Observations of ion acoustic fluctuations in the auroral topside ionosphere by the FREJA S/C

The widespread existence of naturally excited broadband electrostatic wave activity up to 15 kHz, with predominantly parallel polarization, have been detected in the topside auroral ionosphere by the wave experiment onboard the Freja S/C. Amplitudes of a few mV/m electric field fluctuations and a few % density fluctuations are typically observed, but stronger cases are quite common. These emissions are most often observed within the auroral energization region according to the plasma instrument data, and coincide frequently with Alfvén wave activity from a few Hz to tens of Hz. This suggests that the electrostatic emission is linked to the nonlinear evolution of kinetic Alfvén waves.