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

¹ INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: [dspadaro;rventura]@oact.inaf.it
² Dipartimento di Fisica e Astronomia – Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: sur@oact.inaf.it
³ Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375-5320, USA e-mail: [vourlidas;landi]@nrl.navy.mil

Received: 14 May 2007
Accepted: 7 September 2007

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 Lyα and  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, and kinetic temperatures, and outflow velocities were derived from the line intensities and widths, as well as from the  line intensity ratio in the 1.6-5  range of heights, limited to the central region of the streamer. To our knowledge, the  outflow velocities obtained in this work are the first ones determined inside a streamer structure. They are significantly lower than those of the  ions. This, together with the  kinetic temperatures that are much higher than the 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  kinetic temperature and outflow velocity radial profiles are consistent with the results for the other examined streamers.