Quasi-periodic frequency fluctuations observed during coronal radio sounding experiments 1991-2009

Coronal radio sounding experiments were carried out using the dual-frequency signals of the spacecraft Ulysses, Galileo, Mars Express, Venus Express, and Rosetta. The change in differential frequency recorded at the NASA and ESA ground stations, a quantity sensitive only to the plasma along the radio ray path from spacecraft to receiver, has been analyzed in this work. This large volume of observational data provides evidence for the occasional presence of a quasi-periodic component (QPC) in the derived frequency fluctuation spectra. First seen in data from the Mars Express conjunction in 2004, further evidence for the QPC has now been found in data recorded at other solar conjunction opportunities from 1991 to 2009, thereby better defining the statistical characteristics of the QPC. The level of QPC spectral density is a factor of three higher than the expected power-law background level. The characteristic frequency of the spectral density maximum is roughly 4 mHz, corresponding to a QPC fluctuation period of about 4 min. The bandwidth of the spectral line is comparable to the maximum frequency. The QPC are observed at heliocentric distances between 3 and 40 solar radii, both in equatorial regions and at high heliolatitudes. The QPC is detected with an occurrence frequency of about 20% and is occasionally accompanied by its second harmonic. The most likely progenitors of the QPC are quasi-periodic electron density fluctuations associated with magnetosonic waves, which are generated locally from nonlinear interactions of 5-min band Alfvén waves propagating from the coronal base.