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-Moon wake using SELENE (KAGUYA) data. It has beenknown that SW protons flowing around the Moon accessthe central region of the distant lunar wake, while theirintrusion deep into the near-Moon wake has never beenexpected. We show that SW protons sneak into the deepestlunar wake (anti-subsolar region at �100 km altitude), andthat the entry yields strong asymmetry of the near-Moonwake environment. Particle trajectory calculationsdemonstrate that these SW protons are once scattered atthe lunar dayside surface, picked-up by the SW motionalelectric field, and finally sneak into the deepest wake. Ourresults mean that the SW protons scattered at the lunardayside surface and coming into the night side region arecrucial for plasma environment in the wake, suggestingabsorption of ambient SW electrons into the wake tomaintain quasi-neutrality. Citation: Nishino, M. N., et al.(2009), Solar-wind proton access deep into the near-Moon wake,

Geophys. Res. Lett., 36, L16103, doi:10.1029/2009GL039444.

1. Introduction

[2] Studying interaction between the solar wind (SW)and the Moon is important for understanding the lunarplasma environment. The near-Moon space environment ischaracterized by a tenuous tail-like region formed behindthe Moon along the SW flow, which is called the lunar wake[Lyon et al., 1967]. Concerning the near-Moon wakeenvironment, it has been believed that high-energy compo-nent of the SW electrons can approach the lunar nightsidesurface while the SW protons are unlikely to approach thelunar nightside region, because the thermal speed of SWprotons is much lower than the SW bulk speed. Accordingto previous models, the electron-rich status of the lunarwake yields bipolar (inward) electric field at the wakeboundary where SW protons gradually intrude toward thewake center along the interplanetary magnetic field (IMF) inthe distant wake [Ogilvie et al., 1996; Bosqued et al., 1996;Trávnı́èek et al., 2005].[3] Although signatures of electrons and magnetic fields

in the near-Moon wake have been fairly well understood[Halekas et al., 2005], proton behaviors in the near-Moon

wake were not known because there were no observationdata. Recently, a Japanese lunar orbiter SELENE(KAGUYA) performed comprehensive measurements ofthe plasma and electromagnetic environment around theMoon; in particular, entry of SW protons into the near-Moon wake was found [Nishino et al., 2009]. The SWprotons are accelerated by the bipolar electric field aroundthe wake boundary and come into the near-Moon wake bytheir Larmor motion in the direction perpendicular to theIMF. This entry mechanism, which we call ‘Type-I entry’,lets the SW protons come fairly deep into the wake (solarzenith angle (SZA) 150�) at 100 km height.[4] Another outstanding feature of the lunar plasma

environment found by SELENE observations is scattering/reflection of the SW protons at the lunar dayside surface(hereafter, we simply refer it as scattering). About 1 percentof the SW protons that impact against the lunar daysidesurface are scattered and then picked-up by the SW toobtain at most 9 times the original kinetic energy [Saito etal., 2008a]. This energization, which is called ‘self-pick-up’process, occurs on the dayside, and the destination of theself-picked-up protons has never been known.[5] The phenomenon that we deal with in this paper

is unexpected detection of the SW protons in the deepest(anti-subsolar, low-altitude) region of the near-Moon wake.Because Type-I entry cannot let the SW protons come intothe deepest wake, another entry mechanism must be at workfor this phenomenon. In the present study, we show that theSW protons scattered at the lunar dayside surface access thedeepest wake, which should be categorized as ‘Type-IIentry’.

2. Instrumentation

[6] We use data obtained by a Japanese spacecraftSELENE (KAGUYA) which is orbiting the Moon in apolar orbit at �100 km altitude with 2-hour period. TheMAP (MAgnetic field and Plasma experiment) instrumentonboard SELENE performs a comprehensive diagnosis ofelectromagnetic and plasma environment in the near-Moonspace by means of in-situ measurements of electrons, ions,and magnetic field. MAP-PACE (Plasma energy Angle andComposition Experiment)-IMA faces the lunar surface tomeasure up-going positive ions whose energy per charge isbetween 7 eV/q and 29 keV/q with a half-sphere fieldof view (FOV) [Saito et al., 2008b]. MAP-PACE ESA(Electron Spectrum Analyzer)-S1 with a similar FOV asIMA is for measurement of electrons with energy between5 eV and 10 keV. The time resolutions of IMA and ESA-S1data in the present study are 32 sec and 16 sec, respectively.Although IMA is originally designed for detecting tenuousions from the lunar surface, it also measures SW ions

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1Institute of Space and Astronautical Science, Japan AerospaceExploration Agency, Sagamihara, Japan.

2Department of Earth and Planetary Sciences, Tokyo Institute ofTechnology, Tokyo, Japan.

3Department of Earth Sciences, Kumamoto University, Kumamoto,Japan.

4Earthquake Research Institute, University of Tokyo, Tokyo, Japan.

Copyright 2009 by the American Geophysical Union.0094-8276/09/2009GL039444$05.00

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http://dx.doi.org/10.1029/2009GL039444

around the day-night terminator and near the wake bound-ary because of its large FOV (Figure 1). Magnetic field ismeasured by MAP-LMAG (Lunar MAGnetometer) with atime resolution of 32 Hz [Shimizu et al., 2008].

3. Observation

[7] On 24 September 2008 the Moon was located at (X, Y,Z) = (28, �51, 1) RE in the GSE coordinate system,

interacting with the SW flow upstream of the Earth’s bowshock. The SW speed observed by the Wind spacecraft was�300 km/s (�0.47 keV for protons), and the IMF whosestrength was about 5 nT was dominated by the negative BYcomponent. On the day the SELENE spacecraft flew nearthe noon-midnight meridian plane, orbiting from south tonorth on the dayside and going through the tenuous wake onthe nightside (Figure 1). We first mention proton signaturesobserved by SELENE between 09:10–11:10 UT. Before09:38 UT the protons scattered at the lunar dayside surface[Saito et al., 2008a] were detected by IMA (Figure 2a). Thespacecraft passed above the North Pole (NP) at 09:37 UTand crossed the boundary between sunlit and shadowedregions at 09:44 UT. Between 09:36–09:51 UT SELENEobserved SW proton entry near the NP, which is categorizedinto ‘Type-I entry’ [Nishino et al., 2009] by which SWprotons come deep into nightside (deepest at SZA 150�).After SELENE passed through the almost vacuum region inthe northern hemisphere, it began to detect protons around10:08 UT in the deepest wake (SZA 168�) that SW protonsare not anticipated to access. The proton flux in the deepwake was the largest around 10:12 UT, and its energy rangedbroadly between 0.1–1 keV. Between 10:20–10:40 UT theproton energy was higher than the original SW energy, andtook a peak energy of �3 keV around 10:34–10:38 UTnear the South Pole (SP). The increase in energy (factor �6)over the SW is consistent with previous observations bySELENE [Saito et al., 2008a]. Between 10:20–10:30 UT,Type-I entry of the SW protons was again observed, whichseems to be unrelated to the protons found in the deepestwake.[8] When the protons were observed in the deep wake,

the magnetic field was dominated by the BY component(Figure 2c). The averaged magnetic field between 10:08–

Figure 1. A schematic of the SELENE orbit and thefield of view (FOV) of IMA and ESA-S1. SELENE is athree-axis stabilized spacecraft, and IMA and ESA-S1always face upon the lunar surface with an FOV of 2p str.The SW protons come directly into the FOV of IMA nearthe day-night terminator and the wake boundary.

Figure 2. Time series data showing entry of scattered SW protons into the deepest lunar wake in the 1 revolution between09:10 and 11:10 UT on 24 September 2008. (a and b) Energy-time spectrogram (differential energy flux) of protons fromIMA and electrons from ESA-S1, (c) magnetic field, and (d and e) spacecraft location in the SSE coordinate system andsolar zenith angle are shown. Color bars indicate intervals of sunlit/shadowed regions, Type-II entry, proton scattering onthe dayside, and SW proton detection (skyblue) and Type-I entry (dotted skyblue) near the Poles.

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10:30 UT was (�0.3, �6.7, �1.4) nT, which means thatboth ends of the magnetic field at the SELENE locationswould be connected to the SW. The magnetic field in thewake was slightly stronger than the time-propagated IMFobserved by Wind. This feature is similar to a previousobservation in the near-Moon wake [Halekas et al., 2005],and will be considered in the following model calculation.Besides this event, similar proton entry into the deepestwake was at times observed under the non-radially directedIMF condition (data not shown).[9] We stress that the protons in the deepest wake were

detected by the IMA instrument that faces upon the lunarsurface. The proton distribution function obtained by IMAshows that a major part of the protons in the deepest wakewent upward (data not shown). In addition, the SW proton

access into the deepest wake was accompanied by anintrusion of the electrons (Figure 2b). The energy of theelectrons in the deep wake was �0.2–0.3 keV which ishigher than that of the original SW electrons, and theelectron distribution function in the deep wake showedcounter-streaming beams along the magnetic field. Thephysical meaning of the electron signature will be discussedlater. Such enhancement of counter-streaming electronbeams in the wake were observed during similar SW protonentry events.[10] Noting the fact that the protons were continuously

detected from the deepest wake to the dayside region, wepropose that the protons scattered at the lunar daysidesurface are picked up by the SW and come into the deepestwake. (Hereafter, we call this mechanism ‘Type-II entry’.)

4. Model Calculations

[11] To examine the origin of the protons coming into thedeepest wake, we perform following particle trajectorycalculations. Because the observed event occurred underdominant negative BY condition, we assume the simple IMFconfiguration with only BY component and the SW speedsimilar to the observation;

BSW ¼ 0;�5:6; 0ð Þ nT

VSW ¼ �300; 0; 0ð Þ km=s i:e: 0:47 keVð Þ:

8<:

[12] Because the IMF observed in the lunar wake wasslightly stronger than the original IMF, we assume themagnetic field in the shadowed region to be 1.2 timesstronger than the ambient IMF in order to be consistentwith the observations and the previous statistical result[Halekas et al., 2005]. Inward electric fields around thewake boundary are ignored because the energy of protons ofour interest is much larger than the wake potential.[13] We first examine motions of scattered protons in

the noon-midnight meridian plane. Protons scattered at thesub-solar point without energy loss are picked-up by theSW, pass over the SP, and finally reach the anti-subsolarregion (Figure 3a). However, all of them have downwardvelocity in the deepest wake and impact against the night-side surface, and thus they cannot be detected by IMAwhich looks down on the Moon. Because energy lossaccompanied by the scattering at the dayside surface doesnot change proton trajectories so much (Figure 3b), wehereafter consider cases without loss of kinetic energy.Protons scattered in the northern hemisphere do not reachthe nightside region (Figure 3c), and those scattered in thesouthern hemisphere far from the equatorial region comeinto the southern wake but do not access the deepest wake(Figure 3d).[14] Next we investigate motions of protons scattered at

the location off the noon-midnight meridian plane. We findthat some of protons scattered at (Lat. 0�, Lon. 30�E) canreach anti-subsolar region and have upward velocity there(Figures 3e and 3f). Such obliquely-going protons havesmaller velocity in the direction perpendicular to themagnetic field than protons without field-aligned velocity,having smaller Larmor radius that fits into the effectivelunar radius along the trajectories. Part of these protons

Figure 3. Trajectories of scattered protons, and E-t plotsalong virtual spacecraft orbit. Proton motions in the noon-midnight meridian plane are focused on. Each plot showstrajectory of protons scattered at the sub-solar point (Lat. 0�,Lon. 0�) (a) without energy loss at the impact and (b) withenergy loss (coefficient of reflection (COR) � 0.8),scattered at (c) (Lat. 20�N, Lon. 0�) and (d) (Lat. 30�S,Lon. 0�). (e and f) Protons scattered at (Lat. 0�, Lon. 30�E)are considered, and those which access the deepest wakeand have upward velocity are shown. (g) E-t scatter plotalong a virtual spacecraft orbit at 100 km height in thenoon-midnight meridian plane is presented.

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reach the deepest wake and have upward velocity there,because they turn upward just near the nightside surface.The calculation also shows that the source areas of theprotons that access the deepest wake are the daysideequatorial regions off the subsolar point.[15] Finally, we try to reproduce an Energy-time (E-t)

scatter plot along the virtual spacecraft orbit at 100 kmheight in the noon-midnight meridian plane. For simplicity,we assume scattering location at the grid points every 5degrees in the region of 70�N–70�S, 70�E–70�W.Concerning scattering angle, every 2 degrees in the bothlongitudinal and latitudinal directions are assumed. Energyloss by scattering at the dayside surface is not considered inthe calculations. To simulate proton detection by IMAwhich faces upon the Moon, we accumulate protons thathave upward velocity at every locations along the virtualspacecraft orbit. The calculated E-t plot shows patternssimilar to the observations related to scattering on thedayside and Type-II entry (Figure 3g); that is, protons inthe deepest wake, high-energy protons around the SP, andscattered SW protons on the dayside are reproduced. Thesimilarity of the observation and calculation result showsvalidity of our modeling of Type-II entry.

5. Summary and Discussion

[16] In the present study we reported unexpected SWproton coming into the deepest lunar wake. The keymechanisms of this phenomenon, which we call Type-IIentry, are scattering of the SW protons at the lunar daysidesurface and their self-pickup by the SW [Saito et al.,2008a]. Part of scattered and self-picked-up protons withthe specific Larmor radius suitable for the spatial scale of

the Moon can access the deepest lunar wake. In other eventsof Type-II entry, the suitable combination of the IMFdirection, its strength, and the SW speed results in detectionof 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-Moonwake, giving rise to a strong asymmetry of the near-Moonwake environment (Figure 4). In the case with dominantnegative BY, the protons scattered on the dayside come intothe southern hemisphere, while they cannot access thenorthern hemisphere. To maintain quasi-neutrality, thePGR yields outward electric fields to absorb the ambientSW electrons, which are detected as the counter-streamingdistribution. This idea is supported by the fact that both endsof the wake magnetic field at the SELENE location arethought to be connected to the ambient SW. Previousmodels predicted governance of the high-energy electronon the lunar wake environment and negative charging onthe lunar nightside surface [Halekas et al., 2002, 2005;Stubbs et al., 2006], while our results show that SW protonsalso play an important role in the electromagnetic environ-ment there. The SW proton access deep into the near-Moonwake would significantly change the electric field configu-ration and motions of charged particles (including chargeddust particles) in the lunar nightside region.

[18] Acknowledgments. The authors thank the principal investigatorsof Wind SWE instruments for providing the solar wind data via CDAWeb.The authors wish to express their sincere thanks to all the team members ofMAP-PACE and MAP-LMAG for their great support in processing andanalyzing the MAP data. The authors also wish to express their gratefulthanks to all the system members of the SELENE project. SELENE-MAP-PACE sensors were manufactured by Mitaka Kohki Co. Ltd., Meisei Elec.Co., Hamamatsu Photonics K.K., and Kyocera Co.

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Saito, Y., et al. (2008a), Solar wind proton reflection at the lunar surface: Lowenergy ion measurement by MAP-PACE onboard SELENE (KAGUYA),Geophys. Res. Lett., 35, L24205, doi:10.1029/2008GL036077.

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Trávnı́ček, P., P. Hellinger, D. Schriver, and S. D. Bale (2005), Structure ofthe lunar wake: Two-dimensional global hybrid simulations, Geophys.Res. Lett., 32, L06102, doi:10.1029/2004GL022243.

�����������������������K. Asamura, M. Fujimoto, K. Maezawa, M. N. Nishino, Y. Saito,

T. Tanaka, and S. Yokota, Institute of Space and Astronautical Science,

Figure 4. A new model of the near-Moon wake environ-ment under the dominant IMF BY condition, showingasymmetry due to formation of the proton-governed region(PGR). (a) The southern hemisphere of the near-Moon wakeis dominated by the protons due to Type-II entry, while thescattered protons cannot access the northern hemisphere thatthe high-energy component of the SW electrons wouldaccess. (b) The resultant outward electric field generatedaround the PGR absorbs the ambient SW electrons into thewake along the magnetic field.

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Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara,Kanagawa 229-8510, Japan. (nishino@stp.isas.jaxa.jp)M. Matsushima, F. Takahashi, T. Terasawa, and H. Tsunakawa,

Department of Earth and Planetary Sciences, Tokyo Institute of Technology,Ookayama, Meguro-ku, Tokyo 152-8551, Japan.

H. Shibuya, Department of Earth Sciences, Kumamoto University, 2-39-1Kurokami, Kumamoto 860-8555, Japan.H. Shimizu, Earthquake Research Institute, University of Tokyo, 1-1-1

Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.

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