Observation of High-Frequency Electrostatic Waves in the Dip Region Ahead of Dipolarization Front

摘要

Dipolarization front (DF) is a sharp magnetic structure featuring the increase of B_z in the magnetotail. Various types of waves have been observed around the DF. However, in the magnetic dip ahead of the DF, very few waves have been reported so far. In this study, we report broadband highfrequency electrostatic waves in the dip region by using high resolution data from the magnetospheric multiscale (MMS) mission. The frequency range of these electrostatic emissions is higher than electron cyclotron frequency but lower than electron plasma frequency. These high-frequency waves are quasiparallel propagating, and their parallel components dominate. Simultaneously, the measured field-aligned electron distribution function (EDF) has a positive slope (df/dv_(||) > 0). The kinetic theory dispersion relationship curve based on the measured EDF reveals that the positive slope is responsible for the generation of high-frequency electrostatic wave in the magnetic dip ahead of the DF.