Physical parameters along the boundaries of a mid-latitude streamer and in its adjacent regions

Abstract

Context: Coronal streamers appear to be strictly associated with the generation of the slow solar wind, even if a firm identification of the sources of the particle flux within these structures is still an unresolved issue.
Aims: The purpose of this work is to contribute to a better knowledge of the physical characteristics of streamers and of their surroundings in a wide range of heliocentric distances and at both high radial and latitudinal resolutions.
Methods: The analysis is based on spectral observations of a narrow, mid-latitude streamer performed with UVCS/SOHO during one week in May 2004: H i Lyα and O vi resonance doublet line intensities and profiles were obtained at different heliocentric distances and latitudes. In addition, white-light polarized brightness images were taken in the same days of observation, through the LASCO/SOHO C2 coronagraph.
Results: The radial variations in electron density and temperature, H i and O vi kinetic temperatures, and outflow velocities were derived from the observed line intensities, profiles, and O vi line intensity ratios between 1.6 and 5.0 R_⊙, in two regions, 2-3 arcmin wide, located along the boundaries and in a narrow strip (5-10 arcmin) outside the streamer structure. Significantly high kinetic temperatures and outflow velocities were found in the out-of-streamer region above 3.0 R_⊙ for the O vi ions and, for the first time, H i atoms, compared to those obtained along the streamer boundaries. Moreover, the O vi kinetic temperatures and velocities turn out much higher than the H i ones at any heliocentric distance in all the observed regions. A higher anisotropy is also noticed for the O vi kinetic temperature in the region flanking the streamer.
Conclusions: The slow coronal wind is found to flow with significantly different speeds and kinetic temperatures along the boundaries of the streamer and in the out-of-streamer regions at all heights, above 3.0-3.5 R_⊙. This fact, consistent with previous studies, indicates that two components of slow wind probably form in the observed regions: one originates just above the streamer cusp and flows with velocities a little higher than 100 km s^-1, while the other flows along the open magnetic field lines flanking the streamer with velocities slightly lower than the slow wind asymptotic heliospheric value of ~400 km s^-1, around 5.0 R_⊙.