ELF magnetic fluctuations detected by Kaguya in deepest lunar wake associated with type-II protons

Tomoko Nakagawa,Hidetoshi Shibuya,Masaki Matsushima,Hideo Tsunakawa,Tatsuya Nakashima,Takuya Wada,Yoshifumi Saito,Futoshi Takahashi,Hisayoshi Shimizu

doi:10.1186/s40623-015-0196-0

Abstract

Magnetic fluctuations in the extremely low-frequency (ELF) range from 0.1 to 10 Hz were found by the Lunar Magnetometer (LMAG) of the magnetic field and plasma experiment (MAP) on board the spacecraft Kaguya in the deepest wake behind the moon, where the magnetic field is usually quiet. The fluctuations were compressional and non-monochromatic, showing no preferred polarization. They were often accompanied by “type-II entry” solar wind protons that were reflected by the dayside lunar surface or crustal magnetic field, gyrated around the solar wind magnetic field, then entered the deepest wake. The ELF waves persisted for 30 s to several minutes. The duration was often shorter than that of the type-II protons. Most of the waves were detected on the magnetic field lines disconnected from the lunar surface, along which the solar wind electrons were injected into the wake. Since a large cross-field velocity difference is expected between the type-II protons and the solar wind electrons injected along the magnetic field, some cross-field current-driven instability such as the lower hybrid two-stream instability is expected to be responsible for the generation of the waves.

Highlights

Recent observations made by the spacecraft Kaguya revealed that not all solar wind particles are absorbed, but 0.1% to 1% of them are reflected by the lunar surface, the crustal magnetic field, or the anti-moonward electric field over the magnetic anomaly (Saito et al 2008b, 2010, 2012)
Since the main source of these waves is the energy of the solar wind particles reflected at the dayside surface or over crustal magnetic fields of the moon, the wave activity is almost always observed on the dayside of the moon when it is exposed to the solar wind, and is depressed on the nightside
Magnetic fluctuations in the frequency range from 0.1 to 10 Hz were found in the center of the near-moon wake

Summary

Introduction

Recent observations made by the spacecraft Kaguya revealed that not all solar wind particles are absorbed, but 0.1% to 1% of them are reflected by the lunar surface, the crustal magnetic field, or the anti-moonward electric field over the magnetic anomaly (Saito et al 2008b, 2010, 2012). Since the main source of these waves is the energy of the solar wind particles reflected at the dayside surface or over crustal magnetic fields of the moon, the wave activity is almost always observed on the dayside of the moon when it is exposed to the solar wind, and is depressed on the nightside. The center of the wake is magnetically quiet because of the absence of solar wind particles.

Objectives

Findings

Discussion

Conclusion