OBSERVING THE SUN IN H-ALPHA
Compiled by David Knisely
 SOLAR FLARES

ORIGINS AND CLASSIFICATION: 
Flares are intense, abrupt releases of energy which occur in areas where the local magnetic field is rapidly realigning or changing because of magnetic field stress. This stress is usually induced by opposing magnetic flux emerging in or close to an existing active region. The new flux must either cancel the existing fields or push them aside. Since the field lines imbedded in solar plasma can rearrange only slowly in response to these changes, magnetic stress can build up to an extreme point, sometimes resulting in a flare. 
The older flux is pushed aside, creating strong gradients at the edges of the emerging field. As a new flux pushes into existing flux of the opposite sign, there is some immediate realignment or "reconnection" of the fields (marked by H-alpha brightening), but as the material is pushed out of the way, the field lines are sheared or drawn out along the line perpendicular to the motion, and a Neutral Line is formed, defined as the sheared magnetic field boundary dividing regions of opposite magnetic polarity. 
Note: None of this happens if the moving spot pushes into flux of the same polarity. Also, the compressing and shearing of fields generally occurs only with moving sunspots and not with plage fields. 
A Neutral Line Filament sometimes forms along the neutral line, supported by horizontal shearing field lines. At high resolution, the filaments and fibrils in the area tend to be elongated and parallel to the neutral line, leading out to or from the main neutral line filament. Magnetic stress caused by this shearing builds to the breaking point, when a lower energy magnetic connection happens, magnetic energy is released in a flare, usually along or near the part of the neutral line where the stress is the highest. 
The rapid realignment of the magnetic fields induces intense electric currents which heat the plasma and produce extreme H-alpha brightening, which for flare brightness is defined as at least twice the normal chromospheric emission level. As the flare dies down, the sheared field line produced by the original motion and marked by the filament is replaced by loop prominences or field transition arches which arc more directly between two opposite polarity areas, tracing out the new field lines nearly perpendicular to the original neutral line boundary. At that point, the magnetic fields are connected in the lowest possible energy state and, unless further flux emerges, the flares should be over. 

The frequency of solar flares is directly related to sunspot activity, with few occurring near sunspot minimum. Near sunspot maximum, small ones occur almost daily, and major flares can happen several times per week. Flare activity (and often intensity) tends to peak in the years near or just after sunspot maximum. 

Most solar flares occur in or close to growing or disturbed active regions, with the largest flares most often associated with Gamma and (especially) Delta spot groups. Solar flares can often be grouped into two classes: Compact, and Major. Compact flares are usually smaller and somewhat more frequent than major flares. 

They often occur in a pre-existing loop or arch filament system, and little structural change in the area is observed. Compact flares can be seen in or near Emerging Flux Regions, and produce mainly small surges or none at all. Subflares are the smallest of the compact class, and are short-lived, being only slightly brighter than active plage. Major flares are considerably more violent and longer lived, frequently producing large surges or sprays of bright gas. 

They often emit intense X-rays and masses of energetic particles (Coronal Mass Ejections)that later can trigger geomagnetic disturbances on Earth. Major flares often cover large areas of the sun and cause plage brightening expanding outward across the solar disk. Moreton waves can occasionally disturb or disrupt some filaments which lie in their paths, sometimes making them vanish, only to reform later near their original location. 

Flares seen on the solar disk frequently show two areas of emission on either side of the magnetic inversion line, because energy released anywhere in a flux tube will rapidly heat the surface at its two footpoints where it meets the surface. When many lines of force are involved, two ribbons of emission (Twin Ribbon Flare) appear. In great flares, the strands rapidly elongate on either side of the neutral line and separate at 5-20 km/sec while narrow flare loop prominences form to connect them, rising higher in the corona. 

If one ribbon is near a sunspot, it will be small and bright, because many flux lines converge there. The ribbons will not cross the spot since the other side involves magnetic field lines connected away from the flare. In the late stages, the strands evolve into two thin lines formed by the intersection of a thin shell of hot coronal material with the surface. Since reconnection means that two tubes of force interchange their end points, one expects four areas to brighten, and in larger flares these often can be picked out. A few flares will sometimes display only one or even three distinct ribbons instead of two or four, although the reason for this is unclear. 

Solar flares are ranked in importance by optical, X-ray, or radio flux. Soft X-ray intensity is measured in the 1-8 Angstrom range monitored by the GOES weather satellites. The classes are designated by the letters Bn(n x 10-7 w/m2), Cn(n x 10-6 w/m2), Mn(n x10-5 w/m2, or Xn(n x 10-4 w/m2), where n is the integer for each power of ten. Thus a flare classed as a M3 would produce a soft X-ray flux of .00003 watts per square meter. Optically, flares are ranked by the area in square degrees of heliocentric latitude they take up on the disk. A square degree at the center of the solar disk is 12,147 km on a side, or at the sunís mean distance, each side of the square would be about 17 seconds of arc across. The optical class ranges from S (subflares) to 4 (largest). 

AREA(sqr.deg) OPTICAL CLASS TYPICAL SOFT X-RAY CLASS

2.0 or less S(subflares) C2 

2.1-5.1 1 M3 

5.2-12.4 2 X1 

12.5-24.7 3 X5 

More than 24.7 4 X9 

*A suffix(f,n,b)is added if the brightness is faint, normal, or bright, based on a visual estimate.

FLARE SEQUENCE: 
Some gradual H-alpha brightening may often precede many flares. Frequently (especially in major flares), the neutral line filament (or another nearby Active Region Filament), rises tens of minutes before the flare; it may get exeptionally dark, blue-shifted, or broadened in H-alpha. Then, the flare breaks out with brilliant H-alpha emission in what is known as the Flash Phase. 

Flare emission usually consists of three parts: small bright Kernels (often the first feature seen) where the H-alpha line is broad and the intensity is up to three times the photospheric continuum, and extensive area of narrower (approx. 1 Angstrom) emission directly involved with the main energy release, and bright post-flare loops connecting the two ribbons. 

As large flares erupt, the neutral line filament will often blow away, forming a spray, while in other cases, the filament either expands upward into a loop-like eruptive prominence, or it breaks up with considerable twisting and turbulence at the start of the flare. In addition, material dispersed by a flare near the limb may be seen coming down again as "Coronal Rain" after the flare dies down. 

A filament superimposed on plage or a sunspot will usually erupt in a flare because of the conflict between the nearly vertical plage/umbral magnetic field and the horizontal filament field. If the filament does not blow away, the area may flare again (homologous flares), since the magnetic shear stress is still present. 

Frequently, a flare will occur towards the particular end of a neutral line filament where magnetic flux conflict from moving sunspots is the greatest. Occasionally, the neutral line is not marked by any one distinct filament, or has a filament which is very narrow and difficult to see. This often happens when f polarity flux suddenly emerges just ahead of a well developed p spot. Then, the flares seem to come out of nowhere (sometimes producing a surge), however, they are still near a neutral line. Prior clear neutral line filaments may also not be easily seen when an EFR is rapidly replacing weaker existing fields, triggering compact or smaller flares. 

Most flares have a fairly rapid initial rise in brightness, approaching approaching maximum intensity in only a few minutes. The brightness then stays high for a slightly longer period than the rise time before declining slowly. However, a few flares or flare-like phenomenon classed as Long Duration Events (LDEs) have a more gradual rise in brightness and are less impulsive, occasionally lasting up to 12 hours. 

WHITE LIGHT FLARES: 
Intense flares which have bright emission over dark penumbrae or umbrae may occasionally be briefly visible (less than 10 min.) in white light as small bright patches. However, white light flares are rare. White light "light bridges" between umbrae are not white light flares, since they are a relatively long-lived purely photospheric phenomenon and only mark places where sunspot fibrils are weak or absent. 

MAJOR FLARE PRECURSORS: 
Exact flare prediction is difficult at best. However, each of the following circumstances(alone or in combination), may indicate that a big flare may occur in the future. 

1). Delta groups, particularly those of origins 1 and 2. 

2). Sunspot Umbrae obscured by H-alpha emission or large umbrae without penumbrae. 

3). Very bright H-alpha emission which marks flux emergence. 

4). New flux erupting on the Leading side of the penumbra of a dominant p spot. 

5). A filament crossing a delta spot group. 

6). Any strongly sheared magnetic configuration (inverted groups, large-scale highly curved fibril alignment, ect.) 

  

SPOTLESS FLARES: 
Occasionally, flares will occur when a large filament imbedded in extensive plage left over from an old active region whose sunspots have decayed erupts. As the old fields decay, the filament becomes unstable and erupts upward, producing brightening over a fairly wide area. They are not usually as violent as major flares, and little brightening occurs if the filament is very far from the plage. 

  

FOR FURTHER SOLAR INFORMATION: 
Zirinís book Astrophysics of the Sun is probably the best source of detailed information, and I highly recommended it, especially for its photographs. The text is fairly technical in places, containing much undergraduate and graduate-level Physics. The information relevant to the amateur tends to be a bit scattered throughout the book, so be prepared to do some searching. I make absolute guarantees about the accuracy of all the information I have condensed and presented here. It is intended only as a general guide to benefit the amateur H-alpha observer, and may not be reproduced for profit. You may correspond with me about this article at the following address.  
David Knisely 
1616 North 14th Street 
Beatrice, Nebraska 68310 

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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