Solar corona–induced fluctuations on spacecraft signal amplitude observed during solar superior conjunctions of the Cassini spacecraft



[1] During the superior solar conjunctions of interplanetary spacecraft, the emitted radio signals undergo increased amplitude scintillation, phase scintillation, and spectral broadening due to the intervening charged particles of the solar corona. The magnitude of these parameters generally increases as the angle between the spacecraft and the center of the Sun (Sun-Earth probe (SEP) angle) decreases as observed by the tracking station on Earth. During periods of quiescent background, the scintillation and spectral broadening measurements of spacecraft in the ecliptic follow well-defined trends of established models as a function of SEP angle and link frequency. The amplitude data acquired from the Cassini solar superior conjunctions of May 2000 and June 2001 during the maximum of Solar Cycle 23 were analyzed. The X band (8.4 GHz) and Ka band (32 GHz) scintillation measurements from the signal amplitude, the autocorrelation function of the amplitude, and the histogram of the amplitude samples acquired during quiescent periods were found to be consistent with models based on previous studies with other wavelengths. Amplitude autocorrelation functions of the signal amplitude yield information on the limiting scale sizes of the charged-particle blobs which constitute the solar corona. Evidence for very fast fluctuations at X band (∼3 ms) was observed at SEP angles near 0.6°. The histograms of the open-loop receiver amplitude samples were found to be consistent with the Rice distribution. The transition from strong to weak scintillation at Ka band was clearly captured in a Cassini 2001 solar conjunction pass that occurred near an SEP angle of 0.6° (2.3 RS) as was expected for this frequency. The existing scintillation models were validated for Ka band, which is planned as a link frequency for deep space communications for upcoming missions. These results from this study are useful in providing information on solar activity on spacecraft signals at both X band and Ka band, which can be used to develop telecommunication strategies during solar superior conjunctions of planetary spacecraft.