Compiled by David Knisely
(seen in filters with FWHM bandwidths under 0.7 angstrom)

At the edge of the solar limb in H-alpha, the Chromosphere is seen in profile, appearing as an irregular fringe of red light less than 10 seconds of arc high, running all the way around the edge of the disk. At high power (especially in the wings of H-alpha), the individual Spicules making up this fringe are sometimes visible, but they do tend to blend together somewhat. 

Spicules are also visible on the disk as tiny narrow low contrast darker jets which tend to emerge from elements of the Chromospheric Network, an array of long sinuous chains of tiny slightly brighter patches which run over much of the solar disk. The Network is often difficult to see due to its low contrast and the ever-present overlying spicules, but is better shown in the wings of H-alpha, and is enhanced near active regions. 

Also hampering network visibility are the Fibrils, tiny low contrast short narrow filament-like darker features running between nearby points. Groups of longer fibrils which run directly between areas of opposite magnetic polarity are known as Field Transition Arches. Together, spicules and fibrils make up the Dark Mottles, which cover much of the solar disk, and which are often incorrectly referred to as the Network. 

Sunspots are visible in H-alpha, but their prenubras are lower in contrast then in white light. Frequently, fibrils will be seen near sunspots, tracing out the nearby magnetic field lines. Also visible at times in or near active regions is Plage; patchy areas of brightness marking nearly vertical emerging or rapidly realigning magnetic fields. Plage and white-light faculae are related, but are not the same thing, since they often don't occupy exactly the same positions. 




The number and magnetic polarity of sunspots varies according to an approximate 11 year numerical (22 year magnetic) cycle. About 18 months before the old cycle's sunspot minimum, the first new cycle spots may begin to appear near 30 degrees north and south solar latitude, with a few remaining spots from the old cycle straddling both sides of the equator. After the old cycle spots die out, the new ones become more numerous and larger, forming distinct sunspot groups.  

These groups usually consist of a larger leader spot or spots, often followed as the sun rotates by several somewhat smaller trailing spots. Each hemisphere eventually forms an irregular belt of spot activity that slowly drifts towards the equator as the cycle progresses. Near mid-cycle, sunspot number maximum usually occurs, with the main activity belts now being nearly 40 degrees wide, centered around 20 deg. N/S solar latitude (a few short-lived spots have been seen up to 70 deg. N/S) Very large complex groups of spots will be present near and after the maximum, with many having complicated magnetic structure. The number of spots then declines over the next few years, with most forming at lower solar latitudes and fewer large ones developing. As sunspot minimum again approaches, there are few if any spots visible, mostly near 7 deg. N/S. 

Sunspots generally form in magnetically-linked bipolar groups, with each end being one pole of a localized magnetic field called a flux tube. The magnetic configuration of this flux tube (or "dipole") is usually governed by the Hale-Nicholson Rules, which states that the preceding polarity spot is usually the dominant "leader" in most groups for the entire 11 year sunspot cycle. 

For example, in the northern solar hemisphere, the spots leading each group across the sun as the sun rotates (preceding or p) might start out one sunspot cycle having a "north" magnetic polarity. The followers (f) in the same group would then have a "south" polarity. Preceding spots in groups in the southern solar hemisphere would then have a south magnetic polarity and would be followed by the group's north polarity spots. This polarity orientation of sunspot groups will generally be maintained until the next sunspot minimum, when the polarities will reverse for both hemispheres.  

The magnetic axis of the sunspot group is usually slightly inclined to the solar east-west line (Joy's Law), running from 3 degrees near the equator to 11 degrees at latitude 30 N/S, with the preceding polarity spot being slightly closer to the equator. If the axis is highly tilted initially, the group will tend to rotate until the axis is more parallel to the equator. P polarity spots in most bipolar groups tend to be a bit larger and better developed than the somewhat more numerous f polarity spots. 

P spots in developing groups also tend to move westward to the head of the group. If a group starts out with the f polarity leading ("Inverted Polarity"), it will usually die out, or the p spot polarity area behind the f spot will push westward through or past the field of the f spot, creating magnetic shearing and possible flare activity until it regains its rightful place in the leading of the group. Stable sunspots tend to be fairly symmetrical unless there is extensive magnetic shear nearby from emerging magnetic flux or the passing of an area of opposite magnetic polarity. 

Magnetic shearing can cause large portions of sunspot penumbras to distort or vanish. Large spots generally form from the merger of smaller ones. Large spot groups can be over 182,000 km long and usually result from the emergence of several flux tubes, since individual dipoles rarely exceed 50,000 km in length. 

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 
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