A search for a general gradient in the solar wind speed at low solar latitudes

Long-term averages of solar wind speed data obtained in the ecliptic plane from July 1964 through December 1975 have been examined for a regular variation in flow speed associated with the earth's yearly excursion to latitudes ±7¼° about the solar equator. Regular speed modulations of ∼70 km s−1 peaking at the highest latitudes are discernible from mid-1964 through 1966 and from early 1969 to early 1971. During the remainder of this 11½-year interval the modulations in speed seem to be aperiodic. A superposed epoch analysis of all the data fails to reveal a general solar latitude gradient in the solar wind flow for these 11½ years. It is concluded that with near-earth observations a latitude dependence of the flow speed is too small to be detected on a regular basis against a background ‘noise’ of solar wind streams, which vary in longitude, latitude, and time.