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

Jiasheng Wang, Yu Xu, Zhengyong Hou

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~Å 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.

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

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 -- 10~K. Potential field extrapolation identifies two dominant magnetic configurations: low-lying loops with cool plasma (10 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~Å passband ( 10 K), consequently lacking plasma at the typical 10~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.
Paper Structure (10 sections, 2 equations, 7 figures, 1 table)

This paper contains 10 sections, 2 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observations
  3. Data Analysis
  4. DEM Calculation
  5. PFSS Extrapolation
  6. Results
  7. Variation of Intensity and Thermal Energy
  8. Magnetic Field Analysis
  9. Discussion
  10. Summary

Figures (7)

  • Figure 1: An overview the AR 12738 in SDO/AIA EUV channels on Apr. 14 2019. (a-f) show the active region with bright core (peripheral dimming) regions indicated by blue (red) contours in images of 94, 131, 171, 193, 211, and 335 Å respectively. The contours are obtained from AIA 171 Å image with method described in Section \ref{['sec:3']}.
  • Figure 2: Evolution of the AR. (a1-a4) and (c1-c4) show the AR in AIA 171 images at 8 different central meridian crossing times. (b1-b4) and (d1-d4) show magnetograms of the AR at corresponding times. Color bar scales from -50 to 50 G.
  • Figure 3: Synoptic magnetogram and footpoints of field tracing. The black dots in upper panel show the Carrington projected footpoints in the HMI image, each dot indicate a group of 25 footpoints around it. The blue curve indicates PIL across the AR at solar surface. The red (blue) dashed contours indicate core (dimming) region of the AR. The black and purple (cyan) streamlines represent closed and positive (negative) open field, respectively.
  • Figure 4: The average intensity variance (a) in the dimming region and (b) in the AR core region in AIA 171, 193, 211, and 1600 Å. All the intensities are divided by the corresponding value at Time 1 (marked as 0 offset Carrington period) in Table \ref{['tab1']}. Dots represent the relative intensity calculated from observations, and the curves show the varying trend.
  • Figure 5: The DEM curves (a) in the dimming region and (b) in the AR centering region. The grey solid lines in panel (a) and (b) are the time-averaged DEM curves in the selected quiescent region (refers to the text). DEM uncertainties are shown as colored areas. Panel (c) shows the DEM-weighted temperature varying with time in the two sets of regions.
  • ...and 2 more figures