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A Statistical Study of delta-Sunspots from Solar Cycle 23 to 25

Rohan E. Louis

TL;DR

This study analyzes $\delta$-sunspots across solar cycles $23$–$25$ (1996–2024) and their relation to solar flares using SolarMonitor NOAA AR data, GOES X-ray flare catalogs from HEC, Hinode/XRT, and SIDC synoptic sunspot records. It distinguishes $\delta$-phase from non-$\delta$-phase ARs to quantify flare counts and energies as functions of lifetime, latitude, and Hale class, finding that $\delta$-ARs disproportionately produce M- and X-class flares even when their numbers are small (e.g., SC25). The results show a nearly symmetric latitudinal distribution in SCs $23$–$24$ but a flatter, more scattered pattern in SC$25$, with median $\delta$-AR lifetimes around $5$ days and non-$\delta$ AR lifetimes around $8$ days. These findings highlight data-quality biases across catalogs and the need for cross-instrument calibration to reliably compare cycles and interpret trends in $\delta$-AR–flare productivity.

Abstract

Sunspots or active regions (ARs) with a delta-magnetic configuration are known to be associated with strong eruptions such as flares and mass ejections. This article investigates the relationship between delta-ARs and flares over the course of three solar cycles (SCs), from 1996 to 2024, with respect to the former's area, lifetime, latitudinal distribution, and phase of its magnetic complexity. Solar cycle 25 has produced the least number of delta-ARs in comparison to the previous two solar cycles, yet the number of M- and X-class flares exceed that of cycle 24 by 25%. Flare occurrence is higher in C-, M-, and X-class events during the presence of the delta-configuration which is seen in all 3 SCs. The total number of flares produced by delta- and non-delta-ARs were 15875 and 17033, respectively across all 3 SCs. The latter are dominated by B- and C-class flares, while the number of M- and X-class flares across all 3 SCs was significantly less than compared to delta-ARs. The median lifetime of an AR in the delta- and non-delta phase is about 5 and 8 days, respectively. The typical number of flares produced by a delta-AR is 20, with maximum values ranging from 80-156 for lifetimes between 6-13 days. However, about 30% of delta-ARs do not produce flares when their lifetimes are between 6-12 days. The latitudinal distribution of delta-ARs across the northern and southern hemispheres is nearly symmetric on either side of the equator for SCs 23 and 24, peaking around +/-10 deg-20 deg. For solar cycles 23 and 24, about 30% of the host delta-ARs have an area exceeding the mean value over the above latitudinal belt while for SC 25, there is a large scatter possibly due to the cycle still being in progress. It remains to be seen if the latter phase of SC 25 will be as active as its earlier phase and whether the number of delta-ARs emerging during that period scale with the total sunspot number.

A Statistical Study of delta-Sunspots from Solar Cycle 23 to 25

TL;DR

This study analyzes -sunspots across solar cycles – (1996–2024) and their relation to solar flares using SolarMonitor NOAA AR data, GOES X-ray flare catalogs from HEC, Hinode/XRT, and SIDC synoptic sunspot records. It distinguishes -phase from non--phase ARs to quantify flare counts and energies as functions of lifetime, latitude, and Hale class, finding that -ARs disproportionately produce M- and X-class flares even when their numbers are small (e.g., SC25). The results show a nearly symmetric latitudinal distribution in SCs – but a flatter, more scattered pattern in SC, with median -AR lifetimes around days and non- AR lifetimes around days. These findings highlight data-quality biases across catalogs and the need for cross-instrument calibration to reliably compare cycles and interpret trends in -AR–flare productivity.

Abstract

Sunspots or active regions (ARs) with a delta-magnetic configuration are known to be associated with strong eruptions such as flares and mass ejections. This article investigates the relationship between delta-ARs and flares over the course of three solar cycles (SCs), from 1996 to 2024, with respect to the former's area, lifetime, latitudinal distribution, and phase of its magnetic complexity. Solar cycle 25 has produced the least number of delta-ARs in comparison to the previous two solar cycles, yet the number of M- and X-class flares exceed that of cycle 24 by 25%. Flare occurrence is higher in C-, M-, and X-class events during the presence of the delta-configuration which is seen in all 3 SCs. The total number of flares produced by delta- and non-delta-ARs were 15875 and 17033, respectively across all 3 SCs. The latter are dominated by B- and C-class flares, while the number of M- and X-class flares across all 3 SCs was significantly less than compared to delta-ARs. The median lifetime of an AR in the delta- and non-delta phase is about 5 and 8 days, respectively. The typical number of flares produced by a delta-AR is 20, with maximum values ranging from 80-156 for lifetimes between 6-13 days. However, about 30% of delta-ARs do not produce flares when their lifetimes are between 6-12 days. The latitudinal distribution of delta-ARs across the northern and southern hemispheres is nearly symmetric on either side of the equator for SCs 23 and 24, peaking around +/-10 deg-20 deg. For solar cycles 23 and 24, about 30% of the host delta-ARs have an area exceeding the mean value over the above latitudinal belt while for SC 25, there is a large scatter possibly due to the cycle still being in progress. It remains to be seen if the latter phase of SC 25 will be as active as its earlier phase and whether the number of delta-ARs emerging during that period scale with the total sunspot number.
Paper Structure (13 sections, 10 figures)

This paper contains 13 sections, 10 figures.

Table of Contents

  1. Introduction
  2. Data Sources and Methods
  3. SolarMonitor
  4. Flare Catalog
  5. Synoptic Sunspot Data
  6. Results
  7. Number of $\delta$-ARs & Hale Class Association
  8. Latitudinal Distribution of $\delta$-ARs and Area of Host AR
  9. Flare Classes during $\delta$- and Non-$\delta$-Phase of ARs
  10. Flare Distribution in $\delta$-ARs as a Function of Lifetime
  11. Flare Statistics of $\delta$- and Non-$\delta$-ARs
  12. Discussion
  13. Conclusions

Figures (10)

  • Figure 1: Sunspot numbers from 1996 January 01 to 2024 December 31. The grey and red lines correspond to the daily and monthly sunspot numbers from SIDC. The blue triangles indicate the daily number of $\delta$-sunspots recorded in Solar Monitor. The black plus symbols represent the number of sunspots recorded by SIDC but not by Solar Monitor.
  • Figure 2:
  • Figure 3: Distribution of various Hale classes of $\delta$-sunspots from SCs 23--25.
  • Figure 4: Latitudinal distribution of $\delta$-ARs for SCs 23--25. The number of ARs corresponding to the histogram are scaled with respect to the y-axis on the left. The red solid line with triangles corresponds to the maximum area of the host AR in that latitudinal bin while the red dashed line represents the mean. The mean value has been scaled by a factor of two for clarity. Both quantities are scaled with respect to the red y-axis on the right. The blue crosses correspond to the fraction of $\delta$-ARs whose area exceeds the mean value in each bin and its scale is the same as the histogram y-axis on the left. The dotted horizontal line corresponds to a value of 30% for this fraction. The numbers in the top right corner indicate the number of $\delta$-ARs in that cycle.
  • Figure 5: Distribution of flares in various X-ray classes over the three SCs during the non-$\delta$-phase (top panel) and the $\delta$-phase (bottom panel) of an AR.
  • ...and 5 more figures