Investigation of the sources of the slow solar wind

Aim of this analysis is to study the variation of the physical conditions of the coronal plasma across the streamer boundary in order to identify the coronal sources of the slow solar wind during the minimum of solar activity. The analysis is based on the observations of equatorial streamers, obtained in the outer corona during the years 1996 and 1997 with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO. The outflow velocity, the electron density and the oxygen abundance relative to hydrogen of the coronal plasma have been determined, in the range between 1.6 and 3.5 solar radii (R⊙), by means of a spectroscopic analysis of the OVI 1032, 1037 Å and the HI Lyα 1216 Å lines. Coronal expansion at low velocity, in the range 80–100 km/s, is observed along regions 15°–20° wide, surrounding the streamer boundary. Evidence for coronal plasma outflows at low velocity is also found further out in the region along the streamer axis. In this case the outflows become significant beyond 2.7 R⊙. Hence, the slow solar wind during solar minimum flows just outside the denser and brighter zone of a streamer, characterized by closed magnetic field lines and in a lane around the heliospheric current sheet, forming just above the closed field line region.