CLUSTER observations of electrostatic acceleration structures above the polar cap and implications for their origin in the magnetopause boundary layer

During quiet periods of Northward IMF, CLUSTER observed electron acceleration structures at high altitudes along magnetic field lines connected to the polar and extended along the magnetopause boundary layers. The electrons are observed to be successively earthward and outward accelerated, forming current sheets of opposite polarities. The precipitating electrons are accelerated to keV-energies in relatively stable and broad structures. The outflowing electron beams, accelerated to weaker energies (tens of eV), form structures at much smaller scales. These acceleration structures are suggested to result from electrostatic structures generated at the interface between the magnetopause boundary layers and the lobes.