Mini-Magnetospheres at the Moon in the Solar Wind and the Earth's Plasma Sheet

Friday, 19 December 2014
Yuki Harada1, Yoshifumi Futaana2, Stanislav V Barabash2, Martin Wieser2, Peter Wurz3, Anil Bhardwaj4, Kazushi Asamura5, Yoshifumi Saito6, Shoichiro Yokota5, Hideo Tsunakawa7 and Shinobu Machida8, (1)Space Sciences Laboratory, Berkeley, CA, United States, (2)IRF Swedish Institute of Space Physics Kiruna, Kiruna, Sweden, (3)University of Bern, Bern, Switzerland, (4)Vikram Sarabhai Space Center, Thiruvananthapuram, India, (5)ISAS Institute of Space and Astronautical Science, Kanagawa, Japan, (6)Inst Space & Astronautical Sci, Kanagawa, Japan, (7)Tokyo Institute of Technology, Tokyo, Japan, (8)Nagoya University, Nagoya, Japan
Lunar mini-magnetospheres are formed as a consequence of solar-wind interaction with remanent crustal magnetization on the Moon. A variety of plasma and field perturbations have been observed in a vicinity of the lunar magnetic anomalies, including electron energization, ion reflection/deflection, magnetic field enhancements, electrostatic and electromagnetic wave activities, and low-altitude ion deceleration and electron acceleration. Recent Chandrayaan-1 observations of the backscattered energetic neutral atoms (ENAs) from the Moon in the solar wind revealed upward ENA flux depletion (and thus depletion of the proton flux impinging on the lunar surface) in association with strongly magnetized regions. These ENA observations demonstrate that the lunar surface is shielded from the solar wind protons by the crustal magnetic fields. On the other hand, when the Moon was located in the Earth's plasma sheet, no significant depletion of the backscattered ENA flux was observed above the large and strong magnetic anomaly. It suggests less effective magnetic shielding of the surface from the plasma sheet protons than from the solar wind protons. We conduct test-particle simulations showing that protons with a broad velocity distribution are more likely to reach a strongly magnetized surface than those with a beam-like velocity distribution. The ENA observations together with the simulation results suggest that the lunar crustal magnetic fields are no longer capable of standing off the ambient plasma when the Moon is immersed in the hot magnetospheric plasma.