| The dipolarization/jet braking region in the near-Earth magnetotail is a key region associated
during substorms with magnetic field reconfiguration and electromagnetic energy conversion.
While such processes affect large volumes of space, important physics occurs at non-MHD
scales (ion scales and below) making thus crucial to study this region simultaneously at
different scales. Here we focus on the acceleration of energetic electrons, namely electrons
having energies much larger than their thermal energy. We present Cluster observations of
several magnetic field dipolarizations (Bz enhancements) during a substorm interval on
October 27, 2007. Cluster crossed the plasma sheet around XGSM ~ -10 RE with
inter-spacecraft separations ranging from ~ 40 km (electron scales) to ~ 10000 km (fluid
scales). Observations indicate that energetic electrons up to ~ 400 keV are accelerated in the
dipolarization/jet braking region within earthward moving structures (’magnetic bubbles’)
that have a typical size ~few ion scales and are edged by thin current sheets with size
~ few electron scales. Strong electric fields and waves are observed within the
bubbles. We find that electrons are accelerated by both betatron and non-adiabatic
processes. We discuss the role of small-scale electron acceleration processes for
large-scale processes such as the injection of electrons in the inner magnetosphere. |