ULF/ELF Waves Detected by MAP/LMAG Magnetometer Onboard Kaguya around the Moon and in the Lunar Wake (Invited)

Monday, 1 September 2014: 5:30 PM
Regency Ballroom (Hyatt Regency)
Tomoko Nakagawa, Tohoku Institute of Technology, Sendai, Japan, Hideo Tsunakawa, Tokyo Inst. Tech., Tokyo, Japan, Futoshi Takahashi, Kyushu University, Fukuoka, Japan, Hidetoshi Shibuya, Kumamoto Univ, Kumamoto, Japan, Hisayoshi Shimizu, Univ Tokyo, Tokyo, Japan, Masaki Matsushima, Tokyo Tech, Tokyo, Japan and Yoshifumi Saito, Inst Space & Astronautical Sci, Kanagawa, Japan
Abstract:
The moon stands in the solar wind flow as an insulating obstacle. Absorption of the most of the solar wind particles by the lunar surface leads to the formation of the lunar wake, a plasma cavity in the solar wind left on the anti-solar side of the moon. A few percentage of the solar wind particles were found to be reflected by the lunar surface or the lunar crustal field[1], generating magnetic fluctuations in the ultra low frequency (ULF) range and in the extremely low frequency (ELF) range.

The waves repeatedly observed on the dayside of the moon were the ULF waves at 0.01 Hz [2] and the ELF waves of 0.03-10 Hz [3]. Predominance of the two frequency bands is analogous to the low-frequency waves in the upstream of the Earth's bow shock where the solar wind protons are reflected. The two frequency bands detected by Kaguya were also generated by the solar wind protons reflected by the moon. The monochromatic, circularly polarized low frequency waves of 0.01 Hz were generated through the cyclotron resonance of the magnetohydrodynamic waves with the solar wind protons reflected by the moon. The non-monochromatic fluctuations in the range from 0.03 to 10 Hz were whistler waves, and the generator is supposed to be the reflected particles, too, because the detection was concentrated above the magnetic anomaly.

Although the nightside of the moon was essentially quiet because of the absence of access of the solar wind particles, magnetic fluctuations in ELF range of 0.1-10 Hz were occasionally observed in association with the "type-II entry" solar wind protons which were once reflected by the dayside surface and entered the central wake region due to their large Larmour radius [4]. The magnetic fluctuations were detected on the magnetic field lines along which the solar wind electrons were injected into the wake, so it is expected that some cross-field current driven instability such as the lower-hybrid two-stream instability is responsible for the generation of the waves.

[1] Y. Saito, et al., Geophys. Res. Lett., 35, L24205, doi:10.1029/2008GL036077, 2008.

[2] T. Nakagawa, et al., J. Geophys. Res., 117, A04101, doi:10.1029/2011JA017249, 2012.

[3] T. Nakagawa, et al., Earth Planets Space, 63(1), pp. 37-46, doi:10.5047/eps.2010.01.005, 2011.

[4] M. N. Nishino, et al., Geophys. Res. Lett., 36, L16103, doi:10.1029/2009GL039444, 2009.

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