Compiled by David Knisely
Suffixes p or f are used when the preceding or following polarity spot, respectively, is dominant.

ALPHA: A single dominant spot, often linked with a plage of opposite magnetic polarity
BETA: A pair of dominant spots of opposite polarity (Bipolar, ie: a leader and a follower
GAMMA: Complex groups with irregular distribution of polarities.
BETA-GAMMA: Bipolar groups which have more than one clear north-south polarity inversion line.
DELTA: Umbrae of opposite polarity together in a single penumbra. 

Just over half of the observed groups in Beta-p or Alpha-p, with the larger groups most often being Beta-p, Beta-Gamma, or Delta. Delta groups are generally very active and often are the site of major solar flares. 

In white light, an early indication of a developing sunspot group is often the appearance of a few tiny pores, sometimes separated by a short distance, and possibly accompanied by faculae. In H-alpha, a bipolar group begins as a small bright oval area of plage with a few small surges. Several hours later, the pores and Arch Filament System (AFS) superimposed on very bright plage begin to appear, forming what is known as an Emerging Flux Region (EFR). 

The AFS often takes the form of several closely spaced fine filaments that arch between the emerging magnetic flux tubes's poles (often marked by pores), tracing out the field lines like those of a bar magnet. These AFS filaments show upward motion (blue shift) at the tops and downward flow at the ends (red shift). Small bright points of H-alpha emission lasting only a few minutes known as Ellerman Bombs can often be seen near the middle of the EFR.  

Rapid EFR growth then follows with some of the pores intensifying or merging to for the first sunspot umbras. Usually, the p, or preceding polarity spot will form first and then move westward relative to the following (f) polarity spot (spot "Proper Motion") to the eventual head of the group at about 1 km/sec. One to two days after formation, the first spots frequently form penumbras, their proper motion stops or slows, and the arch filaments connected to them usually disappear or erupt upward. 

The p spots may continue to move westward as long as new flux continues to emerge (this usually leaves some plage and a limited AFS near the center of the group where a few more pores or spots might eventually form). Bright plage with arch filaments are good signs of continuing emerging magnetic flux. F spots will either remain relatively fixed or will drift slightly eastward. In large active spot groups, the f spots may occasionally continue to develop until they rival the p spots in size. 

Sometimes, a p spot will develop accompanied by only bright plage and a few small filaments instead of trailing spots (Alpha group). Plage is rarely seen ahead of the p spots unless satellite opposite polarity is emerging. If small emerging flux regions emerge offset from the central AFS axis, a finger of plage forms extending to the new pole until its fields reconnect. If the EFR comes up deep inside a plage, the opposite polarity spot will be surrounded bye Field Transition Arch fibrils connecting to the plage. Beta groups often form when only one or two adjacent flux tubes with the same orientation come up together. Extended groups can form from the emergence of several dipoles end to end, sometimes yielding Beta-Gammas if polarities are mixed. Multiple flux tubes emerging fairly closely together in sequence or tilted to each other can form Gamma or Delta groups.  

As magnetic flux emergence ceases, the individual spots of the dipole(s) frequently interact with local fields and other spots, behaving as if they had never been connected to each other. The main dominant spot will often form magnetic connections to distant fields, sometimes forming a "moat" around itself consisting of a thin patchy ring of brightening and a group of nearly radial H-alpha fibrils roughly twice the penumbral width. Once the active region becomes "mature" (reaches maximum longitudinal spread), it will tend to slowly decline and fade away unless more flux emerges to keep it alive. 

Mature spots rarely move very much, nor do they show much activity unless new flux emerges nearby. The spot motion that leads to magnetic shear and flares is almost always in an Emerging Flux Region. However, flux is somewhat more likely to emerge near where it has previously emerged, so even mature spots bear a little watching. Normal EFR development results in a fairly peaceful group which may have produced a few minor flares. After a few quiet weeks of existence, its spots usually shrink and decay into two diffuse unipolar areas, sometimes marked by weak H-alpha brightening and/or a filament. In declining groups, the smaller f polarity spots usually vanish before the dominant spots begin to decay. Most groups finally decay into a single p spot with no plage, which then slowly shrinks and dies out. 

Deltas are some of the largest and most active areas on the sun. the Delta is defined as two or more umbrae of opposite polarity which are inside a single penumbra or penumbral area. The opposite polarities are generally within two degrees of each other. Deltas usually form in one of three ways. 

1). A single complex emerges at once with the dipoles intertwined and polarities reversed from the Hale-Nicholson rules (ie: f polarity leading p). this is sometimes known as an "Island Delta" group. 

2). Large satellite polarity areas emerge close to existing spots so that the expansion of the emerging flux region pushes a p spot into an f spot or vice versa. 

3). A growing bipolar spot group collides with another separate dipole so that opposite polarities are pushed together (this is the most frequent mode of delta group formation). It only forms from emerging umbrae, not plage. If the new dipole emerges into plage only, modest flares may occur without delta spot group formation. If it emerges under of collides with an umbra of opposite polarity, the delta spot forms and larger flares occur. If the dipole collides with an umbra of the same polarity, the two do not necessarily merge, but coexist peacefully. 

Usually, Deltas show most of the following features: 

1). Delta groups are almost always large, and 90% of those with inverted polarity have a high level of activity, especially in the occurrence of major flares. They often have a complex, irregular, or "broken" umbral look. 

2). Delta groups form by joining opposite polarity spots from different dipoles, which are connected by sheared magnetic field lines, instead of direct (free space) lines of force. All the spots are inside the same prenumbral area. 

3). Delta spots rarely last more than one solar rotation and are somewhat shorter-lived than other spots of the same size (however, new delta spots may emerge in the same complex). 

4). The polarity is generally inverted as compared to the Hale-Nicholson Rules. 

5). Delta spots do not usually separate, but die out locked together (rarely, umbrae are ejected from the group). 

6). Active Delta regions are usually marked by bright H-Alpha emission especially when it occurs over umbrae marking continued flux emergence. Sometimes, a filament can be seen coming out of, or crossing the group. 

David Knisely's Complete H-Alpha Handbook: 
Part #2: Glossary of H-Alpha terms. 
Part #3: Solar Prominences.  
Part #4: Common Visible Disk Features / Solar Activity.  
Part #5. Mt. Wilson Classification of Sunspot Groups.  
Part #6. Solar Flares.  
Back to the Solar Section 
Back to ICSTARS Astronomy