Download Transition Region Exploration (TREx) Mission J. F. Fennell1, J. L.

Transition Region Exploration (TREx) Mission
J. F. Fennell1, J. L. Roeder1, C. Lemon1, P. O’Brien2
1
2
The Aerospace Corporation, MS: M2-260, P.O.Box 92957, Los Angeles, CA 90009-2957
The Aerospace Corporation, MS: CH3-330, 15049 Conference Center Dr., Chantilly, VA 20154-3824
Understanding the transport, acceleration and losses of energetic electrons in the radiation belts is one
strong focus of magnetospheric science. There is evidence that argues for in situ acceleration processes,
involving wave particle interactions, that generate in a peak in electron phase space density (PSD) in the
L region of 5-6 (Green and Kivelson, 2004; Chen et al., 2008). Green and Kivelson [2004] used a single
sensor on the POLAR satellite to examine the energetic electron PSD radial profiles to determine whether
the shapes were consistent with radial diffusion or not. They found that that the data are best explained by
models that require acceleration of an internal source of electrons near L* ~ 5. They also suggested that
outward radial diffusion from a PSD peak near L* ~ 5 could explain the observed correspondence
between flux enhancements at geostationary orbit and increases in ULF wave power. However their radial
profiles were constrained to small pitch angles (large K values) in order to obtain radial profiles over a
sufficient range of L* for constant first and second adiabatic invariants.
Chen et al. [2008] used data collected by multiple satellites in the magnetosphere at different distances
from the Earth and identified frequent and persistent peaks in equatorial electron PSD near
geosynchronous orbit. They argued that their results provided “unambiguous” evidence for local wave–
particle acceleration, consistent with the Green and Kivelson [2004] results. However, as in all such
studies there is difficulty in intercalibrating the different sensors and establishing the accuracy of the
magnetic field models for computing the phase space densities for constant first and second adiabatic
invariants. Chen et al. used two data sets from GPS and GEO that did not have magnetometer data for
comparing against field model values (GOES magnetometer data from different longitudes at GEO were
available). POLAR magnetometer data could be used. However, the highest L* points may or may not
have been taken on closed drift shells. It is hard to know since a range of K values was not examined (see
below).
Fennell and Roeder [2008] used data from the SCATHA satellite whose near equatorial orbit spanned the
5.2 ≤L*≤ 7.3 region to examine the storm time electron response over that range of L* for a wide range of
constant first and second adiabatic invariant values (M=200-2500 MeV/G and K=0.05-0.65 Re√G). They
found that the PSD radial profiles showed a range of features from peaked in L* at small K and M during
the pre-storm period to those in the late storm recovery phase, that were flat, had negative slopes for small
K and had peaks in the L*= 5.2–6.5 range for intermediate to large K. The results implied that radial
diffusion was a reasonable explanation of the near-equatorial post-storm PSD enhancements for L*>5.2
but that either significant electron pitch angle transport, losses, and/or acceleration of off-equatorial
mirroring electrons by waves played an important role in the evolution of off- equator PSD profiles
during storm recovery. Like Green and Kivelson [2004], Fennell and Roeder [2008] used data from a
single instrument and spacecraft. Like the others, though, they had to rely on field models to assess the
phase space densities at constant first and second adiabatic invariants but they were able to intercompare
the measured magnetic field values to the field model outputs along the satellite trajectory (as was Green
and Kivelson [2004]), which is an important constraint for any study of the processes involved. In fact,
one thing that was noted by Fennell and Roeder [2008] was that the PSD profiles had different shapes for
different K’s and that at the larger K values some of the profiles had very sharp negative gradients which
could be consistent with the particles not being on closed drift shells beyond some L*. This would mean
that the model fields used to assess K and L*, which predicted the observations were taken on closed drift
shells, did not match the real conditions.
What were missing in all the studies noted above were good plasma wave measurements. Because they
were lacking, none of the noted investigations could actually point to the existence of waves required for
any in situ acceleration of the particles during the events studied. The wave smoking gun was missing
and what remained were inferences from an incomplete data set. It is clear that what is needed to
definitively determine whether relativistic electrons are accelerated, in situ, from a seed population by
waves, is a mission similar to the SCATHA mission that covers the critical L* range from 4.5<L*<8 that
is properly instrumented to make all the critical measurements. We note that the RBSP mission, which
has an apogee of 5.6 Re may not provide a definitive answer because it may not reach large enough L*
values to observe the PSD peaks noted in the discussion above.
As secondary science, it should be noted that a mission, as described below, could provide the kinds of in
situ plasma boundary condition data needed for ring current modeling. It allows one to set the boundary
condition above geosynchronous, if desired and to track the plasma conditions down towards the
plasmapause boundary during quiet times and well interior to geosynchronous during disturbed times.
Satellite Mission Requirements
A simple low cost mission could be configured to determine whether indeed acceleration of an internal
source of electrons is required to explain the PSD profiles in the 4.5<L*<8 region for a wide range of M
and K, with K~0.0 – 1.0 a goal. The critical measurements needed are as follows. Electron sensor(s) are
required that measure the flux and angular distributions of 20-3000 keV electrons with good statistics and
good angular resolution. E and B wave measurements the cover the frequency range from the ion
cyclotron frequency (if possible) to beyond the electron gyro frequency, at L*=4.5, which, as a minimum,
should be 2-axes in E and 3-axes in B. A measurement of the DC magnetic field vector with a sample rate
sufficient to overlap the wave sensor frequency range at the low end and sufficient to provide detailed
particle pitch angle determinations and B-field variations is required. A plasma electron/ion sensor is
needed to provide the background plasma conditions for the waves, the seed electrons for wave-particle
acceleration, and the ions for estimating warm plasma density and measuring the low energy end of the
ring current. A proton spectrometer is needed to measure the bulk of the proton distribution (30 keV to a
few MeV) that participates in EMIC scattering during storm times and carries the bulk of the ring current
energy at moderate to quiet times. Finally, a plasma density measurement is needed with sensitivity from
as low as reasonable to >100 cm-3. As an option, a ring current composition measurement in the 10 - 200
keV/ion range could be considered, resources permitting.
To obtain the L* and K ranges indicated requires a near equatorial orbit with a perigee and apogee of ~4.4
and ~8.5 respectively with inclination ≤ 5°. This can be attained by piggybacking on a launch to
geotransfer or geosynchronous orbit and then using a small thruster to achieve final orbit configuration.
(Note: such an orbit has a period slightly less than 24 hours such that the apogee will “drift” in longitude
about 10°/day.) The least expensive configuration, from the sensor perspective, is for the vehicle to be a
spinner with the spin axis in the orbit plane. In this way pitch angle distributions can be obtained by
mounting the sensors such that their fields of view are perpendicular to the spin axis, much like those
were on SCATHA, CRRES and those on RBSP. The sensors themselves can be copies of those being
flown on RBSP or THEMIS, as appropriate. Attitude maintenance could be done using a cold gas system
like that on RBSP or POLAR or small plasma thrusters like those used on geosynchronous commercial
communication satellites. Plasma thrusters could even be used to achieve the final orbit configuration as
has be done by recent geosynchronous missions.
This mission is between a small and medium sized mission depending on whether it is a secondary
payload as noted above or requires its own launch.
References;
Chen, Y., Reeves, G. D. and Friedel, R. H. W., The energization of relativistic electrons in the outer Van
Allen radiation belt, Nature Physics, 3, 614, 2007.
Fennell, J. F. and J. L. Roeder, Storm time phase space density radial profiles of energetic electrons for
small and large K values: SCATHA results, Journal of Atmospheric and Solar-Terrestrial Physics, 70,
1760, 2008.
Green, J. C., and M. G. Kivelson, Relativistic electrons in the outer radiation belt: Differentiating between
acceleration mechanisms, J. Geophys. Res., 109, A03213, doi:10.1029/2003JA0, 2004.