Download ARTEMIS writeup

ARTEMIS Science Writeup
1) Wake – To be added to the fairly complete writeup by Jasper, the first part probably in the 3D
structure section, and the second part probably in the dynamics section
Length of the Lunar Wake: The length of the lunar wake is an open question as Wind
observations have clearly shown a density depletion region as far as 25 RL downstream. PIC and
hybrid simulations studies show that the refilling process is due primarily to the flow of ions and
electrons along the IMF into the density depletion region. In addition to the solar wind speed as
an obvious controlling factor, it has also been shown that the direction of the IMF relative to the
flow direction is important [Travnicek et al., 2005]. In a simple minded sense, when the IMF is
normal to the flow direction, plasma flowing along field lines has the shortest distance to travel
across the density cavity from either side, but when the IMF direction is canted towards the flow
direction, the travel distance across the depletion region increases. This is complicated by the
ambipolar electric field that forms due to the charge separation at the edge of the density cavity
where the density gradient is at its largest value. ARTEMIS two point measurements would
allow an examination of the length of the wake for differing solar wind flow speed and IMF
direction.
Lunar Wake Structure within the Terrestrial Magnetosphere: So far the previous discussion has
dealt with the Moon's plasma environment in the wake when it is immersed in the solar wind.
This situation changes drastically when the Moon is in the Earth's magnetosphere. The
following shows Lunar Prospector data, with the panel on the left showing the situation when the
Moon was in the solar wind and on the right shows the Moon was in the Earth's magnetotail.
Figure: Data from Lunar Prospector when the satellite was in the solar wind (left) and in the
Earth's magnetotail (right). The top two panels in each figure show the electron count rate at
various energies and the color energy spectrum from the electron reflectometer instrument,
respectively. The next three panels show the total magnetic field, and the magnetic field polar
and longitudinal directional angles from the magnetometer instrument, respectively. The bottom
two panels show polarity and sun sensor data, respectively.
Lunar Prospector had a precessing polar orbit about the Moon with an orbital period of about 118
minutes and an altitude about 100 km at the time of these measurements. In a Moon-centered
(x,y,z) coordinate system with x along the Moon-Sun line, z along the rotation axis and y
perpendicular to x and z, Lunar Prospector's orbit was approximately in the x-z plane (near y = 0)
at the time of these observations. In the solar wind (left figure), the electron data in the top 2
panels clearly show the plasma density depletion region recurring about every 2 hours when the
satellite passes through the nightside region of the Moon. The purple strips on the bottommost
panel show when the satellite was in the optical shadow, which corresponds with the density
depletion region. The total magnetic field (third panel from top) is relatively steady with some
slight variations, which could be related either to crustal fields, fluctuations due to the density
depletion region, and/or variations of the solar wind IMF. This is contrasted with the right
figure, which shows the Moon in the Earth's magnetotail plasma sheet. While there appear to be
some density depletion regions on the Moon's nightside, there are clearly other times (e.g., about
0200 UT) when a high energy (~ 1 keV) electron population is present within the optical
shadow. This is likely due to a plasma sheet population convecting or flowing earthward at that
time. The total magnetic field is much more variable compared to the case when the Moon was
in the solar wind, perhaps due to the passing of the Moon through different magnetotail plasma
regions (e.g., plasma sheet, plasma sheet boundary layer, lobe, etc.). Clearly the external plasma
conditions around the Moon are very different when the Moon is inside the Earth's
magnetosphere compared to the situation when the Moon is in the solar wind. To date there has
been very little in the way of observations and modeling of the Moon's plasma environment
when it is in the Earth's magnetosphere. All of the different phases of ARTEMIS when the
satellites are in the magnetosphere would provide the first detailed examination of the lunar wake
in a subsonic flow region.
(Note that I can add more here about the modeling approach that can be taken to accompany
ARTEMIS, if needed)
2) Particle Acceleration in the Tail – I will focus here primarily on electrons, but a similar study
could be done for ions.
Electron Acceleration in the Magnetotail: An open question is how electrons are accelerated in
the Earth’s magnetotail, particularly during storm times. Li et al., [1997] have shown that if one
assumes simple radial transport and uses Liouville's theorem where phase space density and the
first adiabatic invariant are conserved, the fluxes of electrons measured in the upstream solar
wind cannot account for electron fluxes for energies above 20 keV measured in the inner plasma
sheet region, nominally located radially at about 10 RE from Earth. This is an important region
known as the “seed region”, since this population of electrons continues convecting earthward
and eventually becomes accelerated to relativistic energies in the radiation belt region. One
possible conclusion of this result is that energization due to non-adiabatic processes (e.g.,
acceleration near reconnection regions or wave-particle interactions) may be occurring as
electrons make their way from the solar wind to the inner magnetosphere. Such a process would
take lower energy solar wind particles (~ 1 keV or less) and accelerate them, leading to an
increase in the flux of electrons with energies > 10 keV in the seed region. It has also been
suggested that electron heating and formation of the seed region occurs during a series of
substorms [e.g., Baker et al., 1998], whereby (adiabatic) cross-tail electron acceleration by the
convection electric field and field collapse or dipolarization of the inner tail occurs [Birn et al.,
1998]. Modeling and data efforts thus far are inconclusive and the acceleration of electrons in
the outer magnetosphere, leading to the formation of the seed population is an open question.
ARTEMIS phase II would allow the study of electron distributions at a relatively large
separation (~ 20 RE) along the magnetotail in a relatively under explored region of the plasma
sheet. The evolution of the electron distribution functions as they are convected earthward (from
a deep tail reconnection region) or tailward (from a reconnection region earthward of the
satellites) could be examined. An indication of this electron distribution evolution can be gained
from large scale 3D modeling of electron trajectories in a magnetotail field configuration with a
deep tail reconnection region located at ~100 RE illustrated in the following figure:
Figure: The top panel shows magnetic field lines from the 1989 Tsyganenko field model in the
nightside magnetotail region in the north-south (Z), sun-Earth (X) plane with X taken positive
downtail (anti-sunward). The Earth is located at (0, 0) and the solid vertical lines in the top panel
show the location of virtual detectors placed at different X locations. The lower panels show
phase space plots of Z vs. parallel velocity (middle panels) and Z vs. perpendicular velocity
(bottom panels) at different X detectors located at 15, 30, 50, and 70 RE downtail. Velocities are
in km/s and Z is in RE.
The electron distributions formed at 50 and 70 RE downtail (at the approximate locations of the
ARTEMIS satellites during Phase II) show clear differences as they convect earthward. In the
deeper tail, closer to the reconnection region, non-adiabatic acceleration of electrons close to the
reconnection line is the dominant process, whereas closer to the Earth adiabatic compression
becomes the more important acceleration process [Schriver et al., 2005]. Features in the
electron distribution functions can be examined at the two different spatial locations using the
ARTEMIS satellites to understand the fundamental electron acceleration processes at play in the
magnetotail plasma sheet.