The Making of Delta Sunspots

TL;DR

This study investigates the formation mechanisms of delta sunspots with sharp polarity inversion lines, finding that most are formed by merging multiple magnetic regions rather than a single flux rope emergence.

Contribution

The paper introduces a classification of delta sunspot genesis into four types and provides observational examples and schematic models for each, highlighting the dominant formation process.

Findings

Few delta sunspots are formed from a single emerging flux rope.

Most delta sunspots result from merging multiple magnetic regions.

Four distinct genesis types are identified and exemplified.

Abstract

We explore what fraction of delta sunspots in which the polarity inversion line (PIL) is sharp in photospheric magnetograms are made from a writhe kink in an emerging twisted flux rope. We searched simultaneous full-disk magnetograms and continuum images from Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO) to find 28 random sharp-PIL delta sunspots that are born well on the disk. Only one of these is made from a single newly emerged bipolar magnetic region (BMR) and therefore is a candidate for being made from a single emerging writhe-kinked flux rope. That outcome indicates that few, if any, sharp-PIL delta sunspots are made by a single emerging writhe-kinked flux rope. That is the main new finding of this paper. Each of the other 27 is made by merging of two or more emerging or emerged BMRs. We name delta-sunspot genesis from a single BMR Type I genesis. We identify another three genesis types among the other 27 delta sunspots: Type II, Type III, and Type IV. We present an observed example genesis for each of the four genesis types, and for each example present schematic drawings depicting our scenario(s) for the cause of that example genesis. The core idea of each scenario is that the delta sunspot is made by packing opposite-polarity magnetic flux together by advection into a convection downflow.