THEMIS observations of plasma bubbles associated with energetic electron acceleration in the inner magnetosphere

Abstract:

Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations, we study the plasma bubbles associated with a transient increase of the magnetic field B_z component in the inner magnetosphere during the substorm expansion phase. Except small electric field, the main characteristics of these plasma bubbles are similar with those associated with dipolarization front (DF) in the mid-tail and near-Earth tail. Based on the different dipolarization of the magnetic field, we defined the plasma bubble with no dipolarization phenomenon as “no dipolarization bubble” (NDB), the plasma bubble with dipolarization phenomenon as “dipolarization bubble” (DB). We find that these plasma bubbles in the inner magnetosphere accompany the energetic electron acceleration. Some pancake-type distributions of energetic electrons inside the NDB in the inner magnetosphere are caused by drift betatron acceleration, other pancake-type distributions of energetic electrons inside the NDB are caused by gyrobetatron acceleration. For the DB in the inner magnetosphere, the cigar-type distributions of energetic electrons are attributed to Fermi acceleration. Our observations suggest that the inner magnetosphere may be a very important source region for energetic electrons, except for a reconnection site in the mid-tail and the plasma sheet in the near-Earth tail.