Long term variability of the solar wind speed

We have reviewed the solar wind data obtained over a period of more than three decades. The data coverage on the Omnitape begins on 27 November 1963 and ends on 31 December 1993, for the version used by us. The coverage is very uneven, ranging from less than 40% to greater than 80%. We find that a correlation continues to exist between the measured values of the solar wind speed (V) and the geomagnetic index Ap. For the period when the coverage is greater than or equal to 85%, the data fit the equation: V (km/s) = 11.7 A(sub p) + 260. The correlation coefficient is 0.8, at a confidence level of 99.95%. However, we find that the predicted value of V may differ from the observed value by as much as 15% for a year in which the coverage is poorer. A comparison of IMP 8 with Pioneer Venus Orbiter (PVO) data indicates that the average values at the former are systematically higher by as much as 35 km/s (approximately 8%) for an overlapping time period. Also, we draw attention to the observed three solar cycle periodicity in A(sub p) data for the 1932 to 1994 period and its implications for forecasting the parameters for solar cycle 23 as well as on the computations of the modulation parameters for cosmic rays.