LOW-LATITUDE CORONAL HOLES, DECAYING ACTIVE REGIONS, AND GLOBAL CORONAL MAGNETIC STRUCTURE

Abstract

We study the relationship between decaying active region magnetic fields,\ncoronal holes and the global coronal magnetic structure using Global\nOscillations Network Group (GONG) synoptic magnetograms, Solar Terrestrial\nRElations Observatory (STEREO) extreme ultra-violet (EUV) synoptic maps and\ncoronal potential-field source-surface (PFSS) models. We analyze 14 decaying\nregions and associated coronal holes occurring between early 2007 and late\n2010, four from cycle 23 and 10 from cycle 24. We investigate the relationship\nbetween asymmetries in active regions' positive and negative magnetic\nintensities, asymmetric magnetic decay rates, flux imbalances, global field\nstructure and coronal hole formation. Whereas new emerging active regions\ncaused changes in the large-scale coronal field, the coronal fields of the 14\ndecaying active regions only opened under the condition that the global coronal\nstructure remained almost unchanged. This was because the dominant\nslowly-varying, low-order multipoles prevented opposing-polarity fields from\nopening and the remnant active-region flux preserved the regions' low-order\nmultipole moments long after the regions had decayed. Thus the polarity of each\ncoronal hole necessarily matched the polar field on the side of the streamer\nbelt where the corresponding active region decayed. For magnetically isolated\nactive regions initially located within the streamer belt, the more intense\npolarity generally survived to form the hole. For non-isolated regions, flux\nimbalance and topological asymmetry prompted the opposite to occur in some\ncases.\n