On the velocity phase space cascade in the inner heliosphere

In space plasma, due to the absence of collisions, the phase space present a complex structuring and strong deviations from thermal equilibrium. Previous works have highlighted this aspect in both magnetosheath data and numerical simulation through an hermite decomposition of the ion velocity distribution function. The hermite spectrum of the vdf is expected to to have a precise spectral slope and to present anisotropy in a magnetic field dominated environment. Such a tool is particularly suited for the vdf representation since each order of the hermite decomposition corresponds to a moment of the vdf.

In this work we study, by using the Parker Solar Probe ion vdfs, the evolution of the hermite spectrum and the vdf fine features with respect to radial distance and solar wind conditions.

These results are useful to understand how the phase space evolve in the inner heliosphere and how this effect the heating in collissionless plasma.