Observations of kinetic Alfvén waves and associated electron acceleration in the plasma sheet boundary layer

Kinetic Alfvén waves(KAWs),with a strong parallel disturbed electric field,play an important role in energy transport and particle acceleration in the magnetotail.On the basis of high-resolution observations of the Magnetospheric Multiscale(MMS)Mission,we present a detailed description of the acceleration process of electrons by KAWs in the plasma sheet boundary layer(PSBL).The MMS observed strong electromagnetic disturbances carrying a parallel disturbed electric field with an amplitude of up to 8 mV/m.The measured ratio of the electric to magnetic field perturbations was larger than the local Alfvén speed and was enhanced as the frequency increased,consistent with the theoretical predictions for KAWs.This evidence indicates that the electromagnetic disturbances should be identified as KAWs.During the KAWs,the energy flux of electrons at energies above 1 keV in the parallel and anti-parallel directions are significantly enhanced,implying occurrences of electron beams at higher energies.Additionally,the KAWs became more electrostaticlike and filled with high-frequency ion acoustic waves.The energy enhancement of electron beams is in accordance with the derived work done with the observed parallel disturbed electric field of KAWs,indicating electron acceleration caused by KAWs.Therefore,these results provide direct evidence of electron acceleration by KAWs embodying electrostatic ion acoustic waves in the PSBL.