Coronal velocity measurements with Ulysses: Multi-link correlation studiesduring two superior conjunctions

A well-known method for studying the solar wind very close to the Sun (heli ocentric distances: 4 to 40 solar radii) is by radio sounding between a spa cecraft at superior conjunction and the Earth. The Ulysses Solar Corona Exp eriment was performed at the spacecraft's two solar conjunctions in summer 1991 and winter 1995, during which dual-frequency ranging and Doppler obser vations were conducted globally on a nearly continuous basis at the NASA De ep Space Network and other ground stations. The dual-frequency Doppler meas urements were used to determine coronal plasma velocities by a cross-correl ation analysis during those occasions when tracking data were recorded simu ltaneously at two well-separated ground stations. A 'filtering' technique w as developed to suppress noise and enhance the 2-station correlations, a pr ocedure particularly effective at small solar offsets. From the electron co ntent measurements during the two solar conjunctions it was found that regi ons of higher electron density tend to occur when the two-station correlati ons yield slower outward flow velocities.