On the Fast Solar Wind Heating and Acceleration Processes: A Statistical Study Based on the UVCS Survey Data

The UltraViolet Coronagraph Spectrometer (UVCS) on board the SOlar and Heliospheric Observatory has almost continuously observed, throughout the whole solar cycle 23, the UV solar corona. This work addresses the first-ever statistical analysis of the daily UVCS observations, performed in the O vi channel, of the northern polar coronal hole, between 1.5 and 3 R⊙, during the period of low solar activity from 1996 April to 1997 December. The study is based on the investigation, at different heights, of the correlation between the variance of the O vi 1031.92 Å spectral line and the O vi 1031.92, 1037.61 Å doublet intensity ratio, which are proxies of the kinetic temperature of the O5+ ions and of the speed of the oxygen component of the fast solar wind, respectively. This analysis allows the clear identification of the sonic point in polar coronal holes at the distance of 1.9 R⊙. The results show that heat addition below the sonic point does not lead to an increase of the outflow speed. As a matter of fact, the coronal plasma is heated more efficiently in the subsonic region, while its acceleration occurs more effectively in the region of supersonic flow. So, within the panorama of the Parker Solar Probe and Solar Orbiter missions, the statistical analysis of the historical UVCS data appears to be very promising in providing unique clues to some still unsolved problems, as the coronal heating, in the solar corona.