CROSS-HELICITY DEPLETIONS IN THE INNER HELIOSPHERE, AND MAGNETIC-FIELD AND VELOCITY FLUCTUATION DECOUPLING

Recent analyses of solar wind data from 0.3 to 20 a.u. have demonstrated that the flow, following initial acceleration out to 20R(s) strongly resembles evolving turbulence. Initially, strongly Alfvenic periods characterized by quasi-planar waves propagate outward along the direction of the averaged magnetic held. As the flow progresses into the heliosphere, this character is destroyed by what appears to be a turbulent cascade into an isotropic or three-dimensional spectrum of fluctuations. In this paper, we use Heli os data to examine the behavior of field and velocity minimum variance directions within fast and slow wind in the inner heliosphere and compare these results to the observed radial evolution of the normalized cross-helicity. We reach the conclusion that the depletion of the Alfvenic character of the outwardly propagating fluctuations, within the hourly frequency range in the spacecraft rest frame, might be driven by their interaction with static structures or magnetosonic perturbations able to modify the homogeneity of the background medium on a scale size comparable with the wavelength of the fluctuations. This interaction, if strong enough, causes the decoupling and successive isotropization of field and velocity fluctuations with the consequent removal of these fluctuations from the Alfvenic regime.