DE‐2 cusp observations: Role of plasma instabilities in topside ionospheric heating and density fluctuations

Observations by the low altitude Dynamics Explorer satellite (DE‐2) in the polar cusp show the ionospheric plasma electron temperature and the ratio of electron to ion temperature to be increased and the electron density fluctuations to be enhanced. Also downward fluxes of energetic electrons and ions increase in the cusp and the magnetic field structure are consistent with the existence of a field‐aligned current. Simultaneously, there is characteristic broadband electrostatic noise (BEN) with amplitudes 1‐10 mV/m peaking in the cusp but extending into the polar cap. These emissions range from far below the local O+ gyrofrequency fO+ to the vicinity of the proton gyrofrequency but below the oxygen lower hybrid frequency. The BEN observations are compared to the predictions of several theories. The amplitude of these waves is shown to be far too small to contribute significantly to the observed ionospheric heating or density fluctuations by local wave‐particle interactions. Rather, the observed spatial variations are attributed to nonlocal field aligned heating processes and reflect the nonuniformity of the magnetosheath’s plasma’s penetration into the ionosphere.