Author: Aimee Norton

Oct 31, 2022

Figure 1 - NOAA 11158 is a δ-spot in the southern hemisphere that produced multiple X-class flares in 2011. It emerged in a single flux emergence event with a quadrupolar magnetic configuration. The top two panels show HMI SHARP continuum intensity and radial field strength from 2011.02.13. The bottom two panels show the same region 26 hours later. The negative (positive) umbrae participating in the ‘magnetic knot’ are outlined in blue (red). The tilt angle calculated as the flux-weighted centroids of all umbrae in the active region obeys Hale’s and Joy’s laws and does not rotate. In contrast the magnetic knot is first identified as anti-Hale and anti-Joy then rotates over 100 degrees at which point it obeys Hale’s and Joy’s law. The configuration of the region and its behavior is consistent with an inverted kink instability.

Active regions in which positive and negative magnetic polarity umbrae share a penumbra are known as δ-spots. They are disproportionately responsible for the most energetic flares in any given solar cycle. During Solar Cycle 24, approximately 8% of active regions were δ-spots and 132 of those were observed by HMI. The observed characteristics of δ-sunspots are not consistent with a single formation mechanism but can be formed by flux rope collisions, multi-segment buoyancy, a kink instability acting on a highly twisted flux tube, or convective buffeting of the flux tube. Using HMI data, we isolate and analyze the ‘magnetic knot’, i.e., the adjacent umbrae creating the δ-configuration, within the active region, as well as analyzing the entire active region in which the knot is embedded. Compared to the more common β-spots in which the magnetic polarities are well-separated, δ-spots have 2.6x the maximum umbral flux, 1.9x the flux emergence rate, and 72x the flare energy. On average, the magnetic knots rotate 17° day-1 while the β-spots rotate 2° day-1. Approximately 72% of the magnetic knots present anti-Hale or anti-Joy tilts. We find that 84% of the δ-spots are formed by single flux emergence events and 58% have a quadrupolar magnetic configuration. The observed δ-spot characteristics are consistent with the formation mechanism signatures as follows: 42% with the kink instability or convective buffeting, 32% with multi-segment buoyancy, 16% with collisions and several regions that are unclassified.

Norton, A. A., Levens, P. J., Knizhnik, K. J., Linton, M. G., & Liu, Y. 'Characterizing the Umbral Magnetic Knots of δ-Sunspots'. ApJ, 938, 117 (2022). DOI:10.3847/1538-4357/ac8eb2

Originally published in the COFFIES Press Newsletter, Volume 2, Issue 2.