CORONAL FARADAY ROTATION FLUCTUATIONS AND A WAVE/TURBULENCE-DRIVEN MODEL OF THE SOLAR WIND

Abstract

Some recent models for coronal heating and the origin of the solar wind postulate that the source of energy and momentum consists of Alfven waves of solar origin dissipating via MHD turbulence. We use one of these models to predict the level of Faraday rotation fluctuations (FRFs) that should be imposed on radio signals passing through the corona. This model has the virtue of specifying the correlation length of the turbulence, knowledge of which is essential for calculating the FRFs; previous comparisons of observed FRFs with models suffered from the fact that the correlation length had to be guessed. We compare the predictions with measurements of FRFs obtained by the Helios radio experiment during occultations in 1975 through 1977, close to solar minimum. We show that only a small fraction of the FRFs are produced by density fluctuations; the bulk of the FRFs must be produced by coronal magnetic field fluctuations. The observed FRFs have periods of hours, suggesting that they are related to Alfven waves which are observed in situ by spacecraft throughout the solar wind; other evidence also suggests that the FRFs are due to coronal Alfven waves. We choose a model field line in an equatorial streamer which has background electron concentrations that match those inferred from the Helios occultation data. The predicted FRFs are found to agree very well with the Helios data. If the FRFs are in fact produced by Alfven waves with the assumed correlation length, our analysis leads us to conclude that wave-turbulence models should continue to be pursued with vigor. But since we cannot prove that the FRFs are produced by Alfven waves, we state the more conservative conclusion, still subject to the correctness of the assumed correlation length, that the corona contains long-period magnetic fluctuations with sufficient energy to heat the corona and drive the solar wind.