[P-EM16] Space Plasma Physics: Theory and Simulation

[PEM16-07] Observations of electron nongyrotropy around the cyclotron resonance velocity in whistler-mode waves

*Naritoshi Kitamura¹, Takanobu Amano¹, Yoshiharu Omura², Scott A. Boardsen^3,4, Daniel J. Gershman³, Yoshizumi Miyoshi⁵, Masahiro Kitahara⁵, Yuto Katoh⁶, Hirotsugu Kojima², Satoko Nakamura⁵, Masafumi Shoji⁵, Yoshifumi Saito⁷, Shoichiro Yokota⁸, Barbara L. Giles³, William R. Paterson³, Craig J. Pollock⁹, Olivier Le Contel¹⁰, Christopher T. Russell¹¹, Robert J. Strangeway¹¹, Per-Arne Lindqvist¹², Robert E. Ergun¹³, Roy B. Torbert¹⁴, James L. Burch¹⁵ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Research Institute for Sustainable Humanosphere, Kyoto University, 3.NASA Goddard Space Flight Center, 4.Partnership for Heliophysics and Space Environment Research, University of Maryland in Baltimore County, 5.Institute for Space-Earth Environmental Research, Nagoya 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.Royal Institute of Technology, 13.Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, 14.Department of Physics, University of New Hampshire, 15.Southwest Research Institute)

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

Electromagnetic whistler-mode waves play important roles in efficient pitch-angle scattering and acceleration of electrons in collisionless space plasma such as solar wind, shock waves, and planetary magnetospheres. The nonlinear wave-particle interaction theory for coherent large amplitude waves predicts that electrons around cyclotron resonance velocity exhibit nongyrotropy due to the phase trapping motion and the nongyrotropic electrons exchange energy and momentum with the waves in the presence of an appropriate magnitude of spatial gradient of magnetic field intensity and/or temporal variation of the wave frequency. In this presentation, we show observational results of nongyrotropic electrons around the cyclotron resonance velocity using the data obtained by the Magnetospheric Multiscale spacecraft during a whistler-mode wave event around the magnetosheath-side separatrix of the dayside magnetopause reconnection. On the basis of measurements of electromagnetic field and electrons, the location of a depression of electron flux in phase space relative to the magnetic field of the whistler-mode wave around the cyclotron resonance velocity agrees well with the prediction by the nonlinear theory, when the magnitude of spatial gradient of magnetic field intensity, which was derived by multipoint measurements, satisfied the condition for occurrence of the phase trapping. The Electron Drift Instrument continuously detected the modulation of electron flux at the cyclotron resonance velocity. These results provide direct evidence of locally ongoing nonlinear wave-particle interaction between the electrons and whistler-mode waves, and demonstrate occurrence of nonlinear wave growth around the reconnection.