Nonlinear Alfvén Wave Interaction with Large-Scale Heliospheric Current Sheet

Abstract

The in-situ measurements of velocity, magnetic field, density and temperature fluctuations performed in the solar wind have
greatly improved our knowledge of MHD turbulence not only from the point of view of space physics but also from the more general
point of view of plasma physics.

These fluctuations which extend over a wide range of frequencies (about 5 decades), a fact which seems to be the signature
of turbulent non-linear energy cascade, display, mainly in the trailing edge of high speed streams, a number of features characteristic
of a self-organized situation: (i) a high degree of correlation between magnetic and velocity field fluctuations, (ii) a very
low level of fluctuations in mass density and magnetic field intensity. These features are locally lost, in the presence of
large scale inhomogeneities of the background medium, like velocity shears due to the stream structure and current sheets
at magnetic sector boundaries. Such nonuniformities generate a coupling among different modes and tend to destroy the equilibrium
solution represented by outward propagating Alfvénic fluctuations. The Alfvénicity is also reduced with increasing distance
from the Sun, and this could be in part due to the effects of the large-scale inhomogeneity related to the solar wind expansion.

The effects of inhomogeneities of the background medium on a MHD turbulence have been studied, from a theoretical point of
view, by a number of numerical models. In this paper we briefly review such models, discussing the main results and their
limitations, and comparing with observed features of the solar wind fluctuations.