GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L16103, doi:10.1029/2009GL039444, 2009 Click Here for Full Article

Solar-wind proton access deep into the near-Moon wake M. N. Nishino,1 M. Fujimoto,1 K. Maezawa,1 Y. Saito,1 S. Yokota,1 K. Asamura,1 T. Tanaka,1 H. Tsunakawa,2 M. Matsushima,2 F. Takahashi,2 T. Terasawa,2 H. Shibuya,3 and H. Shimizu4 Received 3 June 2009; revised 22 July 2009; accepted 24 July 2009; published 28 August 2009.

[1] We study solar wind (SW) entry deep into the near- wake were not known because there were no observation Moon wake using SELENE (KAGUYA) data. It has been data. Recently, a Japanese lunar orbiter SELENE known that SW protons flowing around the Moon access (KAGUYA) performed comprehensive measurements of the central region of the distant lunar wake, while their the plasma and electromagnetic environment around the intrusion deep into the near-Moon wake has never been Moon; in particular, entry of SW protons into the near- expected. We show that SW protons sneak into the deepest Moon wake was found [Nishino et al., 2009]. The SW lunar wake (anti-subsolar region at 100 km altitude), and protons are accelerated by the bipolar electric field around that the entry yields strong asymmetry of the near-Moon the wake boundary and come into the near-Moon wake by wake environment. Particle trajectory calculations their Larmor motion in the direction perpendicular to the demonstrate that these SW protons are once scattered at IMF. This entry mechanism, which we call ‘Type-I entry’, the lunar dayside surface, picked-up by the SW motional lets the SW protons come fairly deep into the wake (solar electric field, and finally sneak into the deepest wake. Our zenith angle (SZA) 150°) at 100 km height. results mean that the SW protons scattered at the lunar [4] Another outstanding feature of the lunar plasma dayside surface and coming into the night side region are environment found by SELENE observations is scattering/ crucial for plasma environment in the wake, suggesting reflection of the SW protons at the lunar dayside surface absorption of ambient SW electrons into the wake to (hereafter, we simply refer it as scattering). About 1 percent maintain quasi-neutrality. Citation: Nishino, M. N., et al. of the SW protons that impact against the lunar dayside (2009), Solar-wind proton access deep into the near-Moon wake, surface are scattered and then picked-up by the SW to Geophys. Res. Lett., 36, L16103, doi:10.1029/2009GL039444. obtain at most 9 times the original kinetic energy [Saito et al., 2008a]. This energization, which is called ‘self-pick-up’ 1. Introduction process, occurs on the dayside, and the destination of the self-picked-up protons has never been known. [2] Studying interaction between the solar wind (SW) [5] The phenomenon that we deal with in this paper and the Moon is important for understanding the lunar is unexpected detection of the SW protons in the deepest plasma environment. The near-Moon space environment is (anti-subsolar, low-altitude) region of the near-Moon wake. characterized by a tenuous tail-like region formed behind Because Type-I entry cannot let the SW protons come into the Moon along the SW flow, which is called the lunar wake the deepest wake, another entry mechanism must be at work [Lyon et al., 1967]. Concerning the near-Moon wake for this phenomenon. In the present study, we show that the environment, it has been believed that high-energy compo- SW protons scattered at the lunar dayside surface access the nent of the SW electrons can approach the lunar nightside deepest wake, which should be categorized as ‘Type-II surface while the SW protons are unlikely to approach the entry’. lunar nightside region, because the thermal speed of SW protons is much lower than the SW bulk speed. According to previous models, the electron-rich status of the lunar 2. Instrumentation wake yields bipolar (inward) electric field at the wake [6] We use data obtained by a Japanese spacecraft boundary where SW protons gradually intrude toward the SELENE (KAGUYA) which is orbiting the Moon in a wake center along the interplanetary magnetic field (IMF) in polar orbit at 100 km altitude with 2-hour period. The the distant wake [Ogilvie et al., 1996; Bosqued et al., 1996; MAP (MAgnetic field and Plasma experiment) instrument Tra´vnı´e`ek et al., 2005]. onboard SELENE performs a comprehensive diagnosis of [3] Although signatures of electrons and magnetic fields electromagnetic and plasma environment in the near-Moon in the near-Moon wake have been fairly well understood space by means of in-situ measurements of electrons, ions, [Halekas et al., 2005], proton behaviors in the near-Moon and magnetic field. MAP-PACE (Plasma energy Angle and Composition Experiment)-IMA faces the lunar surface to

1 measure up-going positive ions whose energy per charge is Institute of Space and Astronautical Science, Japan Aerospace between 7 eV/q and 29 keV/q with a half-sphere field Exploration Agency, Sagamihara, Japan. 2Department of Earth and Planetary Sciences, Tokyo Institute of of view (FOV) [Saito et al., 2008b]. MAP-PACE ESA Technology, Tokyo, Japan. (Electron Spectrum Analyzer)-S1 with a similar FOV as 3Department of Earth Sciences, Kumamoto University, Kumamoto, IMA is for measurement of electrons with energy between Japan. 4 5 eV and 10 keV. The time resolutions of IMA and ESA-S1 Earthquake Research Institute, University of Tokyo, Tokyo, Japan. data in the present study are 32 sec and 16 sec, respectively. Copyright 2009 by the American Geophysical Union. Although IMA is originally designed for detecting tenuous 0094-8276/09/2009GL039444$05.00 ions from the lunar surface, it also measures SW ions

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interacting with the SW flow upstream of the Earth’s bow shock. The SW speed observed by the Wind spacecraft was 300 km/s (0.47 keV for protons), and the IMF whose strength was about 5 nT was dominated by the negative BY component. On the day the SELENE spacecraft flew near the noon-midnight meridian plane, orbiting from south to north on the dayside and going through the tenuous wake on the nightside (Figure 1). We first mention proton signatures observed by SELENE between 09:10–11:10 UT. Before 09:38 UT the protons scattered at the lunar dayside surface [Saito et al., 2008a] were detected by IMA (Figure 2a). The spacecraft passed above the North Pole (NP) at 09:37 UT and crossed the boundary between sunlit and shadowed regions at 09:44 UT. Between 09:36–09:51 UT SELENE observed SW proton entry near the NP, which is categorized into ‘Type-I entry’ [Nishino et al., 2009] by which SW protons come deep into nightside (deepest at SZA 150°). Figure 1. A schematic of the SELENE orbit and the After SELENE passed through the almost vacuum region in field of view (FOV) of IMA and ESA-S1. SELENE is a the northern hemisphere, it began to detect protons around three-axis stabilized spacecraft, and IMA and ESA-S1 10:08 UT in the deepest wake (SZA 168°) that SW protons always face upon the lunar surface with an FOV of 2p str. are not anticipated to access. The proton flux in the deep The SW protons come directly into the FOV of IMA near wake was the largest around 10:12 UT, and its energy ranged the day-night terminator and the wake boundary. broadly between 0.1–1 keV. Between 10:20–10:40 UT the proton energy was higher than the original SW energy, and took a peak energy of 3 keV around 10:34–10:38 UT around the day-night terminator and near the wake bound- near the South Pole (SP). The increase in energy (factor 6) ary because of its large FOV (Figure 1). Magnetic field is over the SW is consistent with previous observations by measured by MAP-LMAG (Lunar MAGnetometer) with a SELENE [Saito et al., 2008a]. Between 10:20–10:30 UT, time resolution of 32 Hz [Shimizu et al., 2008]. Type-I entry of the SW protons was again observed, which seems to be unrelated to the protons found in the deepest wake. 3. Observation [8] When the protons were observed in the deep wake, [7] On 24 September 2008 the Moon was located at (X, Y, the magnetic field was dominated by the BY component Z)=(28,51, 1) RE in the GSE coordinate system, (Figure 2c). The averaged magnetic field between 10:08–

Figure 2. Time series data showing entry of scattered SW protons into the deepest lunar wake in the 1 revolution between 09:10 and 11:10 UT on 24 September 2008. (a and b) Energy-time spectrogram (differential energy flux) of protons from IMA and electrons from ESA-S1, (c) magnetic field, and (d and e) spacecraft location in the SSE coordinate system and solar zenith angle are shown. Color bars indicate intervals of sunlit/shadowed regions, Type-II entry, proton scattering on the dayside, and SW proton detection (skyblue) and Type-I entry (dotted skyblue) near the Poles.

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access into the deepest wake was accompanied by an intrusion of the electrons (Figure 2b). The energy of the electrons in the deep wake was 0.2–0.3 keV which is higher than that of the original SW electrons, and the electron distribution function in the deep wake showed counter-streaming beams along the magnetic field. The physical meaning of the electron signature will be discussed later. Such enhancement of counter-streaming electron beams in the wake were observed during similar SW proton entry events. [10] Noting the fact that the protons were continuously detected from the deepest wake to the dayside region, we propose that the protons scattered at the lunar dayside surface are picked up by the SW and come into the deepest wake. (Hereafter, we call this mechanism ‘Type-II entry’.)

4. Model Calculations

[11] To examine the origin of the protons coming into the deepest wake, we perform following particle trajectory calculations. Because the observed event occurred under dominant negative BY condition, we assume the simple IMF configuration with only BY component and the SW speed similar to the observation; 8 < BSW ¼ ðÞ0; 5:6; 0 nT : VSW ¼ðÞ300; 0; 0 km=siðÞ:e: 0:47 keV :

[12] Because the IMF observed in the lunar wake was slightly stronger than the original IMF, we assume the magnetic field in the shadowed region to be 1.2 times stronger than the ambient IMF in order to be consistent Figure 3. Trajectories of scattered protons, and E-t plots with the observations and the previous statistical result along virtual spacecraft orbit. Proton motions in the noon- [Halekas et al., 2005]. Inward electric fields around the midnight meridian plane are focused on. Each plot shows wake boundary are ignored because the energy of protons of trajectory of protons scattered at the sub-solar point (Lat. 0°, our interest is much larger than the wake potential. Lon. 0°) (a) without energy loss at the impact and (b) with [13] We first examine motions of scattered protons in energy loss (coefficient of reflection (COR) 0.8), the noon-midnight meridian plane. Protons scattered at the scattered at (c) (Lat. 20°N, Lon. 0°) and (d) (Lat. 30°S, sub-solar point without energy loss are picked-up by the Lon. 0°). (e and f) Protons scattered at (Lat. 0°, Lon. 30°E) SW, pass over the SP, and finally reach the anti-subsolar are considered, and those which access the deepest wake region (Figure 3a). However, all of them have downward and have upward velocity are shown. (g) E-t scatter plot velocity in the deepest wake and impact against the night- along a virtual spacecraft orbit at 100 km height in the side surface, and thus they cannot be detected by IMA noon-midnight meridian plane is presented. which looks down on the Moon. Because energy loss accompanied by the scattering at the dayside surface does not change proton trajectories so much (Figure 3b), we 10:30 UT was (0.3, 6.7, 1.4) nT, which means that hereafter consider cases without loss of kinetic energy. both ends of the magnetic field at the SELENE locations Protons scattered in the northern hemisphere do not reach would be connected to the SW. The magnetic field in the the nightside region (Figure 3c), and those scattered in the wake was slightly stronger than the time-propagated IMF southern hemisphere far from the equatorial region come observed by Wind. This feature is similar to a previous into the southern wake but do not access the deepest wake observation in the near-Moon wake [Halekas et al., 2005], (Figure 3d). and will be considered in the following model calculation. [14] Next we investigate motions of protons scattered at Besides this event, similar proton entry into the deepest the location off the noon-midnight meridian plane. We find wake was at times observed under the non-radially directed that some of protons scattered at (Lat. 0°, Lon. 30°E) can IMF condition (data not shown). reach anti-subsolar region and have upward velocity there [9] We stress that the protons in the deepest wake were (Figures 3e and 3f). Such obliquely-going protons have detected by the IMA instrument that faces upon the lunar smaller velocity in the direction perpendicular to the surface. The proton distribution function obtained by IMA magnetic field than protons without field-aligned velocity, shows that a major part of the protons in the deepest wake having smaller Larmor radius that fits into the effective went upward (data not shown). In addition, the SW proton lunar radius along the trajectories. Part of these protons

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the Moon can access the deepest lunar wake. In other events of Type-II entry, the suitable combination of the IMF direction, its strength, and the SW speed results in detection of proton access to the deepest wake. [17] We propose that Type-II entry forms the proton- governed region (PGR) in one hemisphere of the near-Moon wake, giving rise to a strong asymmetry of the near-Moon wake environment (Figure 4). In the case with dominant negative BY, the protons scattered on the dayside come into the southern hemisphere, while they cannot access the northern hemisphere. To maintain quasi-neutrality, the PGR yields outward electric fields to absorb the ambient SW electrons, which are detected as the counter-streaming distribution. This idea is supported by the fact that both ends of the wake magnetic field at the SELENE location are thought to be connected to the ambient SW. Previous models predicted governance of the high-energy electron Figure 4. A new model of the near-Moon wake environ- on the lunar wake environment and negative charging on ment under the dominant IMF BY condition, showing the lunar nightside surface [Halekas et al., 2002, 2005; asymmetry due to formation of the proton-governed region Stubbs et al., 2006], while our results show that SW protons (PGR). (a) The southern hemisphere of the near-Moon wake also play an important role in the electromagnetic environ- is dominated by the protons due to Type-II entry, while the ment there. The SW proton access deep into the near-Moon scattered protons cannot access the northern hemisphere that wake would significantly change the electric field configu- the high-energy component of the SW electrons would ration and motions of charged particles (including charged access. (b) The resultant outward electric field generated dust particles) in the lunar nightside region. around the PGR absorbs the ambient SW electrons into the wake along the magnetic field. [18] Acknowledgments. The authors thank the principal investigators of Wind SWE instruments for providing the solar wind data via CDAWeb. The authors wish to express their sincere thanks to all the team members of MAP-PACE and MAP-LMAG for their great support in processing and reach the deepest wake and have upward velocity there, analyzing the MAP data. The authors also wish to express their grateful because they turn upward just near the nightside surface. thanks to all the system members of the SELENE project. SELENE-MAP- The calculation also shows that the source areas of the PACE sensors were manufactured by Mitaka Kohki Co. Ltd., Meisei Elec. protons that access the deepest wake are the dayside Co., Hamamatsu Photonics K.K., and Kyocera Co. equatorial regions off the subsolar point. References [15] Finally, we try to reproduce an Energy-time (E-t) scatter plot along the virtual spacecraft orbit at 100 km Bosqued, J. M., et al. (1996), Moon-solar wind interactions: First results from the WIND/3DP Experiment, Geophys. Res. Lett., 23, 1259–1262. height in the noon-midnight meridian plane. For simplicity, Halekas, J. S., D. L. Mitchell, R. P. Lin, L. L. Hood, M. H. Acun˜a, and A. B. we assume scattering location at the grid points every 5 Binder (2002), Evidence for negative charging of the lunar surface in degrees in the region of 70°N–70°S, 70°E–70°W. shadow, Geophys. Res. Lett., 29(10), 1435, doi:10.1029/2001GL014428. Halekas, J. S., S. D. Bale, D. L. Mitchell, and R. P. Lin (2005), Electrons Concerning scattering angle, every 2 degrees in the both and magnetic fields in the lunar plasma wake, J. Geophys. Res., 110, longitudinal and latitudinal directions are assumed. Energy A07222, doi:10.1029/2004JA010991. loss by scattering at the dayside surface is not considered in Lyon, E. F., H. S. Bridge, and J. H. Binsack (1967), Explorer 35 plasma measurements in the vicinity of the Moon, J. Geophys. Res., 72, 6113 – 6117. the calculations. To simulate proton detection by IMA Nishino, M. N., et al. (2009), Pairwise energy gain-loss feature of solar which faces upon the Moon, we accumulate protons that wind protons in the near-Moon wake, Geophys. Res. Lett., 36, L12108, have upward velocity at every locations along the virtual doi:10.1029/2009GL039049. spacecraft orbit. The calculated E-t plot shows patterns 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 similar to the observations related to scattering on the wake from the WIND spacecraft on December 27, 1994, Geophys. Res. dayside and Type-II entry (Figure 3g); that is, protons in Lett., 23, 1255–1258. the deepest wake, high-energy protons around the SP, and Saito, Y., et al. (2008a), Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA), scattered SW protons on the dayside are reproduced. The Geophys. Res. Lett., 35, L24205, doi:10.1029/2008GL036077. similarity of the observation and calculation result shows Saito, Y., et al. (2008b), Low energy charged particle measurement by validity of our modeling of Type-II entry. MAP-PACE onboard SELENE, Earth Planet. Sci., 60, 375–386. Shimizu, H., F. Takahashi, N. Horii, A. Matsuoka, M. Matsushima, H. Shibuya, and H. Tsunakawa (2008), Ground calibration of the high- sensitivity SELENE lunar magnetometer LMAG, Earth Planets Space, 5. Summary and Discussion 60, 353–363. Stubbs, T. J., R. R. Vondrak, and W. M. Farrell (2006), A dynamic fountain [16] In the present study we reported unexpected SW model for lunar dust, Adv. Space Res., 37, 59–66, doi:10.1016/j.asr.2005. 04.048. proton coming into the deepest lunar wake. The key Tra´vnı´cˇek, P., P. Hellinger, D. Schriver, and S. D. Bale (2005), Structure of mechanisms of this phenomenon, which we call Type-II the lunar wake: Two-dimensional global hybrid simulations, Geophys. entry, are scattering of the SW protons at the lunar dayside Res. Lett., 32, L06102, doi:10.1029/2004GL022243. surface and their self-pickup by the SW [Saito et al., 2008a]. Part of scattered and self-picked-up protons with K. Asamura, M. Fujimoto, K. Maezawa, M. N. Nishino, Y. Saito, the specific Larmor radius suitable for the spatial scale of T. Tanaka, and S. Yokota, Institute of Space and Astronautical Science,

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Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, H. Shibuya, Department of Earth Sciences, Kumamoto University, 2-39-1 Kanagawa 229-8510, Japan. ([email protected]) Kurokami, Kumamoto 860-8555, Japan. M. Matsushima, F. Takahashi, T. Terasawa, and H. Tsunakawa, H. Shimizu, Earthquake Research Institute, University of Tokyo, 1-1-1 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. Ookayama, Meguro-ku, Tokyo 152-8551, Japan.

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