TY - JOUR
T1 - Kaguya observations of the lunar wake in the terrestrial foreshock
T2 - Surface potential change by bow-shock reflected ions
AU - Nishino, Masaki N.
AU - Harada, Yuki
AU - Saito, Yoshifumi
AU - Tsunakawa, Hideo
AU - Takahashi, Futoshi
AU - Yokota, Shoichiro
AU - Matsushima, Masaki
AU - Shibuya, Hidetoshi
AU - Shimizu, Hisayoshi
N1 - Funding Information:
The authors wish to express their grateful thanks to all the system members of the SELENE project. The Lunar Prospector data and the Wind data are provided via PDS at NASA. Prof. Kazuo Shiokawa and Prof. Masafumi Hirahara are acknowledged for fruitful discussion. This work was supported by Grant-in-Aid for Scientific Research (C) No. 26400477 from Japan Society for the Promotion of Science (JSPS).
Publisher Copyright:
© 2017 Elsevier Inc.
PY - 2017/9/1
Y1 - 2017/9/1
N2 - There forms a tenuous region called the wake behind the Moon in the solar wind, and plasma entry/refilling into the wake is a fundamental problem of the lunar plasma science. High-energy ions and electrons in the foreshock of the Earth's magnetosphere were detected at the lunar surface in the Apollo era, but their effects on the lunar night-side environment have never been studied. Here we show the first observation of bow-shock reflected protons by Kaguya (SELENE) spacecraft in orbit around the Moon, confirming that solar wind plasma reflected at the terrestrial bow shock can easily access the deepest lunar wake when the Moon stays in the foreshock (We name this mechanism ‘type-3 entry’). In a continuous type-3 event, low-energy electron beams from the lunar night-side surface are not obvious even though the spacecraft location is magnetically connected to the lunar surface. On the other hand, in an intermittent type-3 entry event, the kinetic energy of upward-going field-aligned electron beams decreases from ∼ 80 eV to ∼ 20 eV or electron beams disappear as the bow-shock reflected ions come accompanied by enhanced downward electrons. According to theoretical treatment based on electric current balance at the lunar surface including secondary electron emission by incident electron and ion impact, we deduce that incident ions would be accompanied by a few to several times higher flux of an incident electron flux, which well fits observed downward fluxes. We conclude that impact by the bow-shock reflected ions and electrons raises the electrostatic potential of the lunar night-side surface.
AB - There forms a tenuous region called the wake behind the Moon in the solar wind, and plasma entry/refilling into the wake is a fundamental problem of the lunar plasma science. High-energy ions and electrons in the foreshock of the Earth's magnetosphere were detected at the lunar surface in the Apollo era, but their effects on the lunar night-side environment have never been studied. Here we show the first observation of bow-shock reflected protons by Kaguya (SELENE) spacecraft in orbit around the Moon, confirming that solar wind plasma reflected at the terrestrial bow shock can easily access the deepest lunar wake when the Moon stays in the foreshock (We name this mechanism ‘type-3 entry’). In a continuous type-3 event, low-energy electron beams from the lunar night-side surface are not obvious even though the spacecraft location is magnetically connected to the lunar surface. On the other hand, in an intermittent type-3 entry event, the kinetic energy of upward-going field-aligned electron beams decreases from ∼ 80 eV to ∼ 20 eV or electron beams disappear as the bow-shock reflected ions come accompanied by enhanced downward electrons. According to theoretical treatment based on electric current balance at the lunar surface including secondary electron emission by incident electron and ion impact, we deduce that incident ions would be accompanied by a few to several times higher flux of an incident electron flux, which well fits observed downward fluxes. We conclude that impact by the bow-shock reflected ions and electrons raises the electrostatic potential of the lunar night-side surface.
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U2 - 10.1016/j.icarus.2017.04.005
DO - 10.1016/j.icarus.2017.04.005
M3 - Article
AN - SCOPUS:85017415504
VL - 293
SP - 45
EP - 51
JO - Icarus
JF - Icarus
SN - 0019-1035
ER -