Solar-wind speed variations 1964-1967: An autocorrelation analysis

An autocorrelation analysis has been performed on the Vela 2 and 3 solar-wind speed data obtained from July 1964 through December 1967 with two goals: to establish the degree to which speed structures recur from one solar rotation to the next, and to examine the duration and origin of ‘persistence’ in the solar-wind speed. The average correlation between speed measurements separated in time by 1 solar rotation is 0.3, thus implying that most speed structures during the period under consideration do not endure for 28 days; however, the amplitude of the correlation varies with time from less than 0.1 to greater than 0.7. Differential solar rotation serves to weaken the correlation at any particular lag; the data suggest that a wide variety of solar latitudes contribute to the solar wind observed near the earth. Recurrence periods ranging from about 27 to 29 days were observed; these periods differed from those determined from interplanetary-field measurements for the same interval by as much as a day. Correlations associated with the solar rotation are questionable after about August 1966; this fact may be related to increased solar activity associated with the advance of the solar cycle. High-speed streams are not the primary source of correlations between speed measurements separated by about 28 days; rather, low-speed (<400 km sec−1) streams make the major contribution to the correlations observed. Those speed structures which do recur for more than 1 solar rotation have variable lifetimes, but few endure for more than 2 solar rotations. Measurements of the solar-wind speed separated by a day or less are usually highly correlated; the average correlation is 0.94 and 0.64 at lags of 3 hours and 1 day, respectively. It is found that this positive correlation, or persistence, extends out to about 3 days on the average; it is argued that about half this interval is the time over which the solar wind can be considered as ‘steady.’ Persistence is limited by both the spatial scale of coherent emission from the sun and the temporal evolution of the solar atmosphere. It is suggested that 4 days is a typical time interval over which significant changes occur in the flow speed of the wind emanating from a particular region.