Role of Compressibility in the Evolution of Solar Wind Alfvénicity

Solar wind turbulence is determined by the nonlinear interaction between inward and outward propagating Alfvén waves. The normalized cross-helicity, σ_C, is a measure of the predominance of one mode with respect to the other. The evolution of σ_C toward lower values with increasing radial distance is interpreted both as a decrease of the presence of the outward modes or as a continuous production of inward modes within those regions such as stream shears where some plasma instability is active.

On the other hand, it has been found that the decrease of σ_C is sometimes related also to the presence of compressive fluctuations in the form of magnetic field and density enhancements. Thus, compressibility effects might be able to play a non-negligible role in the evolution of the interplanetary plasma not properly considered so far.

The present analysis supports this point of view. We will study the effect of compressibility on Alfvénic correlations presenting both case studies and a statistical analysis over different phases of the solar cycle.

Our findings indicate that the presence of magnetic field compression generally play a relevant role in the depletion of σ_C since it acts on the destruction of the v-b correlation of the outward modes.

Moreover a study on the source regions of the solar wind, both slow and fast, will be presented and discussed.