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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96RS03448

Measurement of the propagation speed of plasma inhomogeneities in the solar corona using an uplink/downlink cross‐correlation method

Radio Sounding of the Solar Wind Acceleration Region with Spacecraft Signals.

Different approaches for determining the velocity and density of a near-solar plasma by the radio-sounding method using high-stability signals from spacecraft are analyzed. The results of studying the solar-wind characteristics at heliocentric distances of 3–50 solar radii are presented. Experimental results on the plasma velocity, density, power, and acceleration for specified distances are given.

Determination of the solar-wind velocity, density, power, and acceleration by the method of radio sounding of the near-solar plasma by the spacecraft signals

Measurement results are presented for the velocity of density irregularities in solar wind plasma, which modulate the frequency of coherent radio signals from the Galileo spacecraft in the S-band. The measurements have been performed by the spaced receiving technique at ground monitoring stations. The data cover a range of heliocentric distances from 7 to 72 solar radii at low heliolatitudes near solar minimum. It is shown that acceleration of slow solar wind continues up to distances of about 30 solar radii. In the overlapping ranges of distances from the Sun, the results are qualitatively consistent with LASCO SOHO data for the same periods, with the previous results obtained by analyzing the amplitude fluctuations of signals from the Venera 15 and 16 spacecraft, and with the more recent measurements acquired using the Mars Express spacecraft.

Velocity of the Inner Solar Wind from Coronal Sounding Experiments with Spacecraft

Measurements of S-band downlink frequency (Doppler) shift were collected for intervals of about 30 days during the 1994 and 1995 solar conjunctions of the Galileo spacecraft. The occultation geometries enabled coronal radio sounding of the heliographic equatorial region over a heliocentric range from about 5 to 20 R s (solar radii) for the 1994 conjunction and from about 4 to 35 R s for the 1995 conjunction. Spectral analysis was carried out with the S-band Doppler scintillations to determine the strength and spectral distribution of coronal electron density fluctuations. Cross-correlations of the Galileo downlink signals received simultaneously at two ground stations were computed to obtain the propagation velocity of plasma disturbances crossing the ray paths.

O. M. Korsak

Papers

Occultation of the plasma near the sun and the speed of the solar wind: Fluctuations in the frequency and phase of radio waves at two separated points