New Solar Research Confirms Why Delta Sunspots Are More Flare Worthy Than Other Magnetic Classes

                         Delta spot group photographed by me in November, 1980

 As I first reported in my original sunspot morphology - SID flare research, e.g.

The d -class  groups, first defined by Kunzel (1960),  have for decades been at the forefront of major flare forecasts and observations related to deformation of the magnetic neutral line.   Those d -class spots  featuring many more large flares erupting than other magnetic class spots.  Computer simulations of such deformations peculiar to the deltas were first done by B..C. Low et al (Solar Phys, 1982) and reveal the extent of neutral line changes below:

Step 1a of mutual polarity intrusion

Step 1b of mutual polarity intrusion

In the latter image (1b) one can see the neutral line almost rebounding onto itself.  far more active than spot groups of other magnetic classes, with many more large flares erupting as well as x-ray flares.  It is believed that the most likely reason  has to do with multiple sites of mutual polarity intrusion (different magnetic polarity areas squeezed into each other – deforming the neutral line. 

The configuration is often associated with a single large area sunspot containing within its penumbra two fields of opposite polarity reversed from the normal Hale law – often betrayed by sheared or twisted penumbral fibrils. Close inspection reveals such fibrils in the sunspot photo at top of post.  The important point to note is that as the deformation progresses the magnetic gradient dramatically increases.  The magnetic gradient is written simply as:

grad B = [+B _N - (-B _N)] / x

 where the numerator denotes the difference in the normal components of the magnetic field B _N (between opposite polarities (+, -)  of the active region as measured by vector magnetograph and the denominator denotes the linear (x) scale separation between them.  As x decreases, as it will with neutral line deformation (Step 1b), the gradient spikes and this signals probable flare conditions, namely for a  d -class sunspot given it is most likely to feature such gradient change.

Thus, if I calculate grad B = 0.1 Gauss/km then I know a flare is 96% probable within 24 hrs. based on previous research (Solar-Terrestrial Prediction Proceedings, 1979)  Now new work by Aimee Norton (Stanford Univ.) and collaborators has shed new light on the d -class  formation process.  This work is based on using data from the Helioseismic and Magnetic Imager aboard the Solar Dynamics Orbiter. 

Their primary aim was to compare the characteristics of sunspot groups that contain magnetic knots (as associated with  d -class  spots ) to those that don't called beta sunspots.   These latter spots, while they have (+) and (-) polarities, do not feature severe deformations of the neutral line such as associated with the delta spots.   They are also more common, making up 64% of all sunspots. 

Most interesting is they found the basic b class (left side) able to evolve to one of  bg class as shown in their graphic below:

Intensity (left column) and magnetic field strength and direction (right column) for a single active region at five different times. The sunspots within the active region move into and out of the delta configuration over several days. [Norton et al. 2022]

The key morphological signature of the d -class spots - as I showed in my own 1984 research-  is that the respective (+) and (-) polarities have begun to intrude on one another as they deform the neutral line.  The nascent mixed polarity development, however, is already evident in the bg class spots which suggests potential for further complex evolution.  The mutual polarity intrusion, the primary signature of the deltas, is often precipitated by a mutual clockwise rotation of opposite polarity sunspots about one another.  This I had also shown in my 1983 paper published in The Journal of the Royal Astronomical Society of Canada, e.g. 

http://adsabs.harvard.edu/full/1983JRASC..77..203S

Refer to page 211 (Fig. 2). Where Norton et al improved on my earlier work is in actually quantifying aspects of the  d -class development, thanks to access to the Helioseismic and Magnetic Imager aboard the Solar Dynamics Orbiter.  Thus, they were able to discern the rotation rate (which I could only roughly estimate in my Fig. 2) being more than 8 times that for the b  -spots.  They also were able to determine the typical umbral flux as 2.6 times higher for the d -class sunspots than for the b  -spots.  

They also used a 3D graphic to show how the magnetic intrusion phenomenon occurs, which I showed earlier (using Low et al's simulations) in 2D.   

As can be seen from their graphic above - if one were to say, imagine looking down from above onto the lower panel-  a similar type of polarity intrusion would be at work.  Thus, the extreme deformation in 2D translates to Norton et al's "magnetic knots" in 3D. 

Given the morphological differences in the delta spots it also makes sense the Norton et al researchers found them most likely to "break the rules" in other respects, diverging with standard trends up to 73 percent of the time compared to just 9 percent for beta class spots. (Such as the leader spot in a group in a group being located closer to the equator than the follower spot.  

In many ways, as my earlier solar cycle 20 research showed, the current work marks an important step in our path to understanding why some sunspots are preferentially associated with solar flares and not others.  To see a complete analysis of the comparative properties of delta and beta sunspots see the original research article in the link below:

“Characterizing the Umbral Magnetic Knots of δ-Sunspots,” Aimee A. Norton et al 2022 ApJ 938 117. doi:10.3847/1538-4357/ac8eb2

See Also:

Shock wave from Sun has opened up a crack in Earth's magnetic field, and it could trigger a geomagnetic storm | Live Science

And:

The Role Of Statistics In My Development Of A Geo-Effective (SID) Solar Flare Trigger