ON THE NATURE OF COMPRESSIVE FLUCTUATIONS IN THE SOLAR-WIND

A statistical analysis of the amplitudes of the thermal pressure and t otal pressure of the solar wind and of several related cross correlati ons between different compressive parameters has been performed with t he plasma and magnetic field data obtained by Hellos 1 and 2 in their primary missions. The statistical analysis is based on small-band aver ages of the relevant spectra over the frequency range (2-5) x 10(-4) H z, corresponding to hourly timescales. The analysis shows that the the oretical values, given by the relation between the fluctuation of tota l pressure and density for perpendicular fast magnetoacoustic waves, p resent an upper limit for the observed amplitudes of the normalized to tal pressure fluctuations. With decreasing ratio between the fluctuati on amplitudes of the total pressure and the density, we found a system atical decrease of the correlation coefficient between density and tot al pressure and of the correlation coefficient between density and mag netic field magnitude. Decreases of the correlation coefficients betwe en temperature and density and between thermal and magnetic pressure a re also found with decreasing ratio of the normalized amplitudes of th e fluctuations of the thermal pressure and the temperature. For high-s peed wind data the pattern of the data distribution in plots of one co rrelation coefficient versus the other correlation coefficient skews s ome systematical changes. Most of these results can be explained quali tatively by a model based on a superposition of small-amplitude perpen dicular fast magnetosonic waves and small-amplitude pressure-balanced structures. We have found a class of data points which seem to represe nt fluctuations dominated by fast magnetosonic waves. In many cases in low-speed wind the correlations between density and total pressure an d between temperature and density are both negative, while the correla tion between temperature and magnetic magnitude is about zero. The nat ure of this phenomenon has not yet been clearified. Some possible expl anations are suggested. The applicability and relevance of the nearly incompressible magnetohydrodynamics theory for the compressive fluctua tions in the inner heliosphere are also discussed.