Long- and Short-Term Evolutions of Magnetic Field Fluctuations in High-Speed Streams

High-speed streams (HSSs) are believed to be only slightly affected by different interactions on their path from the Sun to Earth and thus the analysis of their observations can provide information on the structure and temporal variations of the magnetic field and plasma parameters at the source region. We have chosen three coronal holes supplying 14 HSSs recorded by Wind in 2008. For each HSS, we have calculated the average magnetic field and plasma parameters as well as power spectral densities (PSDs) of magnetic field fluctuations in the MHD and kinetic ranges to investigate their long- and short-term variations. We suggest that long-term variations are connected with a time evolution of the source region on the time scale of solar rotations. On the other hand, the short-term variations would reflect a longitudinal structure of the coronal hole. Our study reveals that coronal holes are very stable source of HSSs and their temporal evolution on short- and long-time scales is negligible. This is true for the average parameters as well as for the fluctuation power and PSDs. Observed correlations between bulk and/or thermal velocity and PSD parameters are consistent with already published results. We suggest that they do not originate in the source region but they can be mostly attributed to interaction with the ambient slow wind that affects even the HSS core.