Multi-Angle Viewing of the Sun and the Inner Heliosphere
Alexander Ruzmaikin and MASSÉ Science Team1
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive,
Pasadena, CA 91109, USA, E-mail: [email protected]
Abstract. We describe the concept of a proposed mission, called Multi-Angle Solar Sources Explorer (MASSÉ), that
would observe the Sun and the inner Heliosphere from an orbit at 0.72 AU over all solar longitudes. It would, in coordination with observations from Earth’s side, investigate the sources of solar activity from their origin deep
within the Sun, their emergence onto the photosphere, and their ejection into the Heliosphere. It carries a Dopplermagnetic imager, and in situ energetic particle, solar wind, and magnetic field detectors. Three-dimensional views of
the convection zone, where solar activity originates, are reconstructed by correlating MASSÉ and earth-side Doppler
signals from acoustic wave packets traversing deep solar layers. Magnetic images reveal the evolution of active
regions over their life-time and allow the study of emerging fields from deep layers. Particle, plasma, and magnetic
field data provide information on the sites and mechanisms of acceleration of hazardous high-energy particles
produced by coronal mass ejections.
Multiviewing of the inner Heliosphere uncovers the
development of space weather related activity and helps
to understand mechanisms of acceleration of energetic
particles by coronal mass ejections.
Here we briefly discuss science benefits of
multiviewing and a potential mission to carry it out.
INTRODUCTION
Viewing only one side of the Sun at a time has
handicapped solar observers. Half a solar rotation is
needed before the farside becomes fully visible. Hence
the processes developing on time scales of weeks, for
example the full evolution of active regions, cannot be
directly observed. Another critical limitation is that we
cannot observe the solar deep interior.
Helioseismology that analyzes acoustic p-waves
traveling within the solar interior has made it possible
to partially overcome both obstacles [1]. Helioseismic
studies have established the thermal structure and
distribution of differential rotation inside the solar
convection zone. Large active regions on the solar
farside can be detected by analysis of
“doublebounced” waves [2].
However, current helioseismology utilizes viewing
from a single point and thus has serious limitations in
probing the 3-D structures of flows and magnetic fields
inside the Sun. For example, the solar core studies are
very limited. The dynamo region located near the
bottom of the convection zone cannot be well
investigated with a narrow field of view or bounced
waves. An effective way to overcome these limitations
is simultaneous observation of the Sun from multiple
longitudes including observations from the earth side.
THE SUN: INTERIOR AND SURFACE
The solar interior can be probed through correlation
of Doppler images taken from two positions (Fig. 1).
This correlation gives travel times (and phases) of
waves traversing different depths of the Sun. The travel
times are influenced by inhomogeneities, flows and
magnetic fields thus carrying information about the
interior structure and dynamics [1].
Solar activity originates in interplay between
magnetic fields and fluid motions within the solar
convection zone [3]. Fields change on time scales from
minutes to the 11-year solar cycle and longer. They
emerge at the solar surface, diffuse, reconnect, and are
transported by the solar wind into the Heliosphere.
Conducting plasma moving in the convection zone is a
generator (dynamo) for the solar magnetic field. Thus
the dynamo is the key to understanding the origins of
solar activity.
1
The MASSE science team includes A. Ruzmaikin (PI), E. Stone (particle lead), J. Harvey (solar magnetic lead), R. Ulrich (helioseismology
lead), K. Ogilvie (in-situ plasma/magnetic lead), M. Acuna, A. Cummings, J. Feynman, B. Goldstein, D. Gough, K. Harvey, A. Kosovichev,
A. Lazarus, C. Lindsey, D. Mewaldt, C. Ng, D. Reames, P. Scherrer, S. Tomczyk, Y. Toomre, T. von Rosenvinge, M. Wiedenbeck, G. Zank.
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
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bracket the solar center, passing both to the east and to
the west of the core center.
Magnetic imaging of the solar surface from
multiple longitudes allows the study of:
Active region evolution. Magnetic field evolution of
a specific area on the Sun can only be observed for
about 10 days before it rotates out of view from the
Earth. Active regions systematically evolve through
significant portions of their life while they are on the
unseen side of the Sun, so their evolution from one
configuration to another can only be described
statistically. There are unanswered questions such as:
What makes some active regions so energetic, while
others are placid? Why do some active regions last for
a long time while others, seemingly similar, quickly
vanish? What is the magnetic flux budget history of
an active region? The uninterrupted view of solar
active regions development allows photospheric
magnetic fields to be studied without confusing its
spatial and temporal changes.
Magnetic clustering. During the rising phase of the
solar cycle magnetic activity clusters at a few solar
longitudes for many months [9,10]. Clusters of
activity are often the sources of fast CMEs and solar
energetic particles (SEPs) and should be monitored
carefully. The cause of this clustering observed in the
distribution of active regions [9] and associated
magnetic fields on the Sun [11, 8] remains unknown;
it may be a manifestation of the low-wavenumber, nonaxisymmetric modes of the dynamo [8]. The preferred
longitudes manifest themselves in the Heliosphere. It
has been found that the radial component of the
interplanetary field and the non-axisymmetric solar
field rotate with a period of 27.03 days [12, 13]. This
period does not coincide with the period of the surface
or core rotation [8]. Multi-angle imaging to probe
different depths of the convection zone is needed in
search for the cause of this periodicity associated with
preferred longitudes.
It has long been known that CMEs tend to occur in
sheared magnetic fields. However, physical interpretation of line-of-sight earth-side observations is
ambiguous (variations of field strength, direction, or
location?) because they provide only the component of
the vector magnetic field in the line of sight. Vector
field measurements suffer from a 180°-direction
ambiguity, and current vector instrumentation is less
sensitive than required to isolate the cause of changes
in weak fields. Much of the ambiguity will be
resolved by making line-of-sight observations of the
same event from different directions. Earth-Sun-spacecraft angles of ~20-100° are especially useful for this.
Two-sided magnetic imaging provides almost fullSun boundary conditions, replacing current synoptic
maps that confuse time and space, and thus greatly
advancing MHD modeling of the Heliosphere [14].
FIGURE 1. Imaging from two positions allows the
probing all depths of the Sun.
One type of motion that is poorly known is called
giant cells. Giant cells are expected to have lifetimes
comparable to the Sun's rotation period; the rotation
and convection at this scale are interrelated and interact
with magnetic fields. Another type of dynamo related
motion is meridional circulation, which is directly
measured on the solar surface. First attempts have
been made to follow it below the photosphere [4] but
available data (SOHO and GONG) do not provide
sufficient accuracy to definitively establish the pattern
of meridional flows deep within the convection zone.
Current modeling of the dynamo [5-7] emphasizes
the importance of a region deep inside the Sun, called
the “tachocline”, a thin layer where differential rotation
of the convective zone, which stretches field lines into
strong toroidal belts, ends and a nearly uniform
rotation of the deeper interior begins. It is here where
the dynamo is believed to work. In each solar cycle
some critical changes occur in the dynamo as the field
switches from a dominantly axisymmetric at solar
minimum to a dominantly non-axisymmetric at solar
maximum. The non-axisymmetic fields are associated
with preferred longitudes of solar activity [8]. The
tachocline can be probed by the Doppler viewing from
different solar longitudes.
The structure and rotation of the Sun’s energygenerating core remain enigmatic. Because magnetic
fields decay very slowly in the core, it can have kept
memory of initial stages of the Sun’s formation in a
field now frozen into the highly-conductive core.
Current models of rotation based on data taken from a
single observation point are limited in depth to >0.4
solar radii. With a spacecraft at the farside, the core
rotation can be measured by using wave packets that
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maximum intensity at or near shock crossing. A study
of plateau intensity versus heliocentric distance has
been made [21] but scant observations of the radial
dependence are available. The spatial dependence of the
early plateau is important because of an impact on
astronauts and spacecraft hardware. It is also important
to study maximum intensities of heavy ions because of
their greater ionizing power.
INNER HELIOSPHERE
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Multiviewing enhances the scientific understanding
needed to forecast space weather. Technological
developments and continuous human presence in space
sharpen the need to accurately predict CME-related
hazardous Solar Energetic Particle (SEP) events on
time scales of days to weeks.
This requires
observation of the solar farside [15]. Several
indications of impending CMEs are seen in
photospheric magnetic fields such as development of
large, long-lived magnetic clusters fed by newly emerging magnetic flux. Many of the most hazardous
SEP events in the past 30 years were associated with
CMEs erupting from these clusters.
Previously the two Helios’ measured high-energy
particles inside 1 AU but only protons, electrons, and
helium ions. Elemental abundances for SEP events
were determined from Helios 1 in only 2 events (21
June 1980 and 3 July 1982). ACE and WIND data
show that composition and spectral signatures are key
to understanding SEP acceleration and transport.
Two main classes of SEP events have been recognized: impulsive and gradual [16]. It is generally
agreed that these involve separate acceleration mechanisms, resulting in different compositions and other signatures. The SEP events that represent a significant
hazard to astronauts and space hardware, are gradual.
Important questions must be answered to verify models
and provide bases for SEP event forecasting:
What is the role of self-amplified waves in
controlling particle transport? Fast CME-driven
shocks may continuously accelerate particles.
According to the quasilinear theory of wave-particle
interaction and models of SEP transport [20,17]
streaming particles produce waves that tend to trap
them near the shock, producing more streaming
particles and waves. These waves can be investigated
by observing of wave intensity and cross (speedmagnetic field) helicity at 0.7 AU, where shocks, wave
and particle intensities are much higher than at 1 AU.
How do CME-driven shocks accelerate particles? A
vantage point inside 1 AU will help to find the answer.
As the shock surface expands past this point to other
spacecraft at 1 AU, correlated observations will allow
evaluation of spatial gradients of shock and energetic
particle characteristics. Models [16-18] will then be applied to study the evolution of SEP abundances,
energy spectra, and maximum particle energies (Fig.
2). Simultaneous measurements of particles from
spacecraft at different longitudes and radii relative
to the Sun will lead to the site of the acceleration (e.g.,
at the nose of the shock or distributed across the shock
front).
How do the early plateau and maximum SEP
intensities scale with radius? Observations at 1 AU
indicate that particle intensity in a given energy range
sometimes rise by more than an order of magnitude to
105
M = 4.7
104
Maximum Energy (keV)
M = 12.3
103
M = 2.2
0.3
0.5
r (AU)
0.7
1
FIGURE 2. Maximum energy of particles accelerated
at interplanetary shocks. Mach number at 0.5 AU is shown
[18]. As shocks propagate into a weaker magnetic field,
maximum energies drop, although particles accelerated
earlier can be trapped behind the shock and then gradually
leak out ahead of the shock.
Are there hybrid SEP events? Advanced
Composition Explorer measurements [22] have shown
that a significant fraction of events have a mixture of
impulsive and gradual characteristics. These events
typically are associated with both a large X-ray flare
and a CME. Joint observations from two or more
spacecraft at different viewing angles can distinguish
whether material in these events is confined to a narrow
cone, as expected for flare-accelerated material, or
whether radial and latitudinal composition variations
are consistent with a CME shock-acceleration model.
MISSION CONCEPT
To carry out the science objectives described above,
a mission, called Multi-Angle Solar Sources Explorer
(MASSÉ), was proposed to NASA. MASSÉ travels to
a circular orbit 0.72 AU from the Sun, using a Venus
gravity assist (Fig. 3). With a 584-day as seen from
the Earth, MASSÉ traverses Earth-Sun-spacecraft
angles at the rate of 225º per year passing the Sun’s far
side 8 months after launch. (The STEREO drift relative
to Earth is only 22.5°/year.) Data are returned via
weekly downlinks to the Deep Space Network.
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A Doppler-Magnetograph has been designed to
perform simultaneous Doppler/magnetic imaging of the
Sun. Doppler data from MASSÉ and GONG (SDO)
would permit the detection of strong magnetic fields in
the interior, while magnetic measurements reveal their
manifestation at the surface. MASSÉ-GONG observations provide unique two-component magnetic field.
Near the limb (90° ± 30°), MASSÉ observes magnetic
fields at sites of the initiation of CMEs seen from
earth-side.
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FIGURE 3. A sample of trajectory and mission phases.
Particle composition measurements and energy
spectra can be made with Low- and High-Energy Telescopes as flown on ACE and designed for STEREO.
These instruments provides spectra from ~2 to >150
MeV/nucleon for ions from H to Ni, as well as ~0.2-5
MeV electrons. Coordinated measurements with an
earthside spacecraft give the gradient; three spacecraft
would give parabolic estimates of the amplitude and
location of the SEP peak; four spacecraft would see inhomogeneities in SEP and in shocks and CMEs.
Solar wind plasma and magnetic measurements can
be made with ion and electron spectrometers, and a
magnetometer that identify CMEs, measure their
speed, and detect interplanetary shocks and waves
associated with high-energy particle acceleration.
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ACKNOWLEDGEMENTS
This research was conducted in part at the Jet
Propulsion Laboratory, California Institute of
Technology, under contract with NASA. We thank S.
Stephens for managing the preparation of the proposal.
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