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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L16103, doi:10.1029/2009GL039444, 2009
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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 nearMoon wake using SELENE (KAGUYA) data. It has been
known that SW protons flowing around the Moon access
the central region of the distant lunar wake, while their
intrusion deep into the near-Moon wake has never been
expected. We show that SW protons sneak into the deepest
lunar wake (anti-subsolar region at 100 km altitude), and
that the entry yields strong asymmetry of the near-Moon
wake environment. Particle trajectory calculations
demonstrate that these SW protons are once scattered at
the lunar dayside surface, picked-up by the SW motional
electric field, and finally sneak into the deepest wake. Our
results mean that the SW protons scattered at the lunar
dayside surface and coming into the night side region are
crucial for plasma environment in the wake, suggesting
absorption of ambient SW electrons into the wake to
maintain 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 lunar
plasma environment. The near-Moon space environment is
characterized by a tenuous tail-like region formed behind
the Moon along the SW flow, which is called the lunar wake
[Lyon et al., 1967]. Concerning the near-Moon wake
environment, it has been believed that high-energy component of the SW electrons can approach the lunar nightside
surface while the SW protons are unlikely to approach the
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
wake yields bipolar (inward) electric field at the wake
boundary where SW protons gradually intrude toward the
wake center along the interplanetary magnetic field (IMF) in
the 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
1
Institute of Space and Astronautical Science, Japan Aerospace
Exploration Agency, Sagamihara, Japan.
2
Department of Earth and Planetary Sciences, Tokyo Institute of
Technology, Tokyo, Japan.
3
Department of Earth Sciences, Kumamoto University, Kumamoto,
Japan.
4
Earthquake Research Institute, University of Tokyo, Tokyo, Japan.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL039444$05.00
wake were not known because there were no observation
data. Recently, a Japanese lunar orbiter SELENE
(KAGUYA) performed comprehensive measurements of
the plasma and electromagnetic environment around the
Moon; in particular, entry of SW protons into the nearMoon wake was found [Nishino et al., 2009]. The SW
protons are accelerated by the bipolar electric field around
the wake boundary and come into the near-Moon wake by
their Larmor motion in the direction perpendicular to the
IMF. This entry mechanism, which we call ‘Type-I entry’,
lets the SW protons come fairly deep into the wake (solar
zenith 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 percent
of the SW protons that impact against the lunar dayside
surface are scattered and then picked-up by the SW to
obtain at most 9 times the original kinetic energy [Saito et
al., 2008a]. This energization, which is called ‘self-pick-up’
process, occurs on the dayside, and the destination of the
self-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 into
the deepest wake, another entry mechanism must be at work
for this phenomenon. In the present study, we show that the
SW protons scattered at the lunar dayside surface access the
deepest wake, which should be categorized as ‘Type-II
entry’.
2. Instrumentation
[6] We use data obtained by a Japanese spacecraft
SELENE (KAGUYA) which is orbiting the Moon in a
polar orbit at 100 km altitude with 2-hour period. The
MAP (MAgnetic field and Plasma experiment) instrument
onboard SELENE performs a comprehensive diagnosis of
electromagnetic and plasma environment in the near-Moon
space by means of in-situ measurements of electrons, ions,
and magnetic field. MAP-PACE (Plasma energy Angle and
Composition Experiment)-IMA faces the lunar surface to
measure up-going positive ions whose energy per charge is
between 7 eV/q and 29 keV/q with a half-sphere field
of view (FOV) [Saito et al., 2008b]. MAP-PACE ESA
(Electron Spectrum Analyzer)-S1 with a similar FOV as
IMA is for measurement of electrons with energy between
5 eV and 10 keV. The time resolutions of IMA and ESA-S1
data in the present study are 32 sec and 16 sec, respectively.
Although IMA is originally designed for detecting tenuous
ions from the lunar surface, it also measures SW ions
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Figure 1. A schematic of the SELENE orbit and the
field of view (FOV) of IMA and ESA-S1. SELENE is a
three-axis stabilized spacecraft, and IMA and ESA-S1
always face upon the lunar surface with an FOV of 2p str.
The SW protons come directly into the FOV of IMA near
the day-night terminator and the wake boundary.
around the day-night terminator and near the wake boundary because of its large FOV (Figure 1). Magnetic field is
measured by MAP-LMAG (Lunar MAGnetometer) with a
time 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,
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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°).
After SELENE passed through the almost vacuum region in
the northern hemisphere, it began to detect protons around
10:08 UT in the deepest wake (SZA 168°) that SW protons
are not anticipated to access. The proton flux in the deep
wake was the largest around 10:12 UT, and its energy ranged
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
near the South Pole (SP). The increase in energy (factor 6)
over the SW is consistent with previous observations by
SELENE [Saito et al., 2008a]. Between 10:20 – 10:30 UT,
Type-I entry of the SW protons was again observed, which
seems to be unrelated to the protons found in the deepest
wake.
[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 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 ð0; 5:6; 0Þ nT
:
Figure 3. Trajectories of scattered protons, and E-t plots
along virtual spacecraft orbit. Proton motions in the noonmidnight meridian plane are focused on. Each plot shows
trajectory of protons scattered at the sub-solar point (Lat. 0°,
Lon. 0°) (a) without energy loss at the impact and (b) with
energy 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 wake
and have upward velocity are shown. (g) E-t scatter plot
along a virtual spacecraft orbit at 100 km height in the
noon-midnight meridian plane is presented.
10:30 UT was (0.3, 6.7, 1.4) nT, which means that
both ends of the magnetic field at the SELENE locations
would be connected to the SW. The magnetic field in the
wake was slightly stronger than the time-propagated IMF
observed by Wind. This feature is similar to a previous
observation 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 deepest
wake was at times observed under the non-radially directed
IMF condition (data not shown).
[9] We stress that the protons in the deepest wake were
detected by the IMA instrument that faces upon the lunar
surface. The proton distribution function obtained by IMA
shows that a major part of the protons in the deepest wake
went upward (data not shown). In addition, the SW proton
VSW ¼ ð300; 0; 0Þ km=s ði:e: 0:47 keVÞ:
[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
with the observations and the previous statistical result
[Halekas et al., 2005]. Inward electric fields around the
wake boundary are ignored because the energy of protons of
our 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 the
sub-solar point without energy loss are picked-up by the
SW, pass over the SP, and finally reach the anti-subsolar
region (Figure 3a). However, all of them have downward
velocity in the deepest wake and impact against the nightside surface, and thus they cannot be detected by IMA
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
hereafter consider cases without loss of kinetic energy.
Protons scattered in the northern hemisphere do not reach
the nightside region (Figure 3c), and those scattered in the
southern hemisphere far from the equatorial region come
into 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 find
that some of protons scattered at (Lat. 0°, Lon. 30°E) can
reach anti-subsolar region and have upward velocity there
(Figures 3e and 3f). Such obliquely-going protons have
smaller velocity in the direction perpendicular to the
magnetic field than protons without field-aligned velocity,
having smaller Larmor radius that fits into the effective
lunar radius along the trajectories. Part of these protons
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Figure 4. A new model of the near-Moon wake environment under the dominant IMF BY condition, showing
asymmetry due to formation of the proton-governed region
(PGR). (a) The southern hemisphere of the near-Moon wake
is dominated by the protons due to Type-II entry, while the
scattered protons cannot access the northern hemisphere that
the high-energy component of the SW electrons would
access. (b) The resultant outward electric field generated
around the PGR absorbs the ambient SW electrons into the
wake along the magnetic field.
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 the
protons that access the deepest wake are the dayside
equatorial 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 km
height in the noon-midnight meridian plane. For simplicity,
we assume scattering location at the grid points every 5
degrees in the region of 70°N – 70°S, 70°E – 70°W.
Concerning scattering angle, every 2 degrees in the both
longitudinal and latitudinal directions are assumed. Energy
loss by scattering at the dayside surface is not considered in
the calculations. To simulate proton detection by IMA
which faces upon the Moon, we accumulate protons that
have upward velocity at every locations along the virtual
spacecraft orbit. The calculated E-t plot shows patterns
similar to the observations related to scattering on the
dayside and Type-II entry (Figure 3g); that is, protons in
the deepest wake, high-energy protons around the SP, and
scattered SW protons on the dayside are reproduced. The
similarity of the observation and calculation result shows
validity of our modeling of Type-II entry.
5. Summary and Discussion
[16] In the present study we reported unexpected SW
proton coming into the deepest lunar wake. The key
mechanisms of this phenomenon, which we call Type-II
entry, are scattering of the SW protons at the lunar dayside
surface and their self-pickup by the SW [Saito et al.,
2008a]. Part of scattered and self-picked-up protons with
the specific Larmor radius suitable for the spatial scale of
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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 protongoverned 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
on the lunar wake environment and negative charging on
the lunar nightside surface [Halekas et al., 2002, 2005;
Stubbs et al., 2006], while our results show that SW protons
also play an important role in the electromagnetic environment there. The SW proton access deep into the near-Moon
wake would significantly change the electric field configuration and motions of charged particles (including charged
dust particles) in the lunar nightside region.
[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
analyzing the MAP data. The authors also wish to express their grateful
thanks to all the system members of the SELENE project. SELENE-MAPPACE sensors were manufactured by Mitaka Kohki Co. Ltd., Meisei Elec.
Co., Hamamatsu Photonics K.K., and Kyocera Co.
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Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara,
Kanagawa 229-8510, Japan. ([email protected])
M. Matsushima, F. Takahashi, T. Terasawa, and H. Tsunakawa,
Department of Earth and Planetary Sciences, Tokyo Institute of Technology,
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H. Shibuya, Department of Earth Sciences, Kumamoto University, 2-39-1
Kurokami, 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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