Electron resonances and heating caused by whistler waves along the separatrix in asymmetric magnetic reconnection

Whistler waves have been observed via many space satellite observations in magnetic reconnection in the Earth′s magnetopause. We study wave–particle interactions between whistler waves and electrons by means of a two-dimensional particle-in-cell simulation of asymmetric reconnection where the guide field strength is 0.3 times the reconnecting magnetic field. The energy transfer between whistler waves and electrons occurs along the separatrix in the magnetospheric side. Tracing trajectories of individual electrons, we investigate how electrons gain energy from whistler waves. The electron velocity distribution functions along the separatrix show electron heating in both perpendicular and parallel to the magnetic field. We investigate two mechanisms (the Landau resonance and the cyclotron resonance) for electron heating through the wave–particle interactions. The electrons satisfying the Landau resonance condition can obtain a positive kinetic energy change due to the parallel electric field. We also discuss a new mechanism to increase the perpendicular energy, in which an increase in perpendicular energy can occur for particles that experience the Landau resonance because of the energy conversion due to the mirror force. Another new finding is that the number of electrons that are accelerated due to the cyclotron resonance is small, and the perpendicular temperature increase is mostly due to the electrons accelerated by the Landau resonance, although this is counterintuitive. As long as reconnection continues and whistler waves are produced, these mechanisms of heating continue and the electron kinetic energy in the outflow is dissipated into heat and the electron temperature increases.