Suprathermal Electron Acceleration Perpendicular to the Magnetic Field in the Topside Ionosphere

Abstract

AbstractWe present the first direct observations of suprathermal (tens to hundreds of eV) electron acceleration perpendicular to the magnetic field in the topside (900–1,500 km) ionosphere. Based on measurements from the suprathermal electron imager (SEI) onboard the Enhanced Polar Outflow Probe satellite over several months, we identify 30 events (28 in the dayside cusp) of enhanced suprathermal electron fluxes peaking at 90° pitch angles, with energies spanning from tens of eV to 325 eV, the upper limit of the SEI measurement. These events take place with a time duration of the order of 0.1 s, corresponding to a spatial scale of less than 1 km across the magnetic field. They are associated with parallel suprathermal electron bursts and upward currents. The correlation between perpendicular and parallel suprathermal electrons suggests a scenario in which downward bursts and the associated Alfvén waves that drive them provide a free energy source to destabilize plasma waves at electron frequencies, which in turn heat electrons in the perpendicular direction. This is a new energy dissipation process for cusp suprathermal electron precipitation in the topside ionosphere.