Statistical Overview of Long-Lived Active Regions Observed Across Multiple Carrington Rotations

TL;DR

The paper tackles the challenge of statistically characterizing long-lived solar active regions (LLARs) by constructing a conservative LLAR catalog, defined as ARs observed across at least two consecutive Carrington rotations without a significant post-emergence flux surge. It integrates full-Sun EUV observations, far-side helioseismic data, and NOAA AR designations to enable robust lifecycle analyses beyond single-rotation observations. The study finds LLARs to be larger and more flux-rich than typical ARs but not notably more magnetically complex, while being substantially more flare-productive across GOES flare classes, suggesting that longevity relates to stronger, more concentrated flux rather than complexity alone. These results have practical implications for space-weather forecasting and AR heating studies, and the authors release the LLAR dataset to the community for further analysis across solar cycles and cycles of solar activity.

Abstract

The study of solar active regions (ARs) is of central importance to a range of fundamental science, as well as the practical applications of space weather. Active region emergence and life cycles are two areas of particular interest, yet the lack of consistent full-Sun observations has made long-term studies of active regions difficult. Here, we present results from a study to identify and characterize long-lived active regions (LLARs), defined as those which were observed during at least two consecutive Carrington rotations and which did not undergo significant successive flux emergence once the decay phase began. Such active regions accounted for 13% of all NOAA-identified ARs between 2011 and 2019, and their distribution closely follows the annual sunspot number. This implies that LLARs are produced by the same basic driving processes as regular ARs. LLAR areas tend to be significantly larger and contain more magnetic flux compared to other ARs, but the two categories have similar magnetic complexity distributions. The most striking result, however, is that LLARs are 3-6 times more likely than other ARs to be the source of a solar flare of GOES class C or greater. This highlights the importance of studying what makes a LLAR and how to identify them at emergence with a view towards improved space weather forecasting. The further implications of these findings for AR heating spatial and temporal patterns will be explored in an upcoming study.

Figures (5)

• Figure 1: Example of the methodology used to identify LLARs. (a) A LLAR's initial NOAA AR designation, as displayed with Helioviewer with AR labels plotted from HEK. The LLAR is highlighted with a cyan box. (b) The same AR, highlighted with a magenta box, as confirmed through visual inspection in the CHMAP annual Carrington map movie. (c) One Carrington rotation later in the CHMAP movie, the LLAR still appears at the same location. (d) Identification of the new NOAA AR designation for the LLAR.
• Figure 2: (a) Plot of the annual sunspot number and the number of LLARs detected in each year from 2011-2019. (b) Median latitudinal distribution of LLARs and non-LLARs in each year over the same period.
• Figure 3: (a) Histogram comparing LLAR and non-LLAR magnetic flux by year and percent of the median AR flux (calculated over all ARs), in the years 2011-2019. (b) Analogous plot, but for area as a percent of the overall median AR size (calculated over all ARs). (c) Scatter plot showing the relationship between AR area and total unsigned flux for both LLARs and non-LLARs, and linear fits to both categories (the x axis is log-scaled). The r-squared value for the LLAR and non-LLAR fits are about 0.91 and 0.78, respectively. Note: the number of overlapping points in the 10-100% range is very high; this plot is not in contradiction to the distributions seen in (b).
• Figure 4: (a) Distribution of magnetic complexity, as measured by Mt. Wilson class at disk-center passage, across LLARs and non-LLARs. (b) The same plot, but for the maximum Mt. Wilson class recorded during front-side passage.
• Figure 5: Sample image of the layout of the Solar Active Region Spotters project on Zooniverse. The top row shows the SDO AIA composite images for the two candidates (this case is not a match), while the bottom left shows a single HMI magnetogram of the original AR and the bottom right shows a short HMI movie of the subsequent AR's evolution. The red flags in the top left of each figure denote that the subject pair shown has already garnered enough classifications to be retired.