Resonant ULF Absorption Revealed by Auroral Doppler Radar Data
- 9 Downloads
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.
The author is grateful to M.G. Deminov and V.A. Pilipenko for helpful discussions.
The STARE instruments were maintained by the Max Plank Institute of Aeronomy (Germany) and Finnish Meteorological Institute (Finland) in cooperation with Trondheim University (Norway).
This work was in part supported by the Russian Foundation for Basic Research (project no. 17-05-00427).
- 6.Badin, V.I., Excitation and absorption of ULF oscillations studied by high-latitude Doppler radar data, Geomagn. Aeron. (Engl. Transl.), 2016, vol. 56, no. 1, pp. 87–94. https://doi.org/10.7868/S0016794016010028
- 14.Gul’el’mi, A.V. and Potapov, A.S., On peculiarities of the MHD-wave field in an inhomogeneous plasma, Issled. Geomagn. Aeron. Fiz. Soln., 1984, no. 70, pp. 149–157.Google Scholar
- 21.Mager, P.N., Berngardt, O.I., Klimushkin, D.Yu., Zolotukhina, N.A., and Mager, O.V., First results of the high-resolution multibeam ULF wave experiment at the Ekaterinburg SuperDARN radar: Ionospheric signatures of coupled poloidal Alfvén and drift-compressional modes, J. Atmos. Sol.-Terr. Phys., 2015, vols. 130–131, pp. 112–126.CrossRefGoogle Scholar
- 23.Pilipenko, V., Belakhovsky, V., Kozlovsky, A., Fedorov, E., and Kauristie, K., Determination of the wave mode contribution into the ULF pulsations from combined radar and magnetometer data: Method of apparent impedance, J. Atmos. Sol.-Terr. Phys., 2012, vol. 77, no. 1, pp. 85–95.CrossRefGoogle Scholar