The Role of Upper Hybrid Waves in the Magnetotail Reconnection Electron Diffusion Region

Plasma waves are believed to play an important role during magnetic reconnection. However, the specific role of upper hybrid (UH) waves in the electron diffusion region (EDR) remains elusive, owing to the absence of high-resolution measurements. We analyze one EDR event in the magnetotail on 2017 July 11 observed by the Magnetospheric Multiscale (MMS) mission. To the best of our knowledge, this is the first time that intense UH waves have been observed in the EDR by MMS. The agyrotropic crescent-shaped electron distributions could result in the observed UH waves. Concomitant with the observations of UH waves, the agyrotropy parameter root Q of the electrons decreases, implying that the UH waves could effectively scatter the electrons in the EDR. The good accordance of positive energy conversion (J . E' > 0, likely dissipation) and the observed UH waves indicates that UH waves may contribute to the energy conversion from the magnetic fields to the plasma particles during magnetic reconnection.