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

Abstract

Kinetic Alfvén waves (KAWs), with a strong parallel disturbed electric field, play an important role in the energy transport and particle acceleration in the magnetotail. Based on the high-resolution observations of the Magnetospheric Multiscale (MMS) mission, we present the detailed acceleration process of electrons by KAWs in the plasma sheet boundary layer (PSBL). MMS observed strong electromagnetic disturbances carrying parallel disturbed electric field with an amplitude up to 8 mV/m. The measured ratio of the electric to magnetic field perturbations is larger than the local Alfvén speed and enhances as the frequency increases, in consistent with the theoretical predictions for KAWs. These evidences indicate 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 direction significantly enhance, implying occurrences of electron beams at higher energies. Meanwhile, the KAWs become more electrostatic-like and filled with high frequency ion acoustic waves. The energy enhancement of electron beams accords to the derived work done by the observed parallel disturbed electric field of KAWs, indicating electron acceleration caused by KAWs. Therefore, the paper provides a direct evidence of electron acceleration by KAWs embodying electrostatic ion acoustic waves in the PSBL.