DENSITY-TEMPERATURE CORRELATION IN SOLAR-WIND MAGNETOHYDRODYNAMIC FLUCTUATIONS - A TEST FOR NEARLY INCOMPRESSIBLE MODELS

The degree of correlation existing between density and temperature fluctuations at MHD scales in the inner solar wind (0.3 to 1 AU) has been investigated as a test for nearly incompressible models. Correlation is evaluated over time intervals of 45 and 180 min for a period of about 3 months in conditions of low solar activity. The number of cases with significant correlation (or anticorrelation) comes out to be quite small. Anticorrelations are mostly observed in slow wind, while correlations do not appreciably depend on flow speed. Relative density fluctuations for intervals with significant correlation or anticorrelation do not scale with turbulent Mach number as expected on the basis of nearly incompressible theory. These findings raise doubts about a general and extended applicability of such theory to the solar wind fluctuations in the inner heliosphere. However, a more comprehensive data set, for instance, one including different solar activity conditions, may be required for definite conclusions to be drawn.