Paper detail

Turbulence-Level Variations and Magnetic Field Orientations in the Fast Solar Wind

The turbulent magnetic fields of a large set of fast solar wind streams measured onboard ACE and STEREO A and B are analyzed in an effort to identify the effects of the turbulence-level broad variations on the orientations of the local, time-averaged magnetic fields. The power level of turbulence, roughly defined as the power in the transverse field fluctuations normalized to the medium-scale average background field, tightly orders the location of the peaks in the probability distribution functions (PDFs) of the angles between local fields and Parker spiral. As a result, the broad variations in the power level of turbulence cause a steep dependence of the average power level of turbulence on the angle of the local field to the Parker spiral, with the highest turbulence levels found near the normal to the Parker spiral and the lowest levels near the Parker spiral direction. Generalized quasilinear estimates of the mean cross-field displacements adapted to intermittent time-varying turbulence lead to accurate fits of the observed angle PDFs at all stable levels of turbulence, supporting the idea that isotropic turbulence could account for the observed angle PDFs. Modeling of the angles alpha_r between local fields and radial direction, from the PDFs of the angles alpha between local and background fields under an assumption of axisymmetry of the turbulent fields around a background field in or near the direction of the Parker spiral, also produces fairly good fits of the observed PDFs of alpha_r, thereby validating the assumption. Finally, local field reversals are found to be quite common even within very broad streams of "unipolar" fast solar wind.