Abstract
The past solar minimum is characterized by several peculiar aspects and by a complex magnetic topology with two different types of coronal streamers: pseudo-streamers and bipolar streamers. Pseudo-streamers or unipolar streamer are coronal structures that separate coronal holes of the same polarity, without a current sheet in the outer corona; unlike bipolar streamers, which 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., Solar Phys. 162, 313, 1995) onboard the Solar and Heliospheric Observatory (SOHO). The solar wind plasma parameters, such as kinetic temperature, electron density, and outflow velocity, were inferred in the extended corona (from 1.7 to \(2.1~{R}_{\odot}\)) by analyzing the O vi doublet and H i Ly \(\upalpha\) line spectra. The coronal magnetic topology was taken into account and was extrapolated with 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 a 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 the slow and fast solar winds. These outflows are a possible source of slow/fast wind in a non-dipolar solar magnetic field configuration.
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Acknowledgements
We thank John Kohl and Larry Gardner for helping us in the careful calibration of the UVCS data. UVCS is a joint project of the National Aeronautics and Space Administration (NASA), the Agenzia Spaziale Italiana (ASI) and Swiss Founding Agencies. We also acknowledge SOHO/EIT and LASCO, STEREO/EUVI and the COR2, NSO/Kitt Peak, Mt. Wilson Observatories for use of their data. The research of LA has been funded through the contract I/023/09/0 between the National Institute for Astrophysics (INAF) and the Italian Space Agency (ASI). L. Abbo, P. Riley and Y.-M. Wang are also grateful to the International Space Science Institute (ISSI) for the fruitful discussions during the international team entitled “Slow solar wind sources and acceleration mechanisms in the corona” and for providing the financial support to it.
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Abbo, L., Lionello, R., Riley, P. et al. Coronal Pseudo-Streamer and Bipolar Streamer Observed by SOHO/UVCS in March 2008. Sol Phys 290, 2043–2054 (2015). https://doi.org/10.1007/s11207-015-0723-y
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DOI: https://doi.org/10.1007/s11207-015-0723-y