Coronal radio sounding experiments with ROSETTA and MARS-EXPRESS during solar conjunctions in 2010/2011

Coronal radio sounding experiments were carried out using two-way coherent dual-frequency carrier signals of the ESA spacecraft ROSETTA (ROS) in 2010 and MARS-EXPRESS (MEX) in 2011. Frequency and ranging measurements recorded at both the NASA and ESA tracking stations (sample rate: 1 Hz) are analyzed in this work. Spectral analysis of the S-band, X-band, and differential-frequency records has shown that the mean frequency fluctuation of each signal can be described by a radial power-law function. The radial falloff exponent for the mean frequency fluctuation during the ROS coronal sounding experiments in October 2010 was close to m = 1.5 for all three data types. The falloff exponent was closer to m = 2.0 during the MEX coronal sounding experiments in February 2011. The mean values of the ratio of the rms of the S-band the frequency fluctuations and the X-band frequency fluctuations are close to the theoretical value 2.161 for both solar conjunctions. Enhanced frequency fluctuations were observed on 11 October 2010 (east limb), 24 October 2010 (west limb), 31 December 2010 (east limb), 28 January 2011 (east limb), 10 February 2011 (west limb). It is suggested that these enhanced fluctuations are connected with the source and formation of the solar wind plasma near the coronal base. Spectral analysis of the differential group delay (ranging) was carried out for the first time to determine the fluctuation characteristics of the integrated electron content. The ratio of the rms ranging fluctuation and the average group delay connected with radio wave propagation in the solar wind plasma, was calculated. It was established that this value is about 1 percent for heliocentric distances between 10 and 25 solar radii. It was also found that quasi-periodic oscillations of the solar wind plasma density were more effectively detected by the ranging measurements than by frequency measurements. The frequency fluctuations, proportional to the product of the plasma density irregularities and the velocity fluctuations, can sometimes occur with opposing phases, thereby decreasing the summed fluctuation intensity.