[P-EM15] Space Plasma Physics: Theory and Simulation

[PEM15-01] Observation of non-gyrotropy of electrons caused by wave-particle interaction with intense whistler mode waves in mirror mode structures in the magnetosheath

*Naritoshi Kitamura¹, Masahiro Kitahara², Scott A Boardsen^3,4, Takanobu Amano¹, Daniel J Gershman³, Yoshiharu Omura⁵, Satoko Nakamura², Masafumi Shoji², Yuto Katoh⁶, Hirotsugu Kojima⁵, Yoshizumi Miyoshi², Yoshifumi Saito⁷, Masafumi Hirahara², Shoichiro Yokota⁸, Barbara L Giles³, William R Paterson³, Craig J Pollock⁹, Olivier Le Contel¹⁰, Christopher T Russell¹¹, Narges Ahmadi¹², Per-Arne Lindqvist¹³, Robert E Ergun¹², James L Burch¹⁴ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Institute for Space-Earth Environmental Research, Nagoya University, 3.NASA Goddard Space Flight Center, 4.Goddard Planetary Heliophysics Institute, University of Maryland, 5.Research Institute for Sustainable Humanosphere, Kyoto University, 6.Department of Geophysics, Graduate School of Science, Tohoku University, 7.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 8.Department of Earth and Space Science, Graduate School of Science, Osaka University, 9.Denali Scientific, 10.Laboratoire de Physique des Plasmas, CNRS/Ecole Polytechnique/Sorbonne Université/Université Paris-Sud/Observatoire de Paris, 11.Department of Earth, Planetary, and Space Science, University of California, Los Angeles, 12.Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, 13.Royal Institute of Technology, 14.Southwest Research Institute)

Keywords:Whistler mode wave, Wave-particle interaction, MMS spacecraft

In the magnetosheath region, magnetic mirror structures often appear due to mirror instability. In many magnetic mirror traps in such mirror structures, whistler mode waves called Lion roars are emitted from the vicinity of the minima of the magnetic field intensity along field lines. The waves propagate in the direction of parallel and antiparallel to magnetic field lines. This situation is similar to the magnetic equator in the inner magnetosphere. Here we report the non-gyrotropy of electrons caused by the non-resonant wave-particle interaction around the source region of the whistler mode waves using the data obtained by the Magnetospheric Multiscale (MMS) spacecraft. The burst mode electromagnetic field data obtained by the search-coil magnetometer and the electric field double probes with the sampling rate of 8192 Hz are used for the analysis of whistler mode waves. The background magnetic field is derived from fast survey mode (16 samples/s) magnetometer data. The electron data (6 eV-30 keV) used in this study is from the Fast Plasma Investigation Dual Electron Spectrometer (FPI-DES). However, the temporal resolution of the burst mode FPI-DES data, which is 30 ms, is not sufficient to resolve fluctuations associated with the whistler mode waves with a frequency of about 100 Hz (= period of about 10 ms). Thus, the data are disassembled to the measurement time of each energy and deflection instrument step. The integration time of each step is 196 micro-seconds, which is only about one fiftieth of the period of waves with a frequency of 100 Hz. Using this disassembled dataset, we investigate the existence of non-gyrotropy and the phase relationship with the electromagnetic fields of the whistler mode wave for various energy and pitch angle bins. In some of energy bins, we discovered non-gyrotropy, which was likely caused by non-resonant interaction, at pitch angles much closer to 90° than the cyclotron resonance condition for electrons moving in the opposite direction to the wave propagation direction. Since the magnetic field amplitude of the whistler mode wave reaches several percent of the background magnetic field intensity, even electrons far from the cyclotron resonance velocity oscillate with a non-negligible extent, mainly in the pitch angle direction in the phase space during the passage of one cycle of the wave seen by the electron. Because the electrons have a butterfly distribution, which is thought to be caused by the growth of the mirror traps, the gradient of the phase space density along the diffusion curve along which the electrons oscillate increases at pitch angles closer to 90° than the peak of the butterfly distribution. This relatively large gradient of the phase space density makes it possible to cause non-gyrotropy observable at the pitch angles.