Evolution of magnetic field fluctuations in high‐speed solar wind streams: Ulysses and Helios observations

Abstract

Ulysses magnetic field measurements in high‐speed streams emanating from the Sun's polar coronal holes reveal a complex combination of fluctuations on different scales. Measurements of the wavenumber dependence of radial and latitudinal power scaling, spectral index, and anisotropy of undisturbed polar magnetic field fluctuations between 1.4 and 4.1 astronomical units (AU) are presented. These results confirm earlier work and reveal the presence of three populations: structures at large scales, Alfvén waves at intermediate scales, and turbulent fluctuations at the smallest scales. A significant decrease in power with latitude, as a result of overexpansion of polar coronal holes, has consequences for energetic particle diffusion coefficients. Other results, in particular those related to field magnitude variations, are not well understood at this time. A similar analysis of magnetic field fluctuations in high‐speed streams at low heliolatitudes between 0.3 and 1 AU, measured by the Helios spacecraft, shows consistency with Ulysses measurements, suggesting a common origin for fluctuations in all high‐speed streams.