Solar wind stream structure at large heliocentric distances: Pioneer observations

Time profiles and histograms of plasma data from Pioneers 10 and 11 are examined for the period between 1975 and 1983. During this time, Pioneer 10 traveled between a heliocentric distance of 8.7 and 30.4 AU. The velocity structure of the solar wind at these heliocentric distances is found to have one of two distinct forms: Approximately 70% of the time the solar wind has a nearly flat velocity profile. Occasionally, this flat velocity profile is accompanied by quasi‐periodic variations in density and in thermal speed consistent with the concept that the “corotating interaction regions” which are produced by the interaction of high‐ and low‐speed streams at intermediate heliocentric distances are replaced by “pressure regions” in the outer heliosphere. The remaining 30% of the time the solar wind is marked by large (50–200 km/s) long‐term (30–120 day) shifts in the average solar wind velocity.