SOLAR-WIND VELOCITY AND TEMPERATURE IN THE OUTER HELIOSPHERE

At the end of 1992, the Pioneer 10, Pioneer 11, and Voyager 2 spacecra ft were at heliocentric distances of 56.0, 37.3, and 39.0 AU and helio graphic latitudes of 3.3-degrees-N, 17.4-degrees-N, and 8.6-degrees-S, respectively. Pioneer 11 and Voyager 2 are at similar celestial longi tudes, while Pioneer 10 is on the opposite side of the Sun. All three spacecraft have working plasma analyzers, so intercomparison of data f rom these spacecraft provides important information about the global c haracter of the solar wind in the outer heliosphere. The averaged sola r wind speed continued to exhibit its well-known variation with solar cycle: Even at heliocentric distances greater than 50 AU, the average speed is highest during the declining phase of the solar cycle and low est near solar minimum. There was a strong latitudinal gradient in sol ar wind speed between 3-degrees and 17-degrees-N during the last solar minimum, but this gradient has since disappeared. The solar wind temp erature declined with increasing heliocentric distance out to a helioc entric distance of at least 20 AU; this decline appeared to continue a t larger heliocentric distances, but temperatures in the outer heliosp here were surprisingly high. While Pioneer 10 and Voyager 2 observed c omparable solar wind temperatures, the temperature at Pioneer 11 was s ignificantly higher, which suggests the existence of a large-scale var iation of temperature with heliographic longitude. There was also some suggestion that solar wind temperatures were higher near solar minimu m.