Quasi-periodic Fluctuations Detected in Mars Express Coronal Radio Sounding Observations

Abstract

Spectral analysis of the high-quality dual-frequency data obtained with the radio sounding experiment MaRS on the ESA spacecraft Mars Express has revealed a quasi-periodic component (QPC) at heliocentric distances from 4 to 10 solar radii. The QPC typically appears as a broad maximum in the temporal frequency fluctuation spectrum centered at a frequency in the range 3.7 mHz<ν<5.3 mHz, and can sometimes be as much as four times stronger than the background power-law spectrum at the same frequency. Occurring sporadically in about 18% of the spectra, the QPC power level can increase rapidly within about 30 minutes, only to vanish again on the same time scale. Two distinct QPCs are evident in a few selected spectra, possibly a main wave and its second harmonic. It is conjectured that MHD waves with periods near 4 minutes (ν~4 mHz) are continuously present in the solar wind acceleration region and can be occasionally detected in radio sounding data under conditions with a favorable observational geometry. The electron density fluctuations are most probably associated with fast magnetosonic waves generated locally via nonlinear interactions of weakly damped Alfvén waves.