Energetic Electrons Observed Inside Magnetic Holes in the Magnetotail

Abstract

Magnetic holes, characterized as magnetic field depressions, have been widely observed in space plasma. Two large-scale magnetic holes, MH1 and MH2, were reported in this paper and the energetic electrons up to 100 keV were detected for the first time inside both holes. The two holes showed many similar features, comparable spatial scale, temperature and total pressure increase, and energetic electrons up to 100 keV with a power-law distribution inside them. On the other hand, distinct features were also found between these two holes. A potential ion flow vortex was detected inside the MH1 and an ion-scale magnetic structure was observed in its core region. The electron flux enhancements were associated with this ion-scale structure and the energetic electrons were nonadiabatic around the ion-scale structure inside MH1, while the energetic electrons were adiabatic inside the MH2. The mirror-mode instability was unstable around MH1 while stable around MH2, which suggested that the two holes might be in a different phase of the mirror-mode instability. The observations suggested that the electrons could be significantly accelerated inside magnetic holes in the different phases.