EXOS‐D observations of electric field fluctuations and charged particle precipitation in the polar cusp

Abstract

EXOS‐D has observed irregular and spiky electric fields at the polar cusp in the altitude range of 4,000 to 10,000 km. The oscillations have amplitude of tens of mV/m and a broadband spectrum. We have found that high‐frequency (> 0.5 Hz) oscillations are associated with the precipitation of magnetosheath‐originated plasma, i.e., ion in the energy range of 0.1 to 1 keV/e and electron above 100eV/e. Oscillatory electric fields can also be observed all along the auroral oval, but they are more intense and more frequently seen at the cusp. The nature of these oscillation is discussed in terms of two possible mechanisms; i.e., 1) Doppler‐shifted quasi‐static structure of field‐aligned current, 2) Alfven waves propagated along magnetic field lines. We suggest that the high‐frequency turbulence in the polar cusp region is due to Alfven waves propagated from the magnetopause where interplanetary and geomagnetic field lines are reconnected and the magnetosheath plasma is injected into the polar cusp.