Variations of solar-wind plasma properties: Vela observations of a possible heliographic latitude-dependence

Twenty-seven-day averages of the solar-wind density and flow speed, observed by Vela 3 and 4 spacecraft between July 1965 and July 1968, are found to vary with the heliographic latitude of observation. High average densities and low average flow speeds were measured near the solar equator; low average densities and high average flow speeds were measured near the northern and southern extremes of the earth's heliographic latitude excursion. Possible instrumental and statistical explanations of this pattern of variations are discussed and found to be unlikely. The variations can be reasonably interpreted in terms of a latitude-dependence in the structure of high-speed solar-wind streams related to solar activity.