Solar Wind Magnetic Field Turbulence over the Solar Activity Cycle Inferred from Coronal Sounding Experiments with Helios Linear-Polarized Signals

Abstract

Results of experiments on polarized radio sounding of the outer solar corona using the Helios spacecraft from 1975 to 1984 are presented. The characteristic parameters of the temporal spectra of fluctuations in the Faraday rotation of the plane of polarization for heliocentric distances from 3.5 to 5.5 solar radii are obtained. The absolute level of these fluctuations and, consequently, the level of fluctuations of the magnetic field, is almost independent of the solar activity. It is well known that the global structure of the solar wind varies with the solar cycle such that there is slow solar wind at low latitudes and fast solar wind at high latitudes during solar minima. In contrast, a slow solar wind dominates at all latitudes during solar maxima. One explanation for the invariance of the fluctuations observed by sounding the circumsolar plasma is that the mean magnetohydrodynamic turbulence of the low-latitude, slow solar wind depends weakly on the phase of the solar cycle.