Nature and Origin of Incompressible MHD Fluctuations

Abstract

In many periods the fluctuations of the magnetic vector δB and the flow velocity δV have been found to be (anti) correlated for Bo_x < 0 (Bo_x > 0) (Belcher and Davis, 1971; Denskat et al., 1981; Denskat and Neubauer, 1982; Marsch and Tu, 1990a; Tu et al., 1989b; Tu and Marsch, 1990a). The natural conclusion is that they are outward propagating Alfvén waves (Belcher and Davis, 1971). If they were a kind of convective structures with such a δB and δV correlation, how could we understand the systematic changes of the polarity of the correlation at magnetic sector boundaries? This argument was already given by Belcher and Davis (1971), however their wave examples observed near 1 AU were not pure Alfvén waves because the magnetic fluctuation energy is dominant. They interpreted this magnetic dominance as an effect of a large anisotropy in the pressure. This requires that 4π (p _II — p _⊥)/ B ²₀ be as large as 0.4. However, the overall plasma anisotropy observed by Helios is very small (Marsch and Richter, 1984a) and cannot cause an Alfvén wave with magnetic energy dominance.