Magnetosonic-mode fluctuations and electron rolling-pin distribution behind anti-dipolarzation front in terrestrial magnetotail

Magnetic reconnection hosts regions of enhanced ion temperature anisotropy that could drive instabilities and excite magnetosonic waves. However, the relation between magnetosonic-mode fluctuations and the anti-dipolarization fronts (ADFs, also known as the leading edge of tailward reconnection jets), is still unclear. Here, for the first time, we provide direct observations of magnetosonic-mode fluctuations behind ADF by the magnetospheric multiscale mission. These compressible waves propagate quasi perpendicularly to the background magnetic fields at very slow phase speeds (~30 km/s) and appears simultaneously with parallel ion temperature anisotropy. Such waves were likely a result of oblique firehose instability. An electron rolling-pin distribution is modulated by such waves behind the ADF: at wave troughs (B minimum), perpendicular electron fluxes are high; at wave crests (B maximum), perpendicular electron fluxes are low. Fermi acceleration and magnetic mirror effect contribute to the formation of such distribution jointly. These findings improve our understanding of wave-particle interactions near the ADFs in Earth's magnetosphere.