Resonant ULF Absorption Revealed by Auroral Doppler Radar Data

Abstract

STARE Doppler data on electron drift velocities detected in the auroral ionosphere for weakly disturbed conditions are analyzed. The Doppler measurements are first averaged along each radar beam. The power spectral density (PSD) as a function of frequency is then calculated by discrete Fourier transforms of averaged signals for every radar beam. Deep stepwise drops (about 10 dB) in spectral powers of the ultra-low-frequency (ULF) range are revealed for all radar beams. These PSD drops are interpreted as manifestations of resonant ULF absorption, which occurs in the eigenfrequency continuum of standing Alfven waves excited on geomagnetic field lines. A variational analysis that models PSD decreases by stepwise profiles of the mean spectral powers is proposed. This analysis provides the least-squares fitting of model profiles to PSD decreases calculated by the data. The frequency of the mean PSD step determined for each radar beam is treated as the minimum frequency of resonant ULF absorption revealed by this beam during the given auroral event. Averaged over all beams, this frequency for the analyzed event is 4.9 ± 0.2 mHz.