Coronal sounding with three ESA spacecraft during solar conjunction: Radial dependence of radio signal fluctuation spectra

Coronal radio sounding experiments were carried out using the coherent dual-frequency carrier signals of the three ESA spacecraft Mars Express (MEX), Venus Express (VEX) and Rosetta (ROS) during their solar conjunctions in 2004, 2006 and 2008/2009. The measurements ana-lyzed in this work are the signal frequency and amplitude recorded at both the NASA and ESA ground tracking stations (sample rate: 1 Hz). The solar activity was quite low during these measurement opportunities, particularly for the conjunction in 2008/2009 (average sunspot number <W> = 2.3). Spectral analysis of the frequency records provides two quantities used to characterize coronal turbulence: the intensity of the differential frequency fluctuations σf and the spectral index of the temporal frequency fluctuation spectra αf . The mean frequency fluctuation can be described by a radial power-law σf ∝ R-a . The spectral index, roughly constant at αf ≈ 0.67 for heliocentric distances beyond a certain solar offset distance RKOL , decreases gradually toward smaller solar offset distances. An unexplained discrepancy is found between the observed difference in amplitude fluctuations from S-Band to X-Band and the difference expected from theory.