Half-year Evolution of a Decaying Solar Active Region and Peripheral Dimming Regions

TL;DR

This study tracks the six-month decay of a solar active region using multi-wavelength SDO data, revealing thermal restructuring, magnetic flux diffusion, and the role of high-lying loops in peripheral dimming.

Contribution

It provides the first long-term analysis of a peripheral dimming region, linking thermal deficits and magnetic configurations to active region decay.

Findings

Peripheral dimming region continuously decreases in area

High-lying loops dominate the dimming, heated above 10^5.8 K

Thermal deficit drives emission reduction, not just plasma absence

Abstract

Using multi-wavelength observations from the Solar Dynamics Observatory (SDO), we investigated the six-month decay process of the solar active region NOAA AR 12738 from April to October 2019. We systematically analyzed the region's evolution by examining extreme ultraviolet (EUV) intensity variations, quantifying magnetic flux diffusion, and investigating thermodynamic changes via Differential Emission Measure (DEM) analysis. This study presents the first long-term tracking of a peripheral dimming region (dark moat), revealing its continuous areal decrease over time. DEM results reveal cooling plasma signatures and thermal restructuring, with the dimming region exhibiting a distinct temperature deficit in range 10$^{5.5}$ -- 10$^{5.9}$~K. Potential field extrapolation identifies two dominant magnetic configurations: low-lying loops with cool plasma ($<$10$^{5.5}$ K), and high-arching structures connecting to the AR core, contributing to localized emission reduction. We found that the dimming is dominated by high-lying loops extending from the AR core, which are heated to temperatures above the main response of the 171~\AA\ passband ($>$ 10$^{5.8}$ K), consequently lacking plasma at the typical 10$^{5.8}$~K formation temperature. The thermal deficit, not just the absence of material, is the key driver of the reduced emission. Our results demonstrate that long-duration dimming provides a valuable diagnostic for understanding active region decay, thermal evolution, and coronal magnetic restructuring.