Optical Solar Flare Classification
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Flares are classified on the basis of area at the time of maximum brightness in H alpha.

Importance 0 (Subflare): < = 2.0 hemispheric square degrees
Importance 1: 2.1-5.1 square degrees
Importance 2: 5.2-12.4 square degrees
Importance 3: 12.5-24.7 square degrees
Importance 4: > = 24.8 square degrees
[One square degree is equal to (1.214 x 10E+4 km)squared = 48.5
millionths of the visible solar hemisphere.]

A brightness qualifier F, N, or B is generally appended to the importance character to indicate faint, normal, or brilliant (for example, 2B).

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