Long-term variations of the solar plasma parameters at 1 AU

Interplanetary magnetic field (IMF) and plasma data taken near 1 AU by a variety of spacecraft have been used to study the variations of the 27-day averages of the plasma parameters during the 38-year period from 1967- 2004. Our analysis reveals the following results: (1) The IMF magnitude B, the solar wind speed (V) times B (VB) and the geomagnetic activity as characterized by the index Ap are positively correlated to the level of solar activity as represented by sunspot number R during the 38-year period, and lag the solar activity by 5, 10 and 36 solar rotations (SRs), respectively. However, Ap and VB are anticorrelated with R during cycles 20 and 21, when they lead solar activity by 5-32 SRs. (2) The solar wind speed, temperature (T), proton flux (NV), thermal pressure (P) and plasma pressure (Pp) are anticorrelated to the level of solar activity, and lead solar activity by ~1-2.5 years. (3) The entropy (S) is positively correlated with solar activity during the ascending phase of the solar cycle. (4) In general high speed solar wind is associated with high temperature and low density; while low speed solar wind is associated with low temperature and high density. However V is positively correlated to N but with time lag during cycles 20, 21 and 22 during which V lags N by ~10-26 SRs. (5) The product VB is better related to the geomagnetic activity than any of the other interplanetary parameters, and is independent upon neither the level of solar activity nor the number of solar cycles