7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Direct observations of energy transfer from resonant electrons to whistler-mode
waves in magnetosheath of Earth

Naritoshi Kitamura1, Takanobu Amano2, Yoshiharu Omura3, Scott A. Boardsen4,5, Daniel J.
Gershman4, Yoshizumi Miyoshi1, Masahiro Kitahara6, Yuto Katoh6, Hirotsugu Kojima3, Satoko
Nakamura1, Masafumi Shoji1, Yoshifumi Saito7, Shoichiro Yokota8, Barbara L. Giles5, William R.
Paterson4, Craig J. Pollock9, Alexander C. Barrie4,10, Daniel G. Skeberdis4,11, Stephen Kreisler4,10,Olivier Le Contel12, Christopher T. Russell13, Robert J. Strangeway13, Per-Arne Lindqvist14, Robert
E. Ergun15, Roy B. Torbert16,17, and James L. Burch17
1Institute for Space-Earth Environmental Research, Nagoya University, 2 Department of Earth andPlanetary Science, Graduate School of Science, the University of Tokyo, 3 Research Institute for
Sustainable Humanosphere, Kyoto University, 4 NASA Goddard Space Flight Center, 5 Goddard
Planetary Heliophysics Institute, University of Maryland, Baltimore County, 6 Department ofGeophysics, Graduate School of Science, Tohoku University, 7 Institute of Space and AstronauticalScience, Japan Aerospace Exploration Agency, 8 Department of Earth and Space Science, Graduate
School of Science, Osaka University, 9 Denali Scientific, 10 Aurora Engineering, 11 a.i. solutions Inc.,
12Laboratoire de Physique des Plasmas, CNRS/Ecole Polytechnique/Sorbonne
Université/Université Paris-Sud/Observatoire de Paris, 13 Department of Earth, Planetary, and Space
Science, University of California, Los Angeles, 14 Royal Institute of Technology, 15 Laboratory for
Atmospheric and Space Physics, University of Colorado, Boulder, 16 Department of Physics,
University of New Hampshire, 17 Southwest Research Institutee-mail (speaker): kitamura@isee.nagoya-u.ac.jp
The interaction between electromagnetic fields andcharged particles is central to collisionless plasma
dynamics in space. Right-hand polarized electromagnetic
whistler-mode waves have been the subject of manystudies due to their efficient pitch-angle scattering andacceleration of electrons and play important roles invarious regions (solar wind, shock waves, and planetary
magnetospheres). A nonlinear wave-particle interaction
theory for coherent large amplitude waves predicts thatelectrons around cyclotron resonance velocity exhibit
nongyrotropy due to the phase trapping motion in thepresence of an appropriate magnitude of spatial gradientof magnetic field intensity and/or temporal variation of
the wave frequency [1]. The nongyrotropic electron
velocity distribution functions can exchange energy andmomentum with waves. Observational identification ofsuch nongyrotropic electron velocity distributionfunctions, however, has been extremely difficult due tothe short time scale of resonant electron dynamics. In
this study, we show an observational result of
nongyrotropic electron velocity distribution functions
close to the cyclotron resonance velocity with ultra-hightemporal resolution data obtained by the MagnetosphericMultiscale spacecraft (MMS) during large-amplitude
whistler-mode wave events around themagnetosheath-side separatrix of the daysidemagnetopause reconnection and the magnetosheath near
the magnetopause. With measurements by the Fast
Plasma Investigation Dual Electron Spectrometer(FPI-DES) and the search-coil magnetometer (SCM), we
identified electron flux hole events that induce wave
growth; a hole at an appropriate relative phase angle to
the whistler-mode wave magnetic field appeared only

close to the cyclotron resonance velocity. The ElectronDrift Instrument also continuously detected modulationof electron flux near the cyclotron resonance velocity,which is consistent with the hole identified by FPI-DESmeasurements. During the time intervals, the magnitudeof the gradient of magnetic field intensity along themagnetic field line, which was derived byfour-spacecraft measurements, was consistent with anappropriate magnitude to cause phase trapping motion ofresonant electrons. The nonlinear growth rate, which iscalculated with observed resonant currents, became~0.005Ωce (0.02–0.07ω), where ω is the angularfrequency of the wave and Ωce is the electron cyclotron
angular frequency. These results provide direct evidenceof locally ongoing nonlinear wave-particle interaction
between whistler-mode waves and cyclotron resonant
electrons due to phase trapping motion [2]. Suchnonlinear wave growth, which has been discussed mainly
for wave growth in the magnetosphere, probably plays arole also around the magnetic reconnection and in themagnetosheath.
References[1] Omura, Y. Nonlinear wave growth theory ofwhistler‑mode chorus and hiss emissions in themagnetosphere. Earth Planet. Space, 73, 95 (2021).
https://doi.org/10.1186/s40623-021-01380-w[2] Kitamura, N. et al. Direct observations of energy
transfer from resonant electrons to whistler-mode waves
in magnetosheath of Earth. Nature Communications, 13,6259 (2022).
https://doi.org/10.1038/s41467-022-33604-2