The inner solar corona seen by SUMER, LASCO/C1, and EIT: Electron densities and temperatures during the rise of the new solar cycle

Abstract

Detailed investigations of the corona and the source regions of the solar wind have become possible with spectroscopic and imaging instruments on the Solar and Heliospheric Observatory (SOHO). We present observations in the extreme ultraviolet (EUV), pertinent to the generation of the slow solar wind, which were obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrograph and by the Extreme-ultraviolet Imaging Telescope (EIT) in early 1998 under relatively quiet solar conditions, but with several active regions of the new solar cycle present. At the same time, forbidden iron lines in the visible were observed by the Large-Angle Spectroscopic Coronagraph (LASCO/C1). We study, in particular, the plasma parameters and the spatial structures of the low-altitude streamer regions, and find an electron density of $n_{\rm e} \approx 2\times 10^8$ cm -3 at 5 Mm above the equatorial limb for the coronal plasma, and $n_{\rm e} = (2~{\rm to}~6)\times 10^9$ cm -3 for the plasma at transition-region temperatures. High-temperature regions have been found at mid-latitudes with electron temperatures of $T_{\rm e} \approx 1.4\times 10^6$ K at heights of about 80 Mm and lower temperatures near the equator.