Moderate Nesting and Cross-Equatorial Asymmetry of Active Regions in Solar Cycle 24

TL;DR

This paper quantifies how sunspot emergence clusters into activity nests during Solar Cycle 24 using the SPEAR and HMI datasets, and it examines how nest detection depends on the chosen rotation frame. It identifies 40–50 nests per hemisphere under Carrington-rate criteria, with up to ~$48$% of AR flux participating in nests, and finds significant nest flux at prograde and retrograde synodic frequencies, suggesting additional dynamical processes beyond rigid Carrington rotation. Hemispheric analysis reveals strong antisymmetry: ~58% ARs co-occupy longitudes across the equator, while nests show far weaker cross-hemisphere intersection (~8%), with random trials rarely reproducing this pattern. The results imply antisymmetric dynamo modes or related mechanisms govern nesting, offering constraints for solar dynamo models and cross-equatorial coupling in stellar magnetism.

Abstract

Solar Cycle 24 data are used to determine how often the Sun emerges sunspots in `activity nests', i.e., regions where sunspots and active regions (ARs) repeatedly emerge. We use the Solar Photospheric Ephemeral Active Region (SPEAR) catalog created from Helioseismic and Magnetic Imager (HMI) data as well as the HMI Carrington Rotation maps of radial magnetic field, $B_r$. The Sun shows moderate nesting behavior with 41\% (48\%) of AR magnetic flux found in Northern (Southern) hemispheric nests that are short-lived ($\sim$4 months). Different rotation rates are used to search for nests that may not be evident `by eye'. The maximum number of nests are found with slightly prograde rotational velocities, with significant nest flux also found at synodic 451--452 nHz prograde and 409--411 nHz retrograde frequencies. Nest patterns show strong hemispheric asymmetry, indicating that the physical origin of nests identified herein must also be asymmetric or antisymmetric across the equator.

Figures (5)

• Figure 1: This cartoon depicts active region distribution similar to the Sun with a moderate degree of activity nesting (left), a moderate degree of activity nesting with active longitudes (middle), and a cool star like the Sun with stronger nesting and active longitudes. Many cool stars show a a modulation of intensity signal with rotation that is much stronger than that of the Sun.
• Figure 2: The Northern hemispheric magnetic activity is shown for Cycle 24, May 2010 through Dec 2020, using HMI Carrington Rotation maps of $B_r$ data (top, panel a). The color scale is such that ARs are shown in light blue (weakest), yellow, green, and red (strongest) on a dark blue background that indicates quiet-Sun. Activity nests are identified in the SPEAR catalog data using the criteria of three ARs found within 4 CRs with $\pm$7.5$^{\circ}$ longitude and $\pm$5$^{\circ}$ latitude of each other with rotational frequency at the b) Carrington rate, and c) slightly prograde with respect to the Carrington rate. Different nests are shown using different colors while the symbol size is proportional to the amount of flux of the region.
• Figure 3: The same as Figure 2 but for the Southern hemisphere.
• Figure 4: The percent of flux found in activity nests with criteria of three ARs within 4 CRs within 4 CRs with $\pm$7.5$^{\circ}$ longitude and $\pm$5$^{\circ}$ latitude of each other is shown as a function of rotational frequency. One, two and three sigma statistical significance levels are plotted as horizontal lines as determined from 10,000 trials where the longitudes of the ARs are randomized. Note that the other quantities associated with the AR - latitude, time and flux - are not randomized. Rotational frequencies are shown in red (orange) for frequencies if the amount of flux found in nests is above three (two) sigma significance levels.
• Figure 5: The distribution of ARs from the North and South hemispheres is shown (top panel) in blue and red symbols with some dilation of $\pm$9$^{\circ}$ longitude and $\pm$3 Carrington rotations. The number of intersections of North and South is noted as 0.579 ($\sim$58%). Nests identified using the Castenmiller criteria of 3 ARs in 4 CRs are shown (middle panel) with only 7.8% of all the ARs in the nests intersecting. Randomization trials show that this hemispheric asymmetry in the nest locations is more asymmetric than $\sim$92% of trials, although this randomization is not a true test of the significance of asymmetry as the identification of nests themselves are already at a 3$\sigma$ level.