Five-minute magnetic field fluctuations in the solar wind acceleration region

A spectral analysis of coronal Faraday rotation (FR) data obtained with the linearly polarized signals of the two Helios spacecraft reveals that about one-third of the temporal FR spectra contain a distinct spectral line supe rposed onto the background power-law spectrum. The most prevalent frequency of this quasi-harmonic component (QHC) is about 4 mHz, corresponding to a 4-5 min periodic oscillation of the coronal magnetic field. Physical reason s for the existence of QHC Alfven fluctuations in the inner solar wind are discussed. FR fluctuations (FRF) are considered to arise from both a turbul ent background as well as an isolated Alfven wave train of finite extent an d duration. An estimate can be made for the conditions under which the isol ated wave train is observed above the ever present background. It is shown that the wave train must have a sufficiently long duration and transverse w avelength. It is suggested that the QHC at periods near 4-5 min in the FRF spectra are most probably produced by outward-propagating Alfven waves exci ted initially in the anisotropic structures of the chromospheric network.