Long-term variations of the solar wind proton parameters

Abstract

Interplanetary plasma data taken near 1 AU by a variety of spacecraft (Imp 1, Vela 2-4, Explorer 33-35, and Heos 1) from 1963 to 1971 are used to study the long-term variations of the solar wind proton properties. An intercalibration among the different experiments is performed in order to obtain a coherent set of data. The most interesting result is a ∼40% reduction of the proton density between the minimum and the maximum of the solar activity cycle. The observed density variation occurs throughout the velocity range; however, a limited sample of data could suggest that at very low velocities (V < 300 km/s) the density is independent of the solar activity. The proton bulk velocity is remarkably constant during the period considered: small increases of the average speed occurring in 1968 and 1971 cannot be attributed to the solar cycle. No clear trend is exhibited by the proton temperature: differences among the various experiments probably have an instrumental origin. The density modulation has been interpreted in terms of the heliographic latitude effect proposed by Hundhausen et al. (1971). It turns out that with increasing solar activity this effect disappears, producing the observed decrease of the average density near the solar maximum.