Anisotropy and minimum variance of magnetohydrodynamic fluctuations in the inner heliosphere

Abstract

In this paper we examine the anisotropy, minimum variance, and related distinguishing plasma parameters of small‐scale fluctuations occurring in the solar wind. We use Helios 2 data taken at solar minimum, at a time when high‐ and low‐speed streams are clearly distinguished and a separation of the characteristics of fluctuations from disparate solar sources is facilitated. We find that while variance directions of fluctuations are generally aligned with the mean magnetic field in regions of high speed and relatively low plasma β, they are more three‐dimensional or isotropic in low‐speed, high‐β intervals. In our analysis period, these latter regions are generally the trailing edges of high‐speed streams and slow‐flow‐containing current sheets. In these low‐speed intervals, we find a tendency for large proton density fluctuations to be associated with a preference for fluctuation variance directions to be three‐dimensional and also a tendency for field and velocity fluctuations to decouple. In the past it has been emphasized that in high‐speed streams, fluctuations are initially outwardly propagating and Alfvénic or two‐dimensional in k space and that this Alfvénicity is destroyed by the production of inwardly traveling waves as the flow evolves. It has been suggested that this admixture of waves is produced in a turbulent cascade initiated by stream shears. Here we suggest that this picture is incomplete, that in addition to inwardly propagating plane waves, compressive waves, convected pressure balances, or other density fluctuations can generate or scatter the original spectrum and produce the observed scattering and decoupling of fluctuation directions. Additionally, while we do not dispute the supposition that the long‐wavelength free energy in stream‐stream interactions can initiate a turbulent cascade in the wind, the role of compressive fluctuations in altering the high‐frequency spectrum cannot be ruled out.