Quiet Sun mini-coronal mass ejections activated by supergranular flows^*

¹ Max-Planck Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: innes@mps.mpg.de
² Université Paul Sabatier, 31062 Toulouse, France
³ Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

Received: 22 September 2008
Accepted: 21 November 2008

Abstract

Context. The atmosphere of the quiet Sun is controlled by photospheric flows sweeping up concentrations of mixed polarity magnetic field. Along supergranule boundaries and junctions, there is a strong correlation between magnetic flux and bright chromospheric and transition region emission.

Aims. The aim is to investigate the relationship between photospheric flows and small flare-like brightenings seen in Extreme Ultraviolet images.

Methods. We describe observations of small eruptions seen in quiet Sun images taken with the Extreme UltraViolet Imager (EUVI) on STEREO. The photospheric flows during the eruption build-up phase are investigated by tracking granules in high resolution MDI continuum images.

Results. Eruptions with characteristics of small coronal mass ejections (CMEs) occur at the junctions of supergranular cells. The eruptions produce brightening at the onset site, dark cloud or small filament ejections, and faint waves moving with plane-of-sky speeds up to 100 km s^-1. In the two examples studied, they appear to be activated by converging and rotating supergranular flows, twisting small concentrations of opposite polarity magnetic field. An estimate of the occurrence rate is about 1400 events per day over the whole Sun. One third of these events seem to be associated with waves. Typically, the waves last for about 30 min and travel a distance of 80 Mm, so at any one time they cover 1/50th of the lower corona.