor can readily accomplish the required outof-band rejection required to image the aurora during sunlit conditions (see Figure 3.2).
Figure 3.2. Modeled UVAMC filter performance based on detector specifications and laboratory test results of
UVAMC manufactured thin film coatings.
Further, the light gathering ability of this system is sufficient to produce a detection threshold of ~20
Rayleighs with a 19 s exposure, at a spatial resolution of ~40 km at apogee.
Instrument conceptual design. The image-forming section of each camera comprises a fast, on-axis
all-reflecting telescope with two elliptical conical mirrors and one hyperbolic mirror (Fig. 3.1). This
type of reflecting telescope is very compact, and has excellent light gathering power. Moreover, it
has excellent resolution over most of its field of view and has a high throughput in the far ultraviolet
part of the spectrum. Each camera has a field of view of 20° × 20°. This field of view must be unobstructed by booms or other protrusions that would scatter sunlight into the optical system and causing possible damage to the detector. Adequate baffling will be implemented, and a reusable aperture door is planned. The baffles depicted in Figure 3.1 are preliminary baffles meant to be representative of a worst case scenario. Definition phase activities would include a baffle design based on
orbital and pointing constraints. Baffles are therefore not included in the volume estimates at this
time.
The imagers will operate in a self-filtering mode, utilizing thin film coatings on each of the four mirror surfaces [Torr et al., 1995]. These have very low reflectivity in Visible and Near-UV region, and
are specifically manufactured for high reflectivity in the LBH-short or LBH-long wavelength region, as
required. The four reflecting surfaces on which to deposit such coatings will help us achieve the required 1010 out-of-band rejection, for an imager sensitivity of ~20 Rayleighs in a 20 s exposure. In
addition, there is a BaF2 window in front of the detector, which provides the short wavelength cutoff. A CsI (solar blind) MCP photocathode provides additional long wavelength cut-off at about 200
nm.
Table 3.2. Preliminary characteristics for UVAMC-0 and -1.
Field of View
20 degrees
Camera speed
CCD format
f/2.5 or better
140-160 nm (UVAMC-1)
160-190 nm (UVAMC-0)
Single-stage microchannel plate intensifier fibre optically coupled to CCD imaging array
1024 × 1024
Pixel size
24.4 × 24.4 µm
Active area
25 × 25 mm
Image format
Sensitivity
1024 × 1024 pixels binned to 341 × 341
~60 R FUV for 1.66 s exposure
Instrument length × cylindrical diameter
23 × 14 cm
Spectral Wavelengths
Detector
Ravens • Auroral and magnetospheric imaging mission • 23
23
The overall design of the UVAMC intensified system is similar to that used for the FREJA instrument
design [Murphree et al., 1994] and utilizes other technologies previously developed by CSA for ultraviolet imaging on India’s ASTROSAT space telescope. UVAMC will adapt these flight proven technologies to meet the current measurement requirements. A LiF window provides a short wavelength
cutoff and seals the intensifier, while the solar blind CsI photocathode provides a longer wavelength
cut-off for visible light rejection. The photocathode material is deposited directly onto the MCP
which is biased with high voltage, approximately 5 kV. The photoelectrons (generated in and accelerated by the MCP) are converted to light again with an aluminum coated phosphor. The aluminum
coating provides light isolation of the CMOS to photons that have come through the MCP. The
phosphor material, P43, was chosen because of the fast decay time (>100ns) due to the Time-DelayIntegration (TDI) imaging mode of the cameras. The photons produced in the phosphor are coupled
through a short fibre optic relay onto the CMOS sensor. The baselined CMOS is a 1024 × 1024 pixels
chip, yielding a ~40 km resolution from apogee, for a 20° field-of-view (when operated in a 3 × 3 pixel binning mode). The CMOS will not require a mechanical shutter and provide increased radiation
hardness compared to a CCD sensor.
Each UVAMC camera is designed to meet and maintain optical performance during assembly, alignment and over the mission life. The design is athermal in that it utilizes a single material for all the
key structural components and the mounting of the optical elements, and this has a nearly identical
thermal expansion rate to the mirror material. The instrument is attached to the spacecraft interface through three bipod kinematic mounts which provides isolation from mounting surface irregularities, interface distortions and any CTE (coefficients of thermal expansion) differences over the
operational temperature range. Thermal stability is further achieved through the use of Multi Layer
Insulation (MLI) covering the unit (excluding the radiator). Due to the orbital variation trim heaters
may be implemented for thermal stability.
Resources. The total UVAMC payload (two imagers and electronics) is expected to be 35 kg (including a 25% contingency). The physical envelope of each imager is 28 cm in length with a 17 cm diameter (cylindrical). With the inclusion of mounting surfaces for co-alignment and stability, a height of
24 cm should be used. A single electronics unit will be utilized for both cameras. The physical dimensions of the unit are 19.1 × 19.1 × 8.9 cm. The average power consumption for the entire payload will be approximately 38 W (again including a 25% contingency).
Interface requirements. The CMOS needs to be operated at some stabilized temperature below 0
degrees C in order to reduce thermal noise. This is accomplished by using a radiator that faces directly towards Earth (see Figure 3.1), supplemented by low-power active cooling (or heating) from a
TEC. The power figures given below include provision for some low-power active cooling. Temperature sensors will be mounted at strategic places, and temperature will be continuously monitored.
The instrument input power supply voltage will be +28 VDC ± 6 V. The instrument will survive indefinite application of input voltages between 0 and +40 V, with no damage.
Heritage and TRL. The UVAMC design has been developed by the Canadian Space Agency and currently contains critical technologies with TRL 4 or greater. All critical technology items within the
UVAMC design have either been previously flown, or are flight qualified and awaiting launch. These
include the UV aspheric mirrors, baffles and internal surfaces, UV mirror coatings, UV transmission
filters, high voltage power supplies, image intensifiers, image sensor, electronics and fibre taper.
The UVAMC design is a reuse of these mature technologies in a new configuration with, in some cases, modifications to achieve specific measurement requirements.
3.3. FUV Spectrographic Imager (Ravens-SI)
Description of the measurement technique. The Ravens-SI is an imaging monochromator. In this
instrument, optical elements are used to isolate one or several wavelength(s) and produce images at
the desired wavelength(s) on spatially separated detectors, one per desired wavelength. In the case
Ravens • Auroral and magnetospheric imaging mission • 24
24
of Ravens-SI, the desired wavelength are the Doppler-shifted auroral Lyman-α produced by auroral
protons having captured and electron, and the oxygen emission at 135.6 nm produced in the aurora
by collisions between auroral secondary electrons and the ambient oxygen [Mende et al., 2000a, b].
Instrument conceptual design. The instrument will rely on the concept of the Spectrographic Imager (SI) onboard the NASA Imager for Magnetopause to Aurora Global Exploration (IMAGE) [Mende
et al., 2000a,b]. In that system, the light enters the instrument optics by an entrance grill and is reflected on a primary mirror producing parallel light from any point in the plane of the grill on a reflecting grating for wavelength selection. The grating is located at a place such that parallel beams
coming through the grill are focused to a point of the grating, which scatters light into a direction
that depends on wavelength. The exit grill (or slit) is located so as to determine the selected wavelength. A second mirror (combined with folding mirror(s) if needed) is then used to produce an image of the grating in the plane of the detector. Two channels were selected in the IMAGE-SI, a first
one centered on 135.6 nm corresponding to the OI-135.6 nm 3s 5S°– 2p4 3P transition with a contribution coming from nearby transitions belonging to the N2 LBH band system. This channel had a
large spectral width and used a simple exit slit. The second channel was centered on 121.8 nm, appropriate for Doppler-shifted HI Lyman-alpha emissions. This latter channel used an exit grill adjusted so that the optical system efficiently rejected the bright geocoronal HI Lyman-alpha emission and
the NI-120 nm 3s 4P – 2p3 4S° triplet. The IMAGE-SI instrument design had the grating located between the entrance slit and the first mirror, so that a hole had to be made in the grating. As a result
the FOV had a hole in its center and the rotation of the spacecraft was used to complete the global
imaging of the Earth. The exposure time was about 5 seconds, while the rotation rate of the spacecraft was 2 rpm. A time-delayed integration was performed onboard to properly accumulate the image signal and correct for the spacecraft motion and rotation.
The Ravens mission will use 3-axis stabilized platforms, so the design needs an adaptation. We will
use a Czerny-Turner design [Mende and Sigler, 2008]. This design (Figure 3.3) is similar to the one
used for the imaging SI onboard the NASA-ICON mission. The ICON SI instrument has a flip-mirror
mechanism that we do not need onboard Ravens, thus simplifying the instrument. The ICON-SI design will have passed full testing soon in 2015. In a Czerny-Turner system, the light passes through an
entrance slit (or grill) and is reflected by a curved mirror towards a reflective grating (which is not
located between the entrance slit and the first mirror), where an image of the outside scene is
formed as parallel beams are focused to a point in the grating plane. The grating performs the wavelength selection and light is reflected under a wavelength-dependent angle to a second curved mirror that directs the light to two exit slits (or grills) suitably located to isolate the desired wavelengths.
In the case of the Ravens mission, the first one will correspond to the 135.6 nm passband, the second will isolate the 121.8 nm wavelength. Light then arrives at curved mirrors (via folding mirrors if
necessary) that form the desired images on the detectors. The 121.8 nm system must be designed as
to reject both the geocoronal Lyman-alpha and the NI-120 nm emissions which is always present in
the electron and proton aurora. In the 135.6 nm channel, the design will be adjusted to keep the
bright, optically thick OI-130.4 nm emission outside of the passband as it was previously done in the
IMAGE-SI. Although the Ravens platforms will be 3-axis stabilized, time delay integration will be
needed to correct for spacecraft motion. The optical design will need to be adapted based on the
ICON-SI and IMAGE-SI instruments as to perform appropriate spectral selection although the CzernyTurner may be subject to larger aberrations than the IMAGE-SI design. The CSL took an active part
to the design and fabrication of both instruments, which can be considered as strong heritages for
the Ravens-SI. The different parts composing the instruments are all at TRL-6 or higher. The Ravens
SI strongly resembles the IMAGE-SI, and our requirements are those that were met by the IMAGE-SI.
Performance assessment with respect to science objectives. The design described has been proven
to properly select the Doppler-shifted auroral Lyman-α while efficiently rejecting the nearby geocoronal Lyman-α and the bright resonance line emission of atomic nitrogen at 120 nm, thus fully allowing the desired imaging of the proton aurora. It was also demonstrated that a ~5 nm FWHM wavelength interval can be selected centered on the oxygen 135.6 nm emission, thus isolating the desired
Ravens • Auroral and magnetospheric imaging mission • 25
25
auroral emission. The sensitivity requirement of the instrument is ~1.8 counts per pixel per 100 Rayleigh per exposure at 121.8 nm and 1.3 counts per pixel per 100 Rayleigh per exposure at 135.6 nm,
as it was for the IMAGE-SI instrument. The instrument will have a 20° field-of-view, and use a 256 ×
256 detector, producing a resolution of ~60 km at ionospheric altitude from apogee at 8 RE (Table
3.3). Time delay integration (TDI) [Mende et al., 2000a] will be used to compensate for the spacecraft motion.
FOV
121.8 nm
135.6 nm
20°
20°
Sensitivity
(counts/pixel/100
R/exposure)
1.8
1.3
Pixel size
Resolution
from apogee
Resolution
from perigee
0.078°
0.078°
60.8 km
60.8 km
2.6 km
2.6 km
Table 3.3. Summary of Ravens-SI characteristics.
Resources. We rely on the previous NASA-IMAGE mission legacy to estimate the properties of the
instrument. The mass of an SI system selecting two wavelengths is approximately 20 kg. The electric
consumption of the working instrument is 3 W (operating). The typical dimension of an SI are 80 ×
51.5 × 30 cm, subject to modification if needed by higher performance requirements.
Pointing and alignment. As no telescope is
needed in the SI, the pointing and alignment issue reduces to properly assessing
the geometry of the instrument within the
spacecraft platform, using reference cubes.
Figure 3.3. Working principle of the CzernyTurner monochromator, reproduced from
Mende and Sigler [2008].
Operating modes. The instrument will be operated as (1) Off, (2) Stand by (these modes can be
needed to protect the instrument while crossing the radiation belts), (3) On (standard operating
mode), and (4) Star field observations: operated with the same electrical voltage as mode (3), and
necessary for in-flight calibration and regular calibration checks using stable stellar emission previously observed with other satellites, like IUE or the Hubble Space Telescope. High voltage tuning can
be decided based on mode (4) observations, in order to compensate for possible sensitivity loss due
to detector ageing.
Specific interface requirements. In order to promote the out-gassing of impurities, which could degrade the optical performance, it is necessary to include heaters in the instrument.
Calibration and other specific requirements. Calibration and testing can be fully conducted at CSL,
Liège, Belgium.
Heritage and TRL. Flown as NASA-IMAGE FUV/SI-12 and -13. CSL (Univeristy of Liège) was extensively involved in the construction, calibration and testing of the NASA IMAGE-SI. However, the optical design will have to be accommodated to allow operation from a 3-axis stabilized platform. We
thus consider Ravens-SI as having TRL-6. Newer and more efficient image sensors will be used in the
Ravens-SI instrument. Slight modifications of the optical design can also be considered to improve
the instrument PSF and increase the field-of-view from 17° (IMAGE-SI) to 20° for Ravens-SI.
Ravens • Auroral and magnetospheric imaging mission • 26
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3.4. X-ray Imager for Ravens (XIR)
Description of the measurement technique. Precipitating energetic electrons produce X-ray bremsstrahlung when colliding with the neutral upper atmosphere, which can be detected at satellite altitude. The imaging technique is based on a pin-hole design and the photons will be detected in a layer of pixelated detector material.
Instrument conceptual design. For XIR the detector material will be Cadmium-Zinc-Telluride (CZT).
CZT semiconductor detectors have a high quantum efficiency for hard X-rays due to their high atomic numbers. They can be operated close to room temperature (typically +10°C). CZT detector systems are operated with modest High Voltage (400 V) supply requirements. Coupled to applicationspecific integrated circuits (ASICs), these systems are compact and ideally suited for space experiments with a low read-out noise and high energy-resolution. XIR will have a simple embedded Data
Process Unit (DPU) and a Power Supply Unit (PSU) for Low and High Voltage supply.
For a 5 mm thick CZT-layer the quantum efficiency is close to 100% below 100 keV (80% at 150 keV).
The lower energy threshold will be ~15 keV. The detector area will be 512 cm2 with 8192 pixels. The
baseline design comprises a pinhole-size of 5 × 5 mm2, pixel-size of 2.5 × 2.5 mm2, and 200 mm between pinhole and detector layer. An adaptive pinhole scheme will be used, with 18 open pinholes
at apogee, and 8 open pinholes at lower altitude.
Table 3.4. XIR counts in one minute integration, per pinhole (4 pixels) per resolution element.
Total counts
/4 pixels*/res
Background counts
/4 pixels*/res
Spatial Resolution
18 pinholes,
18 pinholes
typical X-ray flux
10% of typical flux
Res**: 1 min
Res**: 1 min
600 +/- 25
180 +/- 13
140 +/-12
140 +/-12
500-950 km***
500-950 km***
* 4 pixels correspond to pinhole size
** Res=resolution
*** Spatial resolution from 4-8 Re geocentric distance
From previous measurements of X-ray auroras we estimate a typical flux of photons (15-150 keV) of
3000 ph cm-2 sr-1 s-1. The background count rate is estimated to be 2 cts cm-2 s-1 and will be reduced
by a background rejection design which includes shielding. In addition, a layer of plastic scintillator
parallel to the CZT layer will enable detection of particles by coincident detection in the plastic and
the CZT layer. In addition, all the edge pixels will be used for background rejection. It is estimated
that the background will be reduced by a factor of 5 using this approach. Observations may be integrated for as long as necessary (approx 5 min) to achieve required signal to noise. The XIR count
rates are summarized in Table 3.4.
Resources. XIR will weigh approximately 29 kg, with a power usage of 16 W (operating). The instrument dimensions will be approx 34 × 36 × 30 cm.
Pointing and alignment. 72° × 72° field-of-view.
Operating modes. An adaptive pinhole scheme will be used, with three pinhole configurations (see
above).
Calibration and other specific requirements. Calibration will be carried out on ground and in situ by
weak radioactive sources.
Heritage and TRL. The XIR is similar to the Modular X and Gamma ray Sensor (MXGS), part of the
Atmosphere Space Interactions Monitor (ASIM) instrument, to be flown on the International Space
Station (ISS). Both CZT with ASICs and the read-out electronics are fully developed and tested flight
Ravens • Auroral and magnetospheric imaging mission • 27
27
models will be finished by April 2015. The shielding and pinhole mechanical solution must be designed. The background rejection design must be developed. Current TRL-6; ASIM/MXGS to be TRL9 by April 2015.
3.5. EUV plasmasphere imager (EPI)
Description of the measurement technique. A reflecting telescope will observe the 30.4 nm EUV
line of sunlight resonantly-scattered from singly-charged Helium, He+. He+ is a slowly varying fraction
(~20%) of the cold plasma in the magnetosphere. The source is optically thin and to obtain quantitative 3D global images of the magnetosphere we propose to observe the He 30.4 nm radiation
from the two Ravens spacecraft providing a stereoscopic view.
Instrument conceptual design. The imager consists of seven compact camera units in a common
housing, each with a 30° field-of-view, the individual cameras being canted to cover a combined approximately circular field-of-view ~84° diameter (see Figure 3.4 for the 3-detector head arrangement
used in IMAGE-EUV). The camera optical structure consists of a concave mirror which focuses on a
circular convex detector. The spectral resolution is achieved by the combination of two elements:
the mirror has a multilayer surface which acts as an interference filter with a pass-band centred on
30.4 nm; other pass-bands are eliminated by a filter placed across the aperture. The detector is a
microchannel plate whose spherical surface matches the mirror’s focal plane radius. The detector
readout is a 2D wedge/strip scheme with processing capability of 105 events per second. The camera unit design is based on the IMAGE EUV instrument.
Performance assessment with respect to science objectives. Tables 3.5 and 3.6 indicate the brightnesses of plasmasphere features and the performance of the instrument. The performance will be
similar to the IMAGE EUV instrument which matches the Ravens science requirements.
Table 3.5. Source brightness of plasmaspheric features.
Plasmasphere feature
Plasmasphere structure
Plasmapause location
Detached plasma / drainage plumes
Plasma trough refilling
Source brightness (Rayleighs)
5 – 150
1–5
0.05 – 5
0.05 – 5
Table 3.6. Optical characteristics of EPI.
Sensitivity
Resolution
Dynamic range
1 Rayleigh in 60 s integration
0.4° on axis of each unit, 1.3° at edge
6
~10
Figure 3.4. The IMAGE-EUV instrument, on which Ravens-EPI
will be based [Sandel et al.,
2000]. Ravens-EPI will house 7
detector heads in a hexagonal
arrangement, rather than 3-in-arow arrangement of IMAGEEUV, to provide the required
84° × 84° field-of-view on a nonspinning platform.
Specific interface requirements. The instrument is nadir-pointing. Geodetic position of the emission
element should be known to better than 5 km. Field-of-view to be free of obstruction from other
Ravens • Auroral and magnetospheric imaging mission • 28
28
spacecraft structures. Time resolution to <0.1 second resolution and attitude knowledge at the few
arcmin level is required.
Calibration and other specific requirements. General UV calibration facilities will be set up in a
chamber at MSSL, suitable for calibration and testing including detector noise, gain uniformity, image linearity, efficiency as a function of wavelength, spatial and temporal resolution and temporal
stability. Some opportunities for in-flight calibration exist through the observation of UV stellar
sources (we would presumably require occasional spacecraft repointing to facilitate this).
Resources. Volume, mass and power are scaled up appropriately from the IMAGE-EUV instrument:
the instrument dimensions will be approx 50 × 50 × 50 cm, with mass 32 kg and power requirement
of 31 W, respectively.
Heritage and TRL. The instrument is based on the successfully flown IMAGE-EUV instrument, so in
principle has a TRL of 9. Items which could reduce the TRL would include selecting European suppliers for multi-layer mirrors and filters. Alternatively, US Co-Is might be introduced in the next study
phase who could help provide these from known suppliers as part of a contribution to the instrument.
3.6. High- and Low-Energy Neutral Atom Imager for Ravens (HAIR-Hi and NAIR-Lo)
Description of the measurement technique. Energetic Neutral Atoms (ENAs) are generated in the
terrestrial magnetosphere through charge exchange between magnetically trapped energetic ions
and cool neutral gasses. An ENA camera can record the arrival directions, energies and mass species
of magnetospheric ENAs as well as indirectly provide global images of the spatial, energy and mass
species distributions of their parent ion populations. NAIR consists of two components to detect
high and low energy ENAs, from the ring current/plasma sheet and from low altitudes, NAIR-Hi and
NAIR-Lo.
NAIR-Hi conceptual design. An electrostatic deflection plate assembly with several kV of positive
potential on every other plate (and the rest at ground) removes charged particles. Remaining ENAs
first pass through an ultra-thin (~2 µg cm-2) start foil. The resulting forward-scattered secondary
electrons are guided toward a 1-D position sensitive Start-MCP, which provides 1D start position and
timing pulse. After passing the START-foil, the ENAs continue and penetrate the stop foil and hit the
stop MCP where the 2D stop position and a stop timing pulse are recorded. The start and stop positions are then used to compute the direction of the incoming ENA on board, and the timing pulses
are used to compute the velocity. In addition, back-scattered electrons from the stop foil are used
used as a coincidence measurement to reject false events caused by penetrators and UV. The pulse
height of the STOP signal is used for mass discrimination (separation of H and O) in a manner similar
to that used by HENA/ IMAGE and INCA (Cassini). Figure 3.5 shows the Cassini/INCA design, but for
Ravens a 2nd generation ENA camera would be flown with a ~2-300 keV nuc-1 energy range for H and
O and a G factor of about 1 cm2 sr enabling statistically significant images at a minute time resolution, which far exceeds Ravens measurement requirements.
NAIR-Lo conceptual design. NAIR-Lo, which is based on a surface conversion/reflection technique,
consists of an ion rejection system, ionization surface, photon rejection system (which also performs
rough energy analysis), and a velocity (mass) analysis and detection segment. Neutrals enter the
sensor through an electrostatic charged particle deflector which rejects ambient charged particles by
means of a static electric field. Incoming neutrals are converted to positive ions on an ionization surface. Thereafter, the beam is passed through an Electrostatic-Analyzer (ESA) with a customized
(“wave”) shape that effectively blocks photons.
On exiting the ESA, neutrals are post-accelerated up to 1.5 keV and strike a START surface at less
than the grazing angle (15°). During impact, kinetic secondary electrons are emitted and these are
reflected towards the STOP-MCPs to produce a STOP pulse. Also, secondary electrons from the
START-surfaces are guided to the START-MCPs and produce a START-pulse. The START- and STOPRavens • Auroral and magnetospheric imaging mission • 29
29
timings provide the particle velocity. Combining the TOF measurements and ESA settings allows the
determination of ENA energy and mass (5 values up to Fe). Measuring the radius / azimuth of a neutral impacting on the START-surface by means of the position sensitive START-MCPs allows accurate
determinations of the TOF length and the arrival azimuth of the incoming neutrals.
nd
Figure 3.5. A 2 generation Cassini/INCA design
would be used as a baseline for the NAIR-Hi cameras.
Performance assessment with respect to science objectives. NAIR-Hi has sufficient energy range to
retrieve and reconstruct the 3D plasma pressure that drives the current system of the inner magnetosphere. It has a large G-factor and instantaneous FOV (1/3 duty cycle on spinning platform) that
enables statistically significant images on substorm timescales (minutes). Continuous monitoring/stereoscopic remote sensing of the ring current by NAIR will support the Ravens objective to investigate how the global magnetosphere responds to incoming solar wind disturbances. Joint NAIR
and auroral observations will provide information regarding ongoing magnetospheric-ionospheric
(MI) coupling. NAIR-Lo will provide unique LENA images for energies down to 10 eV resolving hydrogen and oxygen to investigate the ionospheric particle acceleration processes and their response
to the solar wind.
Resources. The mass and dimensions of NAIR-Hi and -Lo are summarized in Table 3.7. Between
them, NAIR will consume 15 W.
Heritage and TRL. Both units of the system (NAIR-Hi and -Lo) are “mission proven” through successful mission operations in space to level TRL-9: NAIR-Hi on HENA/IMAGE and INCA/CASSINI. The 2nd
generation camera of INCA and HENA is also being developed for JUICE. NAIR-Lo is also at TRL-9,
based on successful operations on SARA/Chandrayan-1, BepiColombo/MMO/ENA; Heritage DPUs
provided for ASPERA3/4; SMART-1, PHILAE/ROSETTA.
Calibration and other specific requirements. Calibration will be carried out on the ground.
Table 3.7. Mass and dimensions of NAIR-Hi and -Lo.
NAIR-Hi
NAIR-Lo
Mass (kg)
Dimensions (cm)
Field-of-view
4
3
40 × 40 × 50
24 × 22 × 13
120 × 90
o
o
15 × 160
o
o
3.7. Heritage and Technology Readiness
The Ravens mission is designed around proven technology. The innovation in the mission does not
arise from novel spacecraft design nor, in the main, novel payload design. The novelty arises from
the use of two spacecraft to meet the science requirements of continuous auroral imaging in one
hemisphere, frequent, simultaneous imaging of both hemispheres, and stereoscopic views of the
plasmasphere and ring current. Hence, neither spacecraft design nor instrument design present significant technology challenges.
Most of the instruments proposed for Ravens have flight heritage, or are based on a design that has
flight heritage. These include UVAMC, Ravens-SI, NAIR; EPI is based on technology that has a proven
Ravens • Auroral and magnetospheric imaging mission • 30
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history. The instrument that requires the most development is XIR. XIR (Sect. 3.4) is based on technology that is currently under development for the ASIM X-ray imager, to be flown on the International Space Station (ISS), which is close to completion. XIR is expected to be at TRL 5 or greater by
the end of the mission Definition phase, and is not considered a risk for Ravens.
During the Definition phase the benefit and feasibility of a shared DPU, or some shared DPU functionality, will be considered for the five imaging instruments, in terms of reducing mass and power
requirements.
The spacecraft design is expected to conform to well-understood standards, possibly based on
commercially-available solutions (see Sect. 4). The main technology developments that are required
include the ability to accommodate both spacecraft on the launcher, for both spacecraft to attain
the correct orbits under their own propulsion, and to maintain the correct orbits such that the mission fulfils the main science requirements, that is, continuous imaging coverage of one polar region,
and regular imaging of both hemispheres, together with continuous observations of the plasmasphere and ring current. These have been achieved previously for similar missions (Cluster, IMAGE,
Polar) and it is not expected that these present major technological challenges. The proposed adoption of a central DPU serving multiple imaging instruments requires particular attention to instrument interfaces and data formatting, but does not represent a new technology challenge and offers
substantially increased mass, volume and power efficiency for the spacecraft.
Ravens • Auroral and magnetospheric imaging mission • 31
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4. Proposed Mission Configuration
4.1. Mission philosophy
The science requirements or Ravens are: continuous auroral imaging of the northern and/or southern hemispheres, a maximized duration of simultaneous observations of auroras in both hemispheres, continuous observations of the inner magnetosphere, ring current and plasmasphere, and
maximized duration of stereographic views of the inner magnetosphere.
To achieve this Ravens will comprise a pair of identical spacecraft in near-identical eccentric highinclination orbits; one orbit will have apogee in the northern hemisphere, the other will have apogee
in the southern hemisphere; the orbits will be phased such that one spacecraft is at apogee when
the other is at perigee (Fig. 4.1). This provides continuous views of the auroras, with simultaneous
views of both northern and southern auroras for the majority of the time. This also provides stereoscopic views of the ring current and plasmasphere, for much of the time from advantageous lowand high latitude positions that are optimized for tomographic reconstruction of inner magnetospheric structures (Fig. 4.2).
The spacecraft will be three-axis stabilized, with the instruments nadir-pointing, to maximize observation time and communications opportunities.
A recent ESA design-study for KuaFu-B (a smaller payload but similar platform requirements, orbital
configuration, pointing, communications, and propulsion requirements to Ravens) considered commercially-available platforms to reduce costs. That study identified the SSTL-300, Myriade, and
PROBA platforms as possible solutions, noting that they provided ample opportunities for increased
payload. Without launch and post-launch support costs, the design study concluded that KuaFu-B
could be achieved within a cost of 120 M€. Despite the increased payload we do not anticipate that
the platform cost should increase significantly above this.
Figure 4.1. The planned
orbital configuration of Ravens. Each panel shows the
positions of the two spacecraft at 6 equally spaced
times over a 14.1 hour orbit. The approximate locations of the regions targeted for observation are
shown: the northern and
southern polar auroras
(red), the plasmasphere
(cyan), and ring current/plasma sheet (orange).
L = 4 and 6 magnetic field
lines are indicated as well
as the approximate fieldsof-view of the inner magnetospheric imagers.
4.2. Proposed programme schedule.
Definition phase: 2015-17; Down-selection: 2017; Implementation phases: 2018-2024; Launch: 2025;
Prime mission: 2025-2028.
Ravens • Auroral and magnetospheric imaging mission • 32
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4.3. Orbital configuration
Polar orbits suffer from precession of the line of apsides, such that over the mission lifetime the apogee drifts from an optimal viewing position (e.g. Polar and IMAGE). Consequently, Ravens will
adopt a Molniya-type orbital configuration, that is an orbital inclination of 63.4° which compensates
for the orbit natural drift caused by the Earth J2-term, minimizing precession and maintaining optimal viewing for the duration of the mission.
Figure 4.2. A still from a movie showing the planned orbital configuration of Ravens (full movie available at
ftp://www.ion.le.ac.uk/pub/ets/Ravens/Ravens.mov). Two identical spacecraft are in 8 × 1.3 RE Molniya-type
orbits, one orbit with apogee in the northern hemisphere, the other in the southern hemisphere. The auroral
configuration (red circles) is always visible in one hemisphere (with simultaneous views in both hemispheres
for over 60% of the time) and stereographic views of the ring current (orange) and plasmasphere (blue) from
high and low latitude vantages. For references, circles show fields-of-view with (edge-to-edge) diameters of
30° and 60°.
The planned orbital configuration is that each spacecraft will be in a 8 × 1.3 RE (geocentric) orbit,
with orbital inclination 63.4°, and orbital period of 14.1 h. The argument of periapsis of one spacecraft will be 90° (Ravens-S) and one 270° (Ravens-N), phased in their orbits such that one is at perigee while the other is at apogee. Each spacecraft will be in a position to view the auroras for approximately 10 h of each orbit; Ravens-N observes the northern hemisphere auroras, Ravens-S the
southern hemisphere auroras. Twice every orbit the two spacecraft provide simultaneous observations of the auroras in both hemispheres for an overlapping period of 4.5 h, that is 9 h of interhemispheric observations every 14.1 h (60% of the time). Viewing both northern and southern hemispheres from apogee ensures that for the majority of the time one spacecraft will observe the winter hemisphere and dayglow contamination of auroral imagery will be minimized.
The orbital parameters are driven by a number of constraints: that unbroken auroral imaging is possible, that the duration of simultaneous imaging of northern and southern hemispheres is maximized, and to provide good stereoscopic viewing of the inner magnetosphere. A perigee of 1.3 RE (geocentric) mitigates the need to de-orbit the spacecraft at the end of the mission (see Sect. 4.5). The
parameters will be optimized in the light of additional constraints, such as propulsion and fuel requirements to achieve final orbit during subsequent phases of the mission definition. A brief discussion of the radiation environment of this orbit can be found in Sect. 4.10.
The initial lifetime of the mission is anticipated to be 3 years. Extensions to this lifetime would allow
continued delivery of all the science requirements of the mission. A total mission lifetime of 11
Ravens • Auroral and magnetospheric imaging mission • 33
33
years would allow all phases of the solar cycle to be observed and the magnetosphere’s response to
evolving solar wind conditions to be determined.
4.4 Spacecraft design
Ravens will comprise two identical spacecraft, with identical payload and identical subsystems to
share the same design, assembly, integration, testing, and operations procedures.
The spacecraft do not require booms that must be spin-stabilized, nor do they have instruments (e.g.
plasma analyzers) that require a spinning platform to achieve 4π steradian coverage. As a consequence, the spacecraft will be three-axis stabilized, with the seven instruments of the science payload being nadir-pointing (Fig. 4.3), allowing continuous observing of the auroras and inner magnetosphere. Communications subsystems will also be continuously pointed towards ground-stations.
In this scenario, no subsystem requires articulation and the necessity for rapid spacecraft slewing is
not anticipated. Three-axis stabilization will also facilitate Sun and Moon avoidance by the instruments if necessary.
Figure 4.3. An indication of how the instruments could be accommodated on a nadir-pointing 115 × 115 cm face of the
spacecraft.
We employ the commercially-available SSTL-300 platform produced by Surrey Satellite Technology
Ltd. (UK) as a baseline example of the platform characteristics available which could accommodate
the needs of Ravens. The main payload characteristics of the SSTL-300 are indicated in Table 4.1.
The platform without payload has a dry mass of 218 kg. With a payload of approximately 128 kg
(Sect. 3), the spacecraft dry mass is of order 346 kg. Two Ravens spacecraft are significantly below
the suggested 800 kg dry mass envelope of the M4 Mission Call.
The platform provides more than sufficient power for the payload.
The platform is three-axis stabilized with pointing control, pointing knowledge, jitter, and slew rate
all within the requirements of Ravens.
Magnetic cleanliness is not an issue for the payload. Thermal cooling and heating may be required
for some instruments (see Sect. 3). Cleanliness during the assembly and testing of the spacecraft will
be essential for all imaging instruments. These instruments operate under a vacuum and must be
maintained under constant dry nitrogen purge until launch. The instruments must then be allowed
to out-gas prior to power-up; no instrument should be placed in the path of gas venting from another.
4.5. Launch and transfer to target orbit
Ravens will comprise two identical spacecraft, which will be launched together. They cannot be
launched together directly into their operational orbits (see Sect. 4.3). Instead, they will be launched
into a 1.3 RE altitude, 63.4° inclination circular orbit. The spacecraft will then independently maRavens • Auroral and magnetospheric imaging mission • 34
34
noeuvre into their operational 8 × 1.3 RE (geocentric) orbits under their own power; the mass of apogee kick motor and required fuel will be considered in the Definition Phase.
It is anticipated that the Ravens pair could be launched together onboard a Vega rocket, though detailed study of the accommodation of the spacecraft within the rocket fairing and mass constraints
will be undertaken in the Definition Phase. We baseline costs on a Soyuz launch to provide a worstcase cost scenario (Sect. 6).
Bus dry mass
218 kg
Maximum payload mass
150 kg
Orbit average payload power
140 W
External payload volume
730 mm × 455 mm × 1000 mm
Science data downlink
105 Mbps, X-band
Science data storage
16 Gbytes
Attitude control system
3-axis control, reaction wheels and magnetorquers
Pointing control
360 arcsec all axes
Pointing knowledge
72 arcsec all axes
Pointing stability
2 arcsec s-1
Slew rate
0.75 deg s-1
Solar arrays
Triple-junction GaAs cells, 2.44 m2
Batteries
Li-ion cells, 15 A h
Propulsion
Hot gas Xenon resistojet
Delta-V
15 m s-1
Thermal control
Passive plus heaters
Table 4.1. The main characteristics of an SSTL-300 platform.
4.6. End of life
At the end of the mission, there is no need to de-orbit the spacecraft as the current space debris
mitigation rules do not apply in the proposed Molniya orbits, which remain stable over 25 years.
The spacecraft can therefore be designed without de-orbiting capabilities.
4.7. Instrument operating modes
The platforms are nadir-pointing and the payload is anticipated to operate continuously, all imagers
providing continuous imagery at their design cadences. The images will be stored onboard for
down-link when available. Instrument mode changes will not be necessary routinely. Operating
modes may need to be modified if Sun-avoidance is required, and for in-flight calibration.
4.8. On-board data handling and telemetry
Each auroral camera channel (2 × UVMAC, 2 × Ravens-SI, XIR) produces 8640 images per day, each
image of order 130 kB in size. The other instruments produce considerably less data due to a lower
sampling cadence. The worst-case scenario for data storage is 6 GB day-1 per spacecraft, or ~4 GB
accumulation of data between ground station contacts. This capacity is provided by the SSTL-300
platform. In the worst case scenario (6 GB day-1) this would require 100 min of downlink time per
orbit of each spacecraft at 8 Mb s-1.
Ravens • Auroral and magnetospheric imaging mission • 35
35
4.9. Ground segment
Due to the nature of the eccentric polar orbit of the Ravens spacecraft, it is anticipated that each
spacecraft will be visible from a high latitude ground station (e.g. Kiruna in the northern hemisphere
for Ravens-N and Perth in the southern hemisphere for Ravens-S) for a significant fraction of each
orbit, considerably more than the ~100 min per orbit required to downlink the worst-case scenario
of 6 GB day-1 data collection rate. The availability of secondary ground stations at a locations TBD in
each hemisphere (depending on orbital configuration) is desirable to increase the total duration of
downlink per orbit, and for redundancy. The required bit-rate and end-to-end link budget for the
spacecraft will be considered in the Definition phase study, but in view of the estimated daily volume, S-band communications are likely to meet the mission requirements. The mission contains a
high level of onboard autonomy, and minimal commanding is required (uploaded commands may be
required for e.g. occasional recalibration of instruments).
The ground segment will comprise the Ravens Mission Operations Centre (MOC), ground receiving
stations, communications network, and Ravens Science Archive (RSA) for data distribution to the
scientific community. The mission requires ground stations to communicate with the two Ravens
spacecraft in order to support telemetry, telecommand, and tracking. While the Ravens spacecraft
are manoeuvring into their operational orbits, several ground stations may be necessary; it is anticipated that one or two ESA stations will suffice once the operational phase has commenced.
4.10. Radiation environment
A brief radiation environment study has been undertaken characterise the trapped particle and solar
proton radiation environment expected in the orbit proposed for the two spacecraft making up the
Ravens mission.
The ESA SPENVIS system was used to estimate the radiation environment expected in this orbit. Solar maximum conditions and a mission duration of 3 years were considered.
For trapped particle radiation calculations, standard models were used for proton and electron distributions (AP8 and AE8 respectively). A 50% confidence level was specified for the electron model,
corresponding to the average model flux. Spectra for trapped particles, averaged over one orbit,
were calculated using a 20-orbit spacecraft trajectory to calculate the environment over a range of
locations. The spectra are shown in Figure 4.4. Figure 4.5 shows the trapped proton and electron
flux as a function of time, for a 24 hour period (c.f. the orbital period of 14 hours). The data show
that significant particle flux is observed for less than one half of each orbital period. The long-term
solar particle fluence was calculated using the JPL-91 model, with a 3 year prediction period, resulting in the solar proton spectrum shown in Figure 4.6. These data were generated using a 95% confidence interval, and hence only 5% of missions would be expected to experience a larger solar proton
dose.
These trapped particle and solar proton spectra were used to establish dose-depth and non-ionising
flux values for the Ravens orbit. The ionising radiation dose for a silicon target was calculated as a
function of shielding thickness for two shielding geometries: target behind a finite Al slab, and target
at the centre of an Al sphere. The results suggest that on the basis of the total dose curve, little benefit is obtained in exceeding a shielding thickness of 10 mm. Recommendations for specific shielding
thicknesses would be established in a radiation-mass-volume trade-off exercise during the Definition
Phase.
Ravens • Auroral and magnetospheric imaging mission • 36
36
Figure 4.4. Integral and differential trapped particle flux averaged over 1 orbit. (Left) Protons, (right) electrons.world map of the trapped proton flux for the 20 orbits used to calculate these spectra.
Figure 4.5. Trapped particle flux as a function of orbital time. (left) protons, (right) electrons.
Figure 4.6. Solar proton spectrum calculated using
the JPL-1 model with a prediction period of 3 years.
A 95% confidence interval adopted.
Ravens • Auroral and magnetospheric imaging mission • 37
37
5. Management Scheme
5.1. Overview
It is envisaged that the overarching responsibility for all aspects of the Ravens mission will rest with
the ESA Directorate of Science and Robotic Exploration and its Director. The lead proposer, Prof Steve Milan (University of Leicester, UK), will act as mission PI, and will provide input to the Ravens Project Team and the Ravens Science Office that will be set up by ESA. Prof Milan will support the study
activities by making available at least 20% of his time throughout the study period.
Five scientific payload instrument packages will be provided by consortia selected by ESA via an Announcement of Opportunity. These Instrument Teams will be funded by national agencies, will provide the science payload instruments, calibrate and operate them, process the scientific data, and
deliver the scientific products to ESA.
ESA will convene a Ravens Project Team which will be responsible for the procurement of the spacecraft, instrument integration into the spacecraft bus, system testing and execution of calibration
plan(s), spacecraft launch and operations, and acquisition and transmission of the data to the science data centres. ESA will select a preferred spacecraft design and an industrial Prime Contractor
for the procurement and delivery of the spacecraft, and for other tasks under ESA responsibility.
ESA’s Space Operations Centre (ESOC) will implement the Mission Operations Centre (MOC), operate
the spacecraft, and deliver the raw scientific data to the Instrument Teams. The ESA Ravens Project
Scientist will act as the interface between the ESA Project Team and the Instrument Teams.
Figure 5.1. The information flow during the
Science Operations
phase of the Ravens
mission.
Once Ravens is operational, ESA’s Research and Science Support Department (RSSD) at ESTEC will
develop and manage a Ravens Science Office (RSO). The Science Office will be led by the Ravens
Project Scientist and is the ESA entity charged with scientific aspects and operations of the Ravens
mission. Figure 5.1 outlines the envisaged information flow during the science operations phase of
the mission.
A Ravens Science Team will oversee the preparation and execution of scientific operations, and will
be chaired by the ESA Project Scientist. An Observing Plan will be defined and constructed under the
responsibility of the Science Team, and implemented by the MOC. The Science Team will comprise
the Ravens Principle Investigator, scientists representing the Instrument Teams, and the Ravens Archive Scientist. A Ravens Ground-Based Coordination (RGBC) working group will be formed from
Ravens • Auroral and magnetospheric imaging mission • 38
38
members of the ground-based supporting infrastructure (Sect. 2.5)), and the chair of the RGBC will
be a member of the Ravens Science Team. Due to the interest of Ravens observations to satellite
operators and Space Weather forecasters, a Space Weather working group comprising key stakeholders will be formed, and the chair of that working group will also be a member of the Science
Team.
The data from each instrument will be processed by each Instrument Team during all phases of the
mission. The Instrument Teams will ultimately be responsible for the creation, and delivery to ESA of
the scientific products of the mission.
5.2. Instrument Teams (ITs)
Each of the five instrument packages will be provided by consortia led by Principal Investigators (PIs),
funded by ESA’s member states, and selected via an Announcement of Opportunity (AO) which will
be issued after approval of a Science Management Plan.
The AO will request proposals from consortia of scientific institutes, which will provide both the instruments (one per consortium) and the manpower and facilities needed for their development and
testing, as well as for processing of the scientific and housekeeping data generated by the payload.
Each Instrument Team will be led by a single Principal Investigator (PI), who will act as interface to
ESA and will be a member of the Ravens Science Team. The Instrument Team will include a senior
Instrument Scientist who will provide scientific guidance to the Instrument Team and the Ravens
Science Team, and in particular oversee the development and operation of the data processing
structure proposed by the Team. The Instrument Scientist is expected to be an active member of
the Instrument Team, and to have a significant involvement in the scientific exploitation of the data.
The Instrument Scientist will be a member of the Science Team.
Each Team will also contain a well specified and identified management layer consisting of an experienced Engineering Scientist (responsible for instrument scientific performance), a Data Processing
Manager (responsible for development and operation of the data processing activities and facilities
and interface with the Ravens Science Archive), and a Project Manager (responsible for the overall
management of the instrument development).
Each Team will provide a central location where the data will be delivered from the MOC. Data reduction activities will be carried out within the Instrument Team: this includes daily and longer term
processing of the payload data, delivery of the data to the Ravens Science Archive, and scientific exploitation of the data.
5.3. Ravens Project Team (RPT)
ESA will maintain a Ravens Project Team at ESTEC, directed by a Project Manager, until completion
of the satellite in-orbit commissioning phase. ESA, via the Project Manager and their Project Team,
will retain overall responsibility for the mission.
The Project Team will control the process of definition of mission requirements and payload interfaces, and will finally select a Prime Contractor and a preferred spacecraft design.
In conjunction with the Instrument Teams, the Project Team will be responsible for the procurement
of the spacecraft (with the exception of the instruments), instrument integration into the experiment module and its integration onto the spacecraft bus, system testing and execution of calibration
plans, spacecraft launch and operations, and acquisition and transmission of the data to the Instrument Teams.
The Project Team will monitor and control the work of the Ravens spacecraft industrial contractor(s),
and determine suitable satellite launch dates. During the development phases of the mission, the
Project Team will also monitor the development of the instruments, and ensure their timely readiRavens • Auroral and magnetospheric imaging mission • 39
39
ness by monitoring the adherence of development plans to agreed-to schedules. In addition, the
Project Team will monitor and control all interface specifications, including technical specifications
between the telescope, the instruments, the spacecraft, and any ancillary equipment, as well as data
and information exchange specifications among all parties involved (ESA, the Instrument Teams, and
industry).
The Research and Scientific Support Department of ESA (RSSD) will assume responsibility for management of the Ravens Project at a suitable time after launch.
5.4. Ravens Science Office (RSO)
The Ravens Science Office will support the Ravens Project Scientist in their activities. This will include management of relations with the Science Community throughout the mission.
The Science Office will undertake Ground Segment system engineering tasks, for instance organising
and chairing a Ravens Ground Segment Advisory Group, and managing a Ravens Ground Segment
System Engineering Group.
The Research Office will be responsible for instrument coordination and management of calibration
and cross-calibration activities, and product quality assessment. The Ravens Science Office is also
responsible for the implementation of a Ravens Science Archive (RSA) for distributing data products
to the Research Community, which will be overseen by a Ravens Archive Scientist.
5.5. Ravens Science Team (RST)
The Ravens Science Team will be formed after selection of the Instrument Teams through the AO
process, and will remain in place until the scientific products are delivered to the community. It will
include the ESA Project Scientist as its chairperson, the Ravens Principle Investigator (proposal lead),
the Archive Scientist, and from each of the five Instrument Teams the Principal Investigator, the Instrument Scientist, and the Data Processing Manager.
Ad-hoc experts will be invited to attend Science Team meetings as the need arises. The specific
number and expertise of these experts will vary during the development of the mission to reflect the
current needs of the team.
The ESA Project Manager, Payload Manager, and the Instrument Project Managers will have standing invitations to attend all meetings and participate in all activities of the Science Team.
The Ravens Science Team will mainly rely on the technical support of the Instrument Teams for the
fulfilment of its functions. However, if deemed necessary, the Project Scientist may request external
scientific consultant(s) to conduct an independent review of any of the activities which normally fall
under the responsibility of the Science and Instrument Teams.
The Ravens Science Team will act as a focus for the interest of the scientific community in Ravens,
and will work to maximize the scientific return of Ravens, ensuring that the development of the mission remains compatible with the main scientific objectives. The Science Team will formulate and
optimize the Observing Plan and the calibration strategy, both from the scientific and operational
viewpoints, reviewing the scientific goals of Ravens at regular intervals in the light of recent results.
The Science Team will also oversee the preparation and analysis of data from Ravens, creating and
delivering the final scientific data products to the community. As part of this activity it will oversee
the organization of the Ravens Science Archive. Throughout, the Team will make every effort to
promote public awareness and appreciation of the Ravens mission, supporting ESA in its public relations efforts.
Ravens • Auroral and magnetospheric imaging mission • 40
40
In general, the members of the Science Team will be expected to monitor and advise on all aspects
of Ravens which affect its scientific performance. In particular, they will participate in major project
reviews, and perform specific tasks as needed during the development and operation phases.
5.6. Scientific Operations
Ravens will provide continuous imagery of the Earth’s magnetosphere. Open-access data will be
provided to the Research Community as it is collected. The Ravens project has an open data philosophy, there are no proprietary data or periods.
The main elements of scientific operations which are relevant for the formulation of a management
approach are the following:
•
•
•
•
•
•
•
•
The two Ravens spacecraft will be launched together into a circular orbit, they will separate,
and will then independently manoeuvre into their science orbits.
Routine operations begin ~3 months after launch, once the period of payload commissioning
and performance verification has ended.
The initial mission is expected to last 3 years; however, in anticipation of a possible extension of the mission, spacecraft and instrument consumables are designed to allow for continued operations.
Operations are commanded via a single ground station per spacecraft with daily ground
communication period of ~3 hrs.
The preliminary Observing Plan is to provide continuous imagery from nadir-pointing platforms.
Scientific operations consist of a pre-programmed sequence of manoeuvers to keep the instruments nadir-pointing, and will be executed autonomously by the on-board computer.
Corrections to the sequence may be requested by the Ravens Science Office (RSO).
During routine operations, the instruments will operate uninterruptedly in a unique instrumental mode. Other modes will be required only during the commissioning phase, or for
technical characterization of the instruments.
Data obtained in non-visibility periods is stored in on-board solid-state memory, and is telemetered to ground interleaved with data acquired in real time.
Scientific operations will be supported by four main elements, the Mission Operations Centre (MOC),
the Ravens Science Team (RST), the Ravens Science Office (RSO), the five Instrument Teams. The
Project Scientist will act as chairperson of the Science Team and will be the interface with the MOC.
The MOC will deliver scientific payload and housekeeping data to the Instrument Teams every day
for processing. The Instrument Teams will be responsible for the operation of their respective instruments, continuous data processing during operations (which will include monitoring of the data
quality, calibration, and cleaning) and reduction of the data into the final science products of the
mission, and their delivery to ESA for archival and distribution to the Scientific Community. The Instrument Teams will report regularly to the Ravens Science Team and Science Office on the data
processing activities, and in particular of any anomalies detected. The Instrument Teams will also be
responsible for extracting and publishing scientific results from the processed data.
ESA will be responsible for planning and carrying out Public Relations (PR) activities related to Ravens. The Project Scientist will initiate and identify opportunities for publishing project-related progress reports and scientific results. PR materials suitable for release to the public will be provided by
the members of the Ravens Science Team and the Project Scientist. Public engagement would be
facilitated if real-time auroral images were available for viewing through web- and mobile phonebased technology, with the addition of real-time “aurora alerts” (Figure 5.2).
Ravens • Auroral and magnetospheric imaging mission • 41
41
Figure 5.2. Ravens data products are of interest to the research community, satellite
operators, Space Weather forecasters, and the general public alike. Continuous, realtime auroral images would create a great deal of interest.
5.7. Ravens Science Archive (RSA)
For full and successful exploitation of the Ravens mission products, the development of a Ravens Science Archive is essential. This will be the on-line portal
through which the Scientific Community will access the Ravens mission products, allowing data download or facilitating simple visualization and analysis.
The development of the Archive will be overseen by the Ravens Science Office in coordination with
ESAC, who will form an Archive Development Team. The Project Scientist will be supported by a Ravens Archive Scientist, who will also sit on the Ravens Science Team.
The Ravens Archive Scientist will be responsible for liaison with the Data Processing Manager of each
Instrument Team to ensure smooth and timely delivery of the instrument products to the Archive.
The Archive Scientist will oversee the development of data analysis and visualization tools, and will
actively participate in cross-calibration activities between Instrument Teams. In addition, the Ravens
Archive Scientist will provide a link with the Scientific Community, alerting the community to news
and developments that relate to Ravens, to the Archive, and to Ravens data products.
Data access for the community will be through a web-based interface to the Ravens Science Archive.
The web-interface will provide data from the spacecraft as it becomes available (see below), quicklook data plots, orbital ephemeris data, and other useful supporting information. This portal will
also provide links to supporting information, e.g. instrument descriptions, software archives for plotting and manipulating data, orbit plots and predictions, with the aim of making the data as accessible to users as possible. The archive will also include supporting ground-based data and the results
of modelling runs (Sect. 2.5) where appropriate. The format in which data is distributed will be decided upon during further study phases of the mission, driven by instrument and mission requirements. Possible data formats that will be considered are Common Data Format (CDF), the Cluster
Exchange Format (CEF) developed by ESA for the CAA, or another, possibly bespoke, format which is
optimized for the description of the imagery that will form a major component of the Ravens science
data.
It is expected that “low level” data products will be delivered to the archive within a short period of
time, similar to the two-week period achieved with THEMIS. Such low level data products include
raw auroral, plasmasphere, and ring current images. Supporting data from ground-based observatories will also be archived and made available on-line as rapidly as data collection and processing allows. Higher level data, in which the energy distribution of precipitating particles or the tomographic reconstruction of the 3D structure of the plasmasphere and ring current regions, will require more
detailed processing, but will be archived and made available as rapidly as possible. The highest level
data would involve the assimilation of Ravens and supporting observations into the modelling
framework. This also would be archived and made accessible to the community once it was available.
One key feature that will be explored in the preparatory phase will be the potential for the regular
downlink of real-time images of the auroras, plasmasphere, and ring current. This would be of use
for now- and forecasting magnetospheric state, but also for public engagement purposes, where
these images would be available through web- and mobile phone-based technology.
Ravens • Auroral and magnetospheric imaging mission • 42
42
Annex A
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Ravens Lead Proposer
Prof Steve Milan
Department of Physics and Astronomy
University of Leicester
University Rd
Leicester LE1 7RH
UK
[email protected]
Tel: +44 116 223 1896
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