DENSITY-FLUCTUATIONS AND TURBULENT MACH NUMBERS IN THE INNER SOLAR-WIND

At MHD scales the solar wind behaves as an almost incompressible fluid in both the inner and the outer heliosphere, with density fluctuations that, for a relevant fraction of casks, have a relative amplitude less than 0.1. Nearly incompressible MHD theory would seem then appropriate to describe a major part of the solar wind compressive turbulence at these scales. However, in spite of some examples of good agreement between observations and theoretical predictions, an extended applicability of the nearly incompressible theory to the interplanetary plasma turbulence is far from being firmly established. In the present paper, we focus, as a test of the theory, on the scaling properties of density fluctuations with turbulent Mach number. Interplanetary plasma data in the region from 0.3 to 1 AU in a period of low solar activity have been used. The analysis has been performed on the entire data Sample and on subsets of it with different ranges of solar wind velocity and heliocentric distance. Our results unambiguously indicate a lack of the expected correlations. This negative outcome might be due to the small dynamic range of the analyzed parameters. Nevertheless, in the light also of recent indications about a low level of correlation between density and temperature fluctuations, it appears reasonable to conclude that experimental observations do not seem in favor of a generalized applicability of the nearly incompressible theory to MHD compressive fluctuations in the inner solar wind.