On the nature of compressive fluctuations in the solar wind

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A statistical analysis of the amplitudes of the thermal pressure and total pressure of the solar wind and of several related cross correlations between different compressive parameters has been performed with the plasma and magnetic field data obtained by Helios 1 and 2 in their primary missions. The statistical analysis is based on small‐band averages of the relevant spectra over the frequency range (2‐5) × 10−4 Hz, corresponding to hourly timescales. The analysis shows that the theoretical values, given by the relation between the fluctuation of total pressure and density for perpendicular fast magnetoacoustic waves, present an upper limit for the observed amplitudes of the normalized total pressure fluctuations. With decreasing ratio between the fluctuation amplitudes of the total pressure and the density, we found a systematical decrease of the correlation coefficient between density and total pressure and of the correlation coefficient between density and magnetic field magnitude. Decreases of the correlation coefficients between temperature and density and between thermal and magnetic pressure are also found with decreasing ratio of the normalized amplitudes of the fluctuations of the thermal pressure and the temperature. For high‐speed wind data the pattern of the data distribution in plots of one correlation coefficient versus the other correlation coefficient shows some systematical changes. Most of these results can be explained qualitatively by a model based on a superposition of small‐amplitude perpendicular fast magnetosonic waves and small‐amplitude pressure‐balanced structures. We have found a class of data points which seem to represent fluctuations dominated by fast magnetosonic waves. In many cases in low‐speed wind the correlations between density and total pressure and between temperature and density are both negative, while the correlation between temperature and magnetic magnitude is about zero. The nature of this phenomenon has not yet been clearified. Some possible explanations are suggested. The applicability and relevance of the nearly incompressible magnetohydrodynamics theory for the compressive fluctuations in the inner heliosphere are also discussed.