ORIGINS AND CLASSIFICATION:
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).
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
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:
MAJOR FLARE PRECURSORS:
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.)
FOR FURTHER SOLAR INFORMATION:
Complete H-Alpha Handbook: