Low-latitude coronal holes, decaying active regions and global coronal magnetic structure

TL;DR

This study investigates how decaying active regions influence coronal holes and the global magnetic structure of the Sun, revealing that the large-scale field remains stable despite local changes due to the dominance of low-order multipoles.

Contribution

It provides new insights into the relationship between decaying active regions, coronal holes, and the global magnetic field, emphasizing the role of low-order multipoles in maintaining large-scale stability.

Findings

Coronal fields of decaying regions open only when global structure remains unchanged.

Remnant flux preserves low-order multipole moments long after decay.

Polarity of coronal holes matches the polar field on the streamer belt side.

Abstract

We study the relationship between decaying active region magnetic fields, coronal holes and the global coronal magnetic structure using Global Oscillations Network Group (GONG) synoptic magnetograms, Solar Terrestrial RElations Observatory (STEREO) extreme ultra-violet (EUV) synoptic maps and coronal potential-field source-surface (PFSS) models. We analyze 14 decaying regions and associated coronal holes occurring between early 2007 and late 2010, four from cycle 23 and 10 from cycle 24. We investigate the relationship between asymmetries in active regions' positive and negative magnetic intensities, asymmetric magnetic decay rates, flux imbalances, global field structure and coronal hole formation. Whereas new emerging active regions caused changes in the large-scale coronal field, the coronal fields of the 14 decaying active regions only opened under the condition that the global coronal structure remained almost unchanged. This was because the dominant slowly-varying, low-order multipoles prevented opposing-polarity fields from opening and the remnant active-region flux preserved the regions' low-order multipole moments long after the regions had decayed. Thus the polarity of each coronal hole necessarily matched the polar field on the side of the streamer belt where the corresponding active region decayed. For magnetically isolated active regions initially located within the streamer belt, the more intense polarity generally survived to form the hole. For non-isolated regions, flux imbalance and topological asymmetry prompted the opposite to occur in some cases.