The Occurrences of Coronal Mass Ejection at Solar Minimum and their Association with Surface Activity

We have developed a non-subjective technique for recording the occurrences of coronal mass ejection (CME) in data recorded by the Large Angle Spectrometric Coronagraph experiment (LASCO) aboard the Solar and Heliospheric Observatory spacecraft (SOHO). We have found evidence for, and quantified, an asymmetry in the apparent longitudes at which mass ejections occurred during the first year of LASCO synoptic observations and coinciding with the 1996–1997 solar minimum. Throughout this period the solar surface could loosely be characterized as having both an active and a quiet hemisphere and the observed mass ejection asymmetry is seen to relate closely with the longitudes of most persistent disc activity. However, our best estimate for the centroid of the CME distribution is 45 deg to the west of the brightest regions visible in Fe 195 Å emission on the disc and in an area of reduced coronal emission. This corresponds to the location of a trans-equatorial extension of the northern coronal hole which persisted to some degree throughout the year and was directly associated with the most active region on the disc. We suggest that this indicates magnetic reconnection, which is necessary at the boundaries of coronal holes to maintain their quasi-rigid rotation above the differentially rotating photosphere, could play an important role in triggering the destabilization of nearby structures and result in the observed prevalence of mass ejections. We estimate that the events included in the study could contribute around 8% to the total solar mass loss through the solar wind (which is around 10¹⁴ kg day^–1) and find a scale of asymmetry indicating that close to 70% of this mass is ejected from within a single hemisphere.