Physical parameters of a mid-latitude streamer during the declining phase of the solar cycle

Abstract

Context. Investigating the physical properties of solar coronal streamers is important for understanding their role in the global magnetic structure of the extended solar atmosphere, as well as in the generation of the slow solar wind. Aims. We hope to contribute as completely as possible to the ongoing SOHO instruments campaign devoted to the study of the physical characteristics of coronal streamers at various heliocentric distances. Methods. We analyzed ultraviolet HI Lya and O VI resonance doublet lines observed by UVCS/SOHO in a narrow, mid-latitude streamer structure along different lines of sight during a week in May 2004 and made nearly simultaneous white-light polarized brightness measurements from the LASCO/SOHO C2 coronagraph. Results. Electron densities and temperatures, HI and O VI kinetic temperatures, and outflow velocities were derived from the line intensities and widths, as well as from the O VI line intensity ratio in the 1.6-5 R⊙ range of heights, limited to the central region of the streamer. To our knowledge, the HI outflow velocities obtained in this work are the first ones determined inside a streamer structure. They are significantly lower than those of the O VI ions. This, together with the O VI kinetic temperatures that are much higher than the HI ones, suggest that the absorption of Alfvén waves at the ion cyclotron frequency might also occur inside streamers. Conclusions. In comparison with other streamers described in the literature, the structure examined in this work generally exhibits lower electron density and neutral hydrogen kinetic temperature. Conversely, the O VI kinetic temperature and outflow velocity radial profiles are consistent with the results for the other examined streamers. © ESO 2007.