Background
The Moon has a minimal exosphere and no global magnetic field, and to first order
behaves as a solid obstacle to the flowing plasma encountered in the solar wind and
magnetosheath. Magnetic fields pass through the relatively non-conductive lunar surface
essentially unimpeded, with only a slight inductive interaction with the interior [Sonett,
1982], producing no upstream shock [Colburn et al., 1967]. Meanwhile, plasma impacts
the surface and is absorbed [Goldstein, 1974], leading to the formation of a plasma cavity
behind the lunar obstacle – one of the best natural vacuums in the solar system.
Explorer 35 and the Apollo 15 and 16 sub-satellites [Ness, 1972; Schubert and
Lichtenstein, 1974] first observed the lunar wake (at altitudes of ~700-10,000 km and
~100-160 km respectively) and found enhanced magnetic fields in the central cavity, with
reduced fields in the surrounding expansion region [Ness et. al., 1968; Colburn et al.,
1967, 1971]. These observations have been successfully explained in terms of
diamagnetic current systems at the wake boundary [Colburn et al. 1967], and
equivalently in terms of plasma rarefaction and recompression [Whang, 1968a, 1968b;
Johnson and Midgeley, 1968; Michel, 1968]. Outside of the rarefaction region, many
spacecraft have observed sharply peaked “limb shocks” or “limb compressions” [Ness et
al., 1968; Colburn et al., 1971; Russell and Lichtenstein, 1975; Lin et al., 1998; Halekas
et al., 2006, 2007], clearly associated with remanent crustal magnetic fields [Sonett and
Mihalov, 1972; Russell and Lichtenstein, 1975; Halekas et al., 2006]. A series of lunar
encounters by the Wind spacecraft [Bosqued et al., 1996; Ogilvie et al., 1996; Owen et
al., 1996; Clack et al. 2004] revealed a wake extending >~25 RL downstream, with
anisotropic ion beams propagating from the flanks [Ogilvie et al., 1996; Clack et al.,
2004]. NOZOMI, GEOTAIL, and Wind observed a “forewake”, with counter-streaming
electrons and waves extending well upstream from the wake [Futaana et al., 2001;
Nakagawa et al., 2003; Farrell et al., 1996; Kellogg et al., 1996; Bale et al., 1997]. Most
recently, Lunar Prospector (LP) provided extensive low-altitude (<~100 km)
measurements of suprathermal electrons and magnetic fields at low altitudes and their
response to changing solar wind conditions [Halekas et al., 2005b].
Despite many previous lunar wake encounters, however, a clear understanding of its
3-d structure has not yet emerged. Furthermore, statistical studies indicate that the wake
varies with solar wind conditions [Ogilvie and Ness, 1969; Colburn et al., 1971; Whang
and Ness, 1972; Halekas et al., 2005b], but these studies have all been limited in both
data and altitude coverage, rendering a clear understanding of wake’s response to
external drivers elusive. At present, extensive modern simulations and theoretical work
may represent the best guide to the structure and dynamics of the lunar wake [Farrell et
al., 1998, 2007; Birch and Chapman, 2001, 2002; Borisov and Mall, 2000, 2002; Crow et
al, 1975; Denavit, 1979; Kallio, 2005, Kimura and Nakagawa, 2007, Samir et al., 1983,
Singh and Schunk, 1982; Travnicek et al., 2005]. However, simulation results also differ
on many important points, and we can only resolve these discrepancies with detailed
observations from a mission like ARTEMIS. The wake measurement goals described
here are very consistent with the objectives of the heliospheric NAC subcommittee, who
identified the lunar wake as an ideal natural laboratory for fundamental plasma physics.
ARTEMIS, with full charged particle, magnetic and electric field, and wave
measurements, providing multi-point measurements of the wake at a wide range of
downstream distances with varying solar wind conditions, will finally answer the
many questions which remain about the lunar wake.
Three-Dimensional Structure
Perhaps the most important outstanding question is the wake’s 3-d structure and
extent. Early studies of the wake considered it an MHD structure – a standing tangential
discontinuity [Ness et. al., 1968; Colburn et al., 1967, 1971; Ness, 1972; Schubert and
Lichtenstein, 1974]. Based on early Explorer 35 observations which found no
manifestation of the wake beyond 4 RL [Ness, 1972] (though other authors used data from
the same spacecraft to assert that there was no evidence of cavity closure even at 5.4 RL
[Colburn et al., 1971]), it was assumed that the lunar wake propagated as a magnetosonic
disturbance, closing relatively rapidly in ~3-10 RL, depending on the field orientation and
Mach number, and that a trailing standing shock (never observed) would form at several
RL [Michel, 1968]. Two studies, focusing on the rarefaction wave [Whang and Ness,
1970] and limb shocks [Sonett and Mihalov, 1972], found Mach angles and anisotropies
roughly consistent with magnetosonic wave modes. More recently, though, a series of
Wind orbits around the Moon cut through the wake at large downstream distances, and
discovered a wake extending to as much as 25 RL [Ogilvie et al., 1996; Clack et al.,
2004], more difficult to reconcile with an MHD picture, suggesting that the wake refills
via an ion sonic mode. Concurrent observations of accelerated ion beams [Ogilvie et al.,
1996], and strong plasma wave turbulence [Farrell et al., 1996; Kellogg et al., 1996],
ARTEMIS, with its unique orbit and dual-probe approach, provides the
perfect platform to determine the 3-d structure of the lunar wake and its
response to external drivers. ARTEMIS provides two spacecraft with complete
plasma instrumentation, with orbits cutting through the wake at a wide range
of downstream distances. In addition, ARTEMIS allows multi-point
measurements that cannot be provided by a single spacecraft.
also suggested a more kinetic interaction, though the apparent magnetic field control of
the wake suggests that magnetostatic terms could also prove important [Clack et al.,
2004]. LP data was used to attempt to address this question, but LP lacked the altitude
range necessary to distinguish between magnetosonic and ion sonic recompression wave
velocities [Halekas et al., 2005b]. Currently, ion sonic modes seem most likely to control
the refilling of the wake, but magnetosonic modes could still contribute to refilling, and
also likely control the propagation of the rarefaction wave and limb shocks. In addition,
wake modes may vary as a function of solar wind conditions. No mission to date has had
the orbit and instrumentation necessary to conclusively determine the extent of the lunar
wake and the propagation speed of expansion, recompression, and limb shock waves (or
even if this nomenclature represents an adequate description of the wake interaction).
The unique orbit and complete plasma instrumentation of ARTEMIS will finally
allow us to conclusively settle the question of the wake’s extent and structure.
We currently do not fully understand either the 3-d structure of the wake or the
asymmetries therein. Theoretically, we expect asymmetries whenever the magnetic field
is not parallel to the wake, since diamagnetic current systems (and the resulting magnetic
field perturbations) differ for perpendicular vs. parallel magnetic fields (see figure
below). Furthermore, hybrid simulations predict a twist to the wake [Kallio, 2005] for
non-orthogonal magnetic field directions. In addition, as mentioned above, “limb shocks”
or “limb compressions” are often observed external to the wake cavity and rarefaction
wave, located downstream from crustal magnetic anomalies. Previous measurements
suggest that these features are compressional features [Siscoe et al., 1969] that propagate
Quasi-Perpendicular Case:
Magnetic Perturbations Only
On Two Sides of Wake
Quasi-Parallel Case:
Axisymmetric Magnetic
Perturbations
outward at magnetosonic wave velocities [Whang and Ness, 1970]. One expects them to
propagate downstream from crustal magnetic field regions, producing a highly
asymmetric structure external to the main wake cavity. However, no observations have
clearly confirmed this supposition, or determined how far downstream these features
propagate and to what degree they affect the structure of the wake interior to them. Many
observations have found asymmetric wake characteristics on individual orbits [Ness et
al., 1968], but with only one spacecraft it is always difficult to clearly distinguish
between asymmetries and temporal variations. The two-point measurements provided
by ARTEMIS will for the first time allow us to unambiguously measure the
asymmetries introduced into the wake structure by the perturbing influences of
solar wind and crustal magnetic fields or other effects.
Energetics
Wind observed highly anisotropic ion beams accelerated along magnetic field lines
into the wake from the flanks [Ogilvie et al., 1996; Clack et al., 2004]. These beams
imply an ambipolar potential drop across the wake boundary, which we now recognize as
a natural consequence of the pressure gradient across the wake boundary. Near the lunar
limb, solar wind electrons diffuse rapidly across the low altitude wake boundary ahead of
the slower ions, producing a charge separation electric field that slows the electrons and
accelerates the ions. The electric field thereby produced can have a number of
fundamental effects, including pitch angle diffusion at the wake boundary [Nakagawa
and Iizima, 2006]. Early work approximated the refilling process as a quasi-neutral selfsimilar expansion [Samir et al., 1983], but more detailed PIC and Vlasov simulations of
the wake and more generally of plasma expansion into a vacuum show that the quasineutral approximation breaks down at the edge of the expansion, with an “electron cloud”
preceding the ion front [Farrell et al., 2007; Birch and Chapman, 2001, 2002; Crow et al,
1975; Denavit, 1979; Singh and Schunk, 1982]. Recent PIC simulations [Farrell et al.,
2007] suggest that this charge separation could persist to several lunar radii downstream,
forming a standing double layer with electric fields as high as 0.1-1 V/m. ARTEMIS
could for the first time directly observe non-neutral plasma effects near the wake
boundary, allowing us to investigate fundamental particle acceleration processes.
Secondary electrons from the surface provide another example of a fundamental
acceleration process in the wake. LP commonly observed beams of secondary electrons
produced at the lunar surface and traveling along magnetic field lines into the wake
[Halekas et al., 2002]. It appears that these secondary electrons are generated at low
energies at the surface, and then accelerated through the plasma sheath above the
negatively charged nightside lunar surface. Normally, these accelerated secondaries reach
energies of only a few hundred eV or less, but during plasmasheet passages and solar
storms, when the nightside surface charges to much larger values, LP has observed beams
of upward-going electrons at several keV [Halekas et al., 2005a, 2007a]. These beams
have never been observed at higher altitudes, though, and it is not known what altitude
they reach before beam-plasma instabilities moderate them. The electric fields and
accelerated particles generated in the wake during solar events could have clear
consequences for surface exploration or orbiting spacecraft. ARTEMIS will provide the
first observations of the extent of secondary electron beams and their interaction
with plasma refilling the wake from the flanks.
Both ion and electron beams very likely also produce waves. Indeed, the wake is a
rich laboratory for observing plasma waves, with a broad spectrum of magnetic,
electromagnetic and Langmuir waves previously observed in the central wake [Kellogg et
al., 1996], and even far upstream from the wake (when magnetically connected to the
wake boundary) [Nakagawa et al., 2003; Farrell et al., 1996; Bale et al., 1997]. Some of
these waves have been ascribed to ion acoustic waves and Langmuir waves from
instabilities related to differential ion/electron shadowing [Bale et al., 1997], and
whistlers produced by beam instabilities near the wake boundary [Nakagawa et al., 2003;
Farrell et al., 1996]. Additional waves predicted by simulations include waves from twostream electron instabilities in the central wake [Birch and Chapman, 2001, 2002], bumpon-tail instabilities from particles passing all the way through the wake [Birch and
Chapman, 2001, 2002], ion acoustic-like beam instabilities that slow the ion beams in the
central wake [Farrell et al., 1998], flute instabilities [Borisov and Mall, 2000], and low
frequency electromagnetic turbulence with frequencies near the local proton
gyrofrequency [Travnicek et al., 2005]. This zoo of plasma waves has barely been
explored, and all the wave generation mechanisms and interactions between waves and
particles remain far from understood. The two spacecraft measurements available from
ARTEMIS are especially important here, since previous observers have often had
difficulty discriminating between waves related to the wake interaction and those caused
by other solar phenomena [e.g. Kellogg et al., 1996]. The two-point electric and
magnetic field wave measurements from ARTEMIS will revolutionize our
understanding of the wave environment in and around the lunar wake.
Dynamics
We wish to understand not just the wake structure and energetics, but also their
dynamic response to external drivers. A variety of statistical studies [Ogilvie and Ness,
1969; Colburn et al., 1971; Whang and Ness, 1972; Halekas et al., 2005b] provide
tantalizing hints that the wake responds to changing solar wind conditions, but the limited
data and orbital coverage of all previous missions has limited our knowledge of how the
wake responds to external drivers. During later mission phases, ARTEMIS provides a
perfect platform for exploring this response, since one probe can serve as an upstream
monitor. This avoids difficult considerations required to translate plasma data from
spacecraft far upstream, and provides the best possible determination of the near-local
solar wind conditions. By utilizing dual-probe measurements and extending the
superposed epoch analysis used successfully to analyze LP data [Halekas et al.,
2005b], ARTEMIS will determine the dynamics of the wake and its response to solar
drivers.
The Moon spends ~5-6 days every month in the terrestrial magnetosphere. The lunar
wake in these regions has been briefly described by investigations utilizing Apollo
subsatellite data [Schubert and Lichtenstein, 1974], which found some differences in the
magnetic signature in the lunar wake in the sheath vs. that observed in the solar wind, but
were limited in scope . Recent observations of the moons of Saturn provide tantalizing
hints of a fascinating particle interaction [Khurana et al., 2007] between moons and the
subsonic flowing plasma. In subsonic and/or sub-Alfvenic plasma regimes, it appears
that a qualitatively different interaction may prevail, with a more extended and
asymmetric ion wake and energy-dependent overlapping “shadow wings”. This
interaction, in addition, can be expected to vary significantly in the sheath and
plasmasheet regions, both of which can have significantly flowing plasmas, but with very
different properties. The wake physics in subsonic plasma regimes has never been
studied in detail at the Moon, and the dual-probe approach of ARTEMIS will
provide significant breakthroughs in understanding.
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