CORRELATIONS BETWEEN THE FLUCTUATIONS OF PRESSURE, DENSITY, TEMPERATURE AND MAGNETIC-FIELD IN THE SOLAR-WIND

Compressive fluctuations and the pressure balance in the solar wind ar e investigated with Helios data obtained in 1975-1976 from measurement s in the inner heliosphere between 0.3 and 1.0 AU. Broadband (6 x 10(- 3) Hz-6 x 10(-6) Hz) correlation spectra and hourly correlation coeffi cients for any pair of solar wind parameters like speed, density, temp erature, magnetic field magnitude, thermal and magnetic pressure, and the total pressure of the plasma are calculated and analysed. We find that the nature and intensity of the compressive fluctuations strongly depend on the flow speed and systematically vary with the stream stru cture. A general tendency is observed for the thermal and magnetic pre ssures to be anti-correlated. Particularly in the fast streams, the te mperature fluctuations contribute predominantly to the pressure balanc e. Clear positive correlations between the solar wind (proton) tempera ture and speed are found in the higher-frequency domain for most of th e cases considered. Significant positive correlations between the rela tive fluctuations of the total pressure and the density are obtained, particularly in high-speed streams, a result indicating the existence of fast magnetoacoustic waves. The statistical studies carried out sug gest the notion that the compressive fluctuations are a complex superp osition of magnetoacoustic fluctuations and pressure-balanced structur es, which may be created by interplanetary stream interactions or be o riginally related to plasma flow tubes of coronal origin.