Emerging Dimming as Coronal Heating Episodes

TL;DR

Emerging dimming in solar active regions is caused by coronal heating episodes driven by magnetic reconnection between emerging flux and ambient magnetic fields, leading to observable EUV emission changes.

Contribution

This study links emerging dimming to coronal heating and magnetic reconnection, providing detailed thermodynamic and magnetic analysis of 18 events.

Findings

Coronal plasma heats from sub-MK to 1-2 MK during dimming.

Magnetic connectivity changes with longer loops after flux emergence.

Dimming correlates with heating episodes caused by reconnection.

Abstract

Emerging dimming occurs in isolated solar active regions (ARs) during the early stages of magnetic flux emergence. Observed by the Atmospheric Imaging Assembly, it features a rapid decrease in extreme-ultraviolet (EUV) emission in the 171 \r{A} channel images, and a simultaneous increase in the 211 \r{A} images. Here, we analyze the coronal thermodynamic and magnetic properties to probe its physical origin. We calculate the time-dependent differential emission measures for a sample of 18 events between 2010 and 2012. The emission measure (EM) decrease in the temperature range $5.7 \le \log_{10}T \le 5.9$ is well correlated with the EM increase in $6.2 \le \log_{10}T \le 6.4$ over eight orders of magnitude. This suggests that the coronal plasma is being heated from the quiet-Sun, sub-MK temperature to 1-2 MK, more typical for ARs. Potential field extrapolation indicates significant change in the local magnetic connectivity: the dimming region is now linked to the newly emerged flux via longer loops. We conclude that emerging dimming is likely caused by coronal heating episodes, powered by reconnection between the emerging and the ambient magnetic fields.