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 self- similar 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 quasi- neutral 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 two- stream electron instabilities in the central wake [Birch and Chapman, 2001, 2002], bump- on-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.

References Bale, S.D., C.J. Owen, J.-L. Bougeret, K. Goetz, P.J. Kellogg, R.P. Lepping, R. Manning, and S.J. Monson (1997), Evidence of currents and unstable particle distributions in an extended region around the lunar plasma wake, Geophys. Res. Lett., 24, 1427-1430. Birch, P. C., and S. C. Chapman (2001), Detailed structure and dynamics in particle-in- cell simulations of the lunar wake, Phys. Plasmas, 8, 4551-4559. Birch, P.C., and S.C. Chapman (2002), Two-dimensional particle-in-cell simulations of the lunar wake, Phys. Plasmas, 9, 1785-1789. Borisov, N., and U. Mall (2000), Plasma distribution and electric fields behind the Moon, Phys. Lett. A, 265, 369-376. Bosqued, J. M., N. Lormant, H. Reme, C. d’Uston, R. P. Lin, K. A. Anderson, C. W. Carlson, R. E. Ergun, D. Larson, J. McFadden, M. P. McCarthy, G. K. Parks, T. R. Sanderson, and K. P. Wenzel (1996), Moon-solar wind interaction: First results from the WIND/3DP experiment, Geophys. Res. Lett., 10, 1259-1262. Clack, D., J. C. Kasper, A. J. Lazarus, J. T. Steinberg, and W. M. Farrell (2004), Wind observations of extreme ion temperature anisotropies in the lunar wake, Geophys. Res. Lett., 31, L06812, doi: 10.1029/2003GL018298. Colburn, D. S., R. G. Currie, J. D. Mihalov, and C. P. Sonett (1967), Diamagnetic solar- wind cavity discovered behind moon, Science, 158, 1040. Colburn, D. S., J. D. Mihalov, and C. P. Sonett (1971), Magnetic observations of the lunar cavity, J. Geophys. Res., 76, 2940-2957. Crow, J. E., P. L. Auer, and J. E. Allen (1975), The expansion of plasma into a vacuum, J. Plasma Phys., 14, 65-76. Denavit, J. (1979), Collisionless plasma expansion into a vacuum, Phys. Fluids, 22, 1384-1392. Farrell, W.M., T.J. Stubbs, J.S. Halekas, G.T. Delory, R.R. Vondrak, and R.P. Lin (2007), The loss of solar wind plasma neutrality near the lunar terminator and polar regions, Geophys. Res. Lett. Submitted. Farrell, W. M., M. L. Kaiser, J. T. Steinberg, and S. D. Bale (1998), A simple simulation of a plasma void: Applications to Wind observations of the lunar wake, J. Geophys. Res., 103, 23653-23660. Farrell, W.M. R.J. Fitzenreiter, C.J. Owen, J.B. Byrnes, R.P. Lepping, K.W. Ogilvie, and F. Neubauer (1996), Upstream ULF waves and energetic electrons associated with the lunar wake: Detection of precursor activity, Geophys. Res. Lett., 23, 1271-1274. Futaana, Y., S. Machida, Y. Saito, A. Matsuoka, and H. Hayakawa (2001), Counterstreaming electrons in the near vicinity of the Moon observed by plasma instruments on board NOZOMI, J. Geophys. Res., 106, 18729-28740. Goldstein, B. E. (1974), Observations of electrons at the lunar surface, J. Geophys. Res., 79, 23-35. Halekas, J.S., G.T. Delory, D.A. Brain, R.P. Lin, M.O. Fillingim, C.O. Lee, R.A. Mewaldt, T.J. Stubbs, W.M. Farrell, and M.K. Hudson, Extreme lunar surface charging during solar energetic particle events, Geophys. Res. Lett., 34, L02111, doi:10.1029/2006GL028517, 2007a. Halekas, J.S., Brain, D.A., Lin, R.P., Mitchell, D.L., 2007b, Solar wind interaction with lunar crustal magnetic anomalies, J. Adv. Space Res., in press (available online), doi:10.1016/j.asr.2007.04.003. Halekas, J.S., D.A. Brain, D.L. Mitchell, and R.P. Lin, Whistler waves observed near lunar crustal sources, Geophys. Res. Lett., 33, L22104, doi:10.1029/2006GL027684, 2006b. Halekas, J.S., Brain, D.A., Mitchell, D.L., Lin, R.P., Harrison, L., 2006a, On the occurrence of magnetic enhancements caused by solar wind interaction with lunar crustal fields, Geophys. Res. Lett., 33, L01201, doi:10.1029/2006GL025931. Halekas, J.S., R.P. Lin, and D.L. Mitchell (2005a), Large negative lunar surface potentials in sunlight and shadow, Geophys. Res. Lett., 32, L09102, doi:10.1029/2005GL022627. Halekas, J.S., S.D. Bale, D.L. Mitchell, and R.P. Lin, (2005b) Electrons and magnetic fields in the lunar plasma wake, J. Geophys. Res., 110, A07222, doi:10.1029/2004JA010991. Halekas, J. S., D. L. Mitchell, R. P. Lin, L. L. Hood, and M. H. Acuña (2002), Evidence for negative charging of the lunar surface in shadow, Geophys. Res. Lett., 29, doi:10.1029/2001GL014428. Hood, L. L., A. Zakharian, J. Halekas, D. L. Mitchell, R. P. Lin, M. H. Acuña, and A. B. Binder (2001), Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector data, J. Geophys. Res., 106, 27825-27840. Johnson, F., and J. E. Midgeley (1968), Notes on the lunar magnetosphere, J. Geophys. Res., 73, 1523. Kallio, E. (2005), Formation of the lunar wake in quasi-neutral hybrid model, Geophys. Res. Lett., 32, L06107, doi:10.1029/2004GL021989. Kellogg, P.J., K. Goetz, and S.J. Monson (1996), Observations of plasma waves during a traversal of the Moon’s wake, Geophys. Res. Lett., 23, 1267-1270. Khurana, K.K., C.T. Russell, and M.K. Dougherty (2007), Magnetic portraits of Tethys and Rhea, Icarus, in press. Kimura, S., and T. Nakagawa (2007), Electromagnetic full particle simulation of the electric field structure around the Moon and lunar wake, Earth Planets Space, submitted. Lin, R. P., D. L. Mitchell, D. W. Curtis, K. A. Anderson, C. W. Carlson, J. McFadden, M. H. Acuña, L. L. Hood, and A. Binder (1998), Lunar surface magnetic fields and their interaction with the solar wind: Results from Lunar Prospector, Science, 281, 1480-1484. Michel, F. C. (1968), Magnetic field structure behind the Moon, J. Geophys. Res., 73, 1533-1541. Nakagawa, T., Y. Takahashi, and M. Iizima (2003), GEOTAIL observation of upstream ULF waves associated with the lunar wake, Earth Planets Space, 55, 569-580. Nakagawa, T., and M. Iizima, (2006), A reexamination of pitch angle diffusion of electrons at the boundary of the lunar wake, Earth Planets Space, 58, e17-e20. Ness, N. F. (1972), Interaction of the solar wind with the Moon, in Solar Terrestrial Physics/1970, Part II, edited by Dyer, pp. 159-205, D. Reidel Publ. Comp., Dordrecht. Ness, N. F., K. W. Behannon, H. E. Taylor, and Y. C. Whang (1968), Perturbations of the interplanetary magnetic field by the lunar wake, J. Geophys. Res., 73, 3421-3440. Ogilvie, K. W., and N. F. Ness (1969), Dependence of the lunar wake on solar wind plasma characteristics, J. Geophys. Res., 74, 4123-4128. Ogilvie, K. W., J. T. Steinberg, R. J. Fitzenreiter, C. J. Owen, A. J. Lazarus, W. M. Farrell, and R. B. Torbert (1996), Observations of the lunar plasma wake from the WIND spacecraft on December 27, 1994, Geophys. Res. Lett., 10, 1255-1258. Owen, C. J., R. P. Lepping, K. W. Ogilvie, J. A. Slavin, W. M. Farrell, and J. B. Byrnes (1996), The lunar wake at 6.8 RL: WIND magnetic field observations, Geophys. Res. Lett., 10, 1263-1266. Russell, C. T., and B. R. Lichtenstein (1975), On the source of lunar limb compression, J. Geophys. Res., 80, 4700. Samir, U., K. H. Wright, Jr., and N. H. Stone (1983), The expansion of a plasma into a vacuum: Basic phenomena and processes and applications to space plasma physics, Rev. Geophys., 21, 1631-1646. Schubert, G., and B. R. Lichtenstein (1974), Observations of Moon-plasma interactions by orbital and surface experiments, Rev. Geophys., 12, 592-626. Singh, N., and R. W. Schunk (1982), Numerical calculations relevant to the initial expansion of the polar wind, J. Geophys. Res., 87, 9154-9170. Siscoe, G. L., E. F. Lyon, J. H. Binsach, and H. S. Bridge (1969) Experimental evidence for a detached lunar compression wave, J. Geophys. Res., 74, 59-69. Sonett, C. P. (1982), Electromagnetic induction in the Moon, Rev. Geophys. Space Phys., 20, 411-455. Sonett, C. P., and J. D. Mihalov (1972), Lunar fossil magnetism and perturbations of the solar wind, J. Geophys. Res., 77, 588-603. Travnicek, P., P. Hellinger, D. Schriver, and S.D. Bale (2005), Structure of the lunar wake: Two-dimensional global hybrid simulations, Geophys. Res. Lett., 32, L06102, doi:10.1029/2004GL022243. Whang, Y. C. (1968a), Interaction of the magnetized solar wind with the Moon, Phys. Fluids, 11, 969-975. Whang, Y. C. (1968b), Theoretical study of the magnetic field in the lunar wake, Phys. Fluids, 11, 1713-1719. Whang, Y. C., and N. F. Ness (1970), Observations and interpretation of the lunar Mach cone, J. Geophys. Res., 75, 6002-6010. Whang, Y.C., and N. F. Ness (1972), Magnetic-field anomalies in the lunar wake, J. Geophys. Res., 77, 1109-1115.