Scaling of density fluctuations with Mach number and density‐temperature anticorrelations in the inner heliosphere

We present observations of field and plasma fluctuations of duration of a few hours or less of spacecraft time frame as observed by Helios 2 in the inner heliosphere. Two distinct turbulent plasma regimes are found. In the regions between high‐speed streams, small‐amplitude density fluctuations show an excellent correlation with plasma frame Mach number (M), and they are often anticorrelated with small‐scale proton temperature fluctuations. In high‐speed flows, however, density fluctuations have several scales, both M and M2, and density and temperature fluctuations are more positively correlated. Our observations cast in a new light the distinctly different nature of high‐speed flows from coronal holes and flows from other equatorial regions. They provide a characterization of small‐scale fluctuations in the inner heliosphere, and we interpret the results in terms of the nearly incompressible fluid description of the solar wind (Zank and Matthaeus, 1990). Additionally, we compare our results with Voyager observations in the outer heliosphere (Matthaeus et al., 1991).