Spatial features of mid-latitude field-line resonances from simultaneous ground-satellite measurements

Abstract

Three series of simultaneous pulsation measurements (f<0.06 Hz) on the Freja satellite and at the Budkov Observatory have been spectrally processed (FFT) in 6-min intervals of Freja’s transits near the local Budkov field line. Doppler-shifted, weighted spectral-peak frequencies, determined in both transverse magnetic components in the mean field-aligned coordinate system on Freja, allowed the estimation, by comparison with the stable frequency at Budkov, of fundamental frequencies of the local magnetic-field-line resonance ranged from 13 to 17 mHz in two pulsation events analyzed, with K _p =2₊ to 0₊. The ratio of total amplitudes of the spectral-pulsation components on the ground and on Freja at an altitude of \sim1700 km (values <0.7) characterizes the transmissivity of the ionosphere. In the Pc3 frequency range this correlates well with simulation computations using models of the ionosphere under low solar activity.