Anisotropy and minimum variance directions of solar wind fluctuations in the outer heliosphere

Abstract

We systematically examine Voyager 2 magnetic field and plasma data over time intervals of 1 to 12 hours in the heliospheric range of 1 to 10 AU to study the evolution of the anisotropy of solar wind fluctuations. Consistent with previous results, we find that, on average, the directions of minimum variance vectors of magnetic fluctuations are close to the mean magnetic field direction with an increasing component of the variance along the field at larger scales. In both compression regions and rarefactions at large radial distances there is more spread in the minimum variance directions about the mean field than at smaller radial distances. The power in smaller‐scale fluctuations in the magnetic field components perpendicular to the local mean field B0 is in the ratio of about 5∶1 near 1 AU (compared to the power in fluctuations parallel to the field) at the scale of 1 hour but decreases to about 3∶1 farther out. We find no evidence for selective enhancement of out‐of‐the‐ecliptic components of fluctuations as predicted by a possible geometrical coupling between refracting Alfvén waves and compressive modes. In contrast to results for field fluctuations, our analysis of fluctuations in the velocity shows that the minimum variance direction systematically remains more radially oriented and becomes increasingly less oriented along B0 with increasing heliocentric distance. The velocity fluctuations are also generally more isotropic than the magnetic fluctuations at all distances examined. Our observations cannot be explained by a superposed wave picture, and thus are consistent with the view that nonlinear turbulent evolution is responsible for the anisotropy in the fluctuations.