Electron Firehose Fluctuations Behind Anti-Dipolarization Front

Magnetic reconnection hosts regions of enhanced temperature anisotropy that could excite various kinetic instabilities and waves. However, the relation between electron firehose instability and the leading edge of tailward reconnection jets, as referred to anti-dipolarization fronts (ADFs), is still unclear. Here, for the first time, we provide direct observations of electron firehose fluctuations behind ADF by the Magnetospheric Multiscale (MMS) mission. These compressible fluctuations converted by the bulk flow have zero frequency in the plasma frame and large wave normal angles with respect to the background magnetic fields, and appears simultaneously with parallel electron temperature anisotropy. Our results found that the wave properties of the observed fluctuations largely agree with kinetic theoretical predictions of the non-propagating electron firehose mode. Specifically, the threshold of electron oblique firehose instability is exceeded under local plasma parameters and such instability has a growth rate of $\gamma \sim 0.01{\omega }_{ce}$. These findings improve our understanding of wave dynamics near the ADFs.