International Workshop on Instrumentation for Planetary Missions (2012)
1032.pdf
A NEUTRAL ATOM INSTRUMENT FOR IO OBSERVATIONS AND OTHER PLANETARY
APPLICATIONS. M. R. Collier1, J. W. Keller1, and M. D. Shappirio1, 1NASA’s Goddard Space Flight Center,
Greenbelt, Maryland 20771
Introduction: We describe an approach to remotely image the dynamics of the Io plasma torus using low energy neutral atoms. Such a capability will
improve understanding of the interrelation between
volcanic, plasma torus, and Jovian magnetospheric
mass and energy-exchange processes.
Charge Exchange and Energetic Neutral Atom
(ENA) Imaging: Energetic neutral atoms (ENAs) are
produced when an energetic ion (greater than a few eV
- for example, a corotating ion in the Io plasma torus)
undergoes a charge exchange reaction with a cold neutral atom (such as the neutral oxygen and sulfur that
populate the torus). The cold neutral atom gives up an
electron to the ion, neutralizing it. As a result, a cold
ion and an energetic neutral atom are created. The resulting ENA is no longer confined by the magnetic
field and, in the case of charge exchange in the Jovian
Io torus, has sufficient energy to escape Jupiter’s gravity. The energetic neutral atom travels ballistically
away from the location of the charge exchange reaction which is generally energy and momentum conserving. Thus, measurement of ENA fluxes allows us
to image remotely plasma populations that in the past
had to be studied in-situ.
In particular, ENA imaging allows (i) direct observation of charge exchange processes that result in the
transfer of both mass and energy, for example the introduction of cold ions into the Io torus via charge exchange, (ii) remote monitoring of plasma populations
from a distance, for example outside of the Jovian
magnetosphere, (iii) the acquisition of global information about plasma populations, for example, their time
and spatial variability and
(iv) the ability to study
fundamental
plasma
physics through temporal
variations in ENA flux,
for example the effects of
wave processes [1].
In the case of the Jovian Io plasma torus,
ENA imaging will allow
us to study the torus
morphology,
composition, and time-variability
remotely. Here we disFigure 1 - Concept for a low
cuss an instrument conenergy neutral atom imager for
observing Io torus composition cept based on flightand variability.
proven technology to
image remotely the Jovian Io plasma torus from even
as far as outside of the Jovian magnetosphere.
Instrument Description: The Jovian low energy
neutral atom instrument concept is shown in Figure 1.
The top section of the instrument consists of a passive
conversion surface neutral atom imager with a 360
degree field-of-view. In this design, energetic neutral
atoms enter the instrument and hit a venetian-blind
assembly of passive conversion surfaces at a small
angle.
The venetian-blind assembly increases the effective
surface area of the passive conversion surfaces. A fraction of the incident neutral atoms then become converted to negative ions
and traverse a torroidal
analyzer, for energy
measurement (shown in
purple in Figure 1). The
negative ions then interact with a thin carbon foil to generate
secondary
electrons
that are steered to hit an
array of six microchannel plates to produce a
start signal and to determine the direction of
the incoming neutral
atom. The ions upon Figure 2 - The MINI-ME instruexiting the carbon foil ment (upper near corner of spacecraft) on the FASTSAT spacecraft
fly through the time-of- prior to launch.
flight chamber at the
bottom of the figure and are turned around in a quadratic potential which, like an harmonic oscillator, produces a time-of-flight (i.e. period of oscillation) independent of energy but dependent on the mass of the
incident ion. The ions then hit a stop microchanel plate
allowing a determination of their time-of-flight and
hence mass.
As described above, the Io ENA telescope employs
a quadratic potential high resolution mass analyzer (the
chamber at the bottom in the concept figure). The high
resolution mass spectrometer which takes the place of
the simple detector plane used on previous ENA instruments represents a major advance for this type of
instrumentation. Its performance has been electrostatically modeled using simion code. The simulations
show that for a linear electric field design we will
achieve M/ΔM~50, remarkably good for an energetic
neutral atom imager (as opposed to a thermal quad-
International Workshop on Instrumentation for Planetary Missions (2012)
rupole mass analyzer), and necessary for separating
molecular species.
Estimate of Iogenic Neutral Atom Flux and Instrument Count Rate: Eviatar and Barbosa [2] based
on charge exchange of heavy ions in the Io torus calculated a creation rate Sn for fast (~70 km/s) neutral atoms from the Io torus of Sn=5.7x1028 s-1. They model
this oxygen and sulfur neutral wind as flowing out in
the form of a cylinder of total height 2 RJ based on the
geometry of the torus. Thus, if the neutral atom camera
were observing the Io torus from outside the Jovian
magnetosphere at even as far as 100 RJ, the neutral
flux, ΦN~Sn/2π/(100RJ)/(2RJ)=9x105/cm2/s. This is a
very large flux and a very observable flux - for comparison, it is about six times the solar wind oxygen flux
[3].
The top end of the Io ENA telescope concept
shown in Figure 1 has flight heritage based on the
Miniature Imager for Neutral Ionospheric atoms and
Magnetospheric Electrons (MINI-ME) instrument
launched on NASA’s FASTSAT-HSV01 satellite (Fast
Affordable Science and Technology Satellite
Huntsville-01 mission) by the DoD Space Test Program-S26 (STP-26) as a secondary payload from
Kodiak, Alaska on 19 November 2010 [4]. MINI-ME,
shown on the FASTSAT spacecraft in Figure 2, has
collected data in full science mode as of the time of
this writing for over 18 months. It responds to neutral
atoms in the energy range from a few eV up to about
700 eV, although the same design could go significantly higher in energy. Note that energetic neutral
oxygen from the Io torus will be at about 300 eV and
energetic neutral S from the Io torus will be at about
600 eV, well within the energy range of the MINI-ME
instrument.
The first science results from the FASTSAT/MINIME instrument were reported at the Fall American
Geophysical Union 2011 meeting [5]. One of the
strengths of the MINI-ME design that will prove particularly useful for observing energetic Iogenic molecules such as SO2 is its ability to detect not only elements but also molecules. Because some of the incident energetic molecules dissociate on the highly polished tungsten conversion surfaces and the dissociation
results in the products moving at the same velocity as
the original molecule, MINI-ME observes distinct
peaks at energies in the ratios of the individual specie
mass to the molecular mass. For example, the top two
plots in Figure 3 show flight spectra from a MINI-ME
pass of the aurora oval, based on FASTSAT/PISA and
AMPERE data shown in the lower figures. In particular, the spectrum on the right shows two distinct peaks
with energies of about 100 eV, the dissociated oxygen,
and 200 eV, the undissociated O2. This characteristic
response to neutral molecules, which was observed in
1032.pdf
Figure 3 - Data from the March 31, 2011 event including
MINI-ME neutral atom data, PISA electrometer data, and
AMPERE current profiles.
the MINI-ME calibration data, will allow us to infer
the presence of molecular neutral atoms from the Io
torus at a high cadence while the time-of-flight unit
will provide composition information on the observed
molecules.
The top end of the Io ENA camera shown in Figure
1 is very similar to MINI-ME and provides compelling, flight-based, evidence for the soundness of the
design. Based on the calibration of the
FASTSAT/MINI-ME instrument, the total instrument
efficiency (including conversion, transmission, and
MCPs) is ~10-3, and the aperature size is ~1 cm2 for a
single angular sector. Thus, the count rate, RN, observed by the Io ENA camera in the sector facing the
Io torus at 100 RJ would be RN~9x105/cm2/s•10-3•1
cm2 = 900 counts/s. This rate would, of course, be
larger at closer distances.
Assuming the efficiency of the time-of-flight unit is
about 10%, this means even at 100 RJ the Io ENA
camera would be observing about 100 counts per second in the time-of-flight spectrum, statistics that would
make it extremely easy to observe time variability in
the composition of the Io torus and link it to both subsequent Jovian magnetospheric activity as well as geologic activity on Io itself.
References: [1] Espley J. R. et al. (2005) J. Geophys. Res., 110, A09S33, doi: 10.1029/2004JA010935
[2] Eviatar A. and Barbosa D. D. (1984) J. Geophys.
Res., 89, 7393. [3] Bame S.J. et al. (1975) Solar Phys.,
43, 463-473. [4] Rowland, D. E. et al. (2011) IEEEAC
paper #1425. [5] Collier, M. R. et al. (2011) AGU Fall
Meeting Abstract SM31A-2087.
Acknowledgments: This work was supported in
part by the Proposal Design Lab program of the Planetary Division at Goddard Space Flight Center.