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

Ulysses magnetic field measurements in high-speed streams emanating from th e 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 po lar 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, Alfven waves at intermedi ate scales, and turbulent fluctuations at the smallest scales. A significan t decrease in power with latitude, as a result of overexpansion of polar co ronal 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 Hellos spacecraft, shows consistency with Ulysses meas urements, suggesting a common origin for fluctuations in ail high-speed str eams.