Formation of holes in the solar corona

Abstract

SOFT X-ray spectroheliograms obtained from Skylab¹ and rocket flights² show many bright filamentary features which map out the magnetic structure of the solar corona. Additional interesting features are the extensive regions from which the emission is extremely low. These long, but relatively narrow, ‘coronal holes’ can stretch much of the way across the solar disk in a roughly N–S direction. The photosphere and chromosphere beneath are relatively featureless and have low magnetic flux density. The magnetic structure of these lower levels is not untypical of other quiet regions on the Sun where coronal holes have not developed. This suggests that the magnetic configuration associated with the holes might not have emerged from the solar surface with any particularly unusual geometry, but this might have developed later due to some rearrangement within the atmosphere. Changes in field structure within the corona have been regarded for more than a decade as part of the normal process of development during the solar cycle³.