Study of the relationship between large-amplitude neutral sheet oscillations and oblique firehose instabilities during fast flows in the terrestrial magnetotail

Abstract

Flapping motions of the current sheet are a large-scale dynamic process in the terrestrial magnetotail. Some large-amplitude neutral sheet oscillations during fast flows are regarded as flapping motions. In recent years, simulation results suggested that oblique firehose instabilities might be one of their generation mechanisms during fast flows, but this lacks sufficient observational evidence. Here, we statistically investigate the relationship between large-amplitude neutral sheet oscillations and oblique firehose instabilities during fast flows based on observations of the magnetospheric multiscale mission. We find that they have a broad range of period T from 4 to 2000 s. In addition, their propagation velocities (wavelengths) tend to decrease (increase) with the increase in T. About ∼17.6% of the large-amplitude neutral sheet oscillation events are approximately stationary relative to the flow and are linear or quasi-linear polarization, which meets the characteristics of the magnetic field fluctuations generated by oblique firehose instabilities. The probabilities of the fast flows accompanied by large-amplitude neutral sheet oscillations and the probabilities of Cof < 0 (oblique firehose unstable condition) near the neutral sheet have a similar tendency. In addition, T tends to be smaller when Cof is more negative during fast flows. These results suggest that Cof can not only affect the occurrence of the large-amplitude neutral sheet oscillations in the fast flow, but also affect their periods. Our above findings support that oblique firehose instabilities are a generation mechanism of some large-amplitude neutral sheet oscillations during fast flows.