Observations of the beam-driven whistler mode waves in the magnetic reconnection region at the dayside magnetopause

We report observations of the whistler mode waves in the magnetic reconnection region at the dayside magnetopause using the magnetospheric multiscale (MMS) mission on January 11, 2016. In this event, whistlers mostly occur on the magnetospheric side of the reconnection central plane and are closely related to counter-streaming electron beams in the medium energy range (200-300 eV). These counter-streaming electron beams can be attributed to the accelerated magnetosheath electrons by the reconnection. Through quantitatively testing the Landau resonance condition and solving the kinetic dispersion relations, we present the evidence for whistler excitation by electron beams in the medium energy range. Additionally, the whistlers are observed simultaneously with the pitch-angle scattering of electrons in the high-energy range (300-3,000 eV). The calculation results show that this scattering is likely to occur through electron cyclotron wave-particle interactions, which can isotropize electrons and exchange energy between plasmas and waves.