Radio sounding investigations were performed over a period of about 30 days during the 1991 solar occultation of the Ulysses spacecraft (August and September 1991). Dual-frequency Doppler and ranging measurements were obtained using the NASA Deep Space Network. For this experiment the radio ray path from Ulysses to Earth moved essentially parallel to the solar equator, sounding the circumsolar plasma on the east (ingress phase) and west (egress phase) solar limbs at solar offset distances from about 4 to 40 RS (solar radii). Dual-frequency, two-way Doppler data were used to determine the propagation speed of coronal plasma inhomogeneities by cross-correlation analysis between the uplink and downlink ray paths. The motion of Earth and spacecraft with respect to Sun results in a spatial separation (typically 20,000 km) between the uplink and downlink ray path during the round-trip travel time of the signal. The specific configuration of the Ulysses radio system (dual-frequency S/X band downlinks coherently locked to a S band uplink) provides the means for separating the uplink and downlink plasma contributions. The propagation speed of plasma inhomogeneities intersecting both ray paths can be determined by computing the cross-correlation of the Doppler data from both ray paths as a function of time lag. The time lag of maximum cross-correlation was used to calculate the propagation speed of plasma inhomogeneities. By evaluating the Ulysses data using this uplink and downlink cross-correlation method, significant cross-correlation peaks were obtained in the range of 7 to 34 RS. A high symmetry of the propagation speeds for both sides of the solar limb was found. An increase in propagation speed with increasing solar distance can be seen, starting from about 50 km/s at a distance of 7 RS up to 390 km/s at a distance of 29.7 RS (east) and from about 120 km/s at 9.8 RS up to 415 km/s at 33.8 RS (west). A significant enhancement (550–660 km/s) was observed at a distance of about 18 RS on both sides of the solar limb due to a solar event still being investigated.
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