The correlation between coronal Doppler shifts and the supergranular network

¹ Kiepenheuer Institut für Sonnenphysik (KIS) Schöneckstraße 6, 79104 Freiburg, Germany e-mail: tayeb@kis.uni-freiburg.de
² Institut d'Astrophysique Spatial (IAS), CNRS-Université Paris XI, 91405 Orsay Cedex, France

Received: 11 October 2004
Accepted: 28 January 2005

Abstract

We examine properties of line profiles as found with large raster scans of the solar corona acquired by the UV spectrometer SUMER on board SOHO. The observed regions include an equatorial coronal hole, a polar coronal hole, as well as surrounding quiet Sun areas. In order to reveal the network and remove strong local brightenings, a filter is applied to a continuum image. The filtered continuum image, the intensity image and the dopplergram are used to produce “scatter diagrams” (dispersion plots). We find correlations between the chromospheric network, the  (770 Å) intensity and the  (770 Å) Doppler shift in quiet Sun areas and in coronal holes. We establish that the maximum outflow (blue-shift) at low corona temperatures does not appear in the centre of the network but rather near network boundaries. Furthermore the maximum blue-shift seems to appear in the dark regions in  line intensity, which is in agreement with Wilhelm (2000). The opposite correlation appears for very low intensities (less than half of the average intensity), revealing in these regions a lack of energy to either accelerate the solar wind or produce any detectable radiation. The absence of magnetic field concentration in these regions in a reconstructed magnetogram from a MDI/SOHO series seems to confirm the lack of energy.