Solar coronal holes as sources of recurrent geomagnetic disturbances

Abstract

Observations of the solar corona by Oso 7 have been used in a superposed epoch analysis to study the relationships between classes of coronal features and geomagnetic activity. Both bright coronal regions (associated with solar activity) and regions of less than average brightness (coronal holes) were investigated. It was found that for the period from January 1972 through January 1973 a significant enhancement in geomagnetic activity (Ap increase of nearly a factor of 2) occurred 2–3 days after central meridian passage of large coronal holes that extended to within 5° of the solar subearth point when they were on the meridian. Neither bright coronal features nor sunspots nor bright calcium plage regions showed such a correlation. Large coronal holes, which are frequently recurrent long-lived features, appear to satisfy the requirements for ‘M regions,’ which were hypothesized to be responsible for recurrent geomagnetic disturbances (Bartels, 1934). If solar wind high-speed streams originate preferentially in these regions, their velocity at the base of the corona will be substantially higher than that expected from an axisymmetric solar wind model.