Coronal Pseudo-Streamer and Bipolar Streamer Observed by SOHO/UVCS in March 2008

Abstract

The last solar minimum is characterized by several peculiar aspects and by the presence of a complex magnetic topology with two different kinds of coronal streamers: pseudo-streamers and bipolar streamers. Pseudo-streamers or unipolar streamer are coronal structures which separate coronal holes of the same polarity, without a current sheet in the outer corona; unlike bipolar streamer that separate coronal holes of opposite magnetic polarity. In this study, two examples of these structures have been identified in the period of Carrington rotation 2067, by applying a potential-field source-surface extrapolation of the photospheric field measurements. We present a spectroscopic analysis of a pseudo-streamer and a bipolar streamer observed in the period 12-17 March 2008 at high spectral and spatial resolution by the Ultraviolet Coronagraph Spectrometer (UVCS; Kohl et al., 1995) onboard Solar and Heliospheric Observatory (SOHO). The solar wind plasma parameters, such as kinetic temperature, electron density and outflow velocity, are inferred in the extended corona (from 1.7 to 2.1 Rsun) analysing the O VI doublet and Ly alpha line spectra. The coronal magnetic topology is taken into account and has been extrapolated by a 3D magneto-hydrodynamic model of the global corona. The results of the analysis show some peculiarities of the pseudo-streamer physical parameters in comparison with those obtained for bipolar streamers: in particular, we have found higher kinetic temperature and higher outflow velocities of O VI ions and lower electron density values. In conclusion, we point out that pseudo-streamers produce a "hybrid" type of outflow that is intermediate between slow and fast solar wind and they are a possible source of slow/fast wind in not dipolar solar magnetic field configuration.Comment: 14 pages, 8 figures, in press in Solar Physics, 201