Solar Flare Intensity

Several systems are in use for measuring the intensity of solar flares. Older systems are based on the appearance of the flare through ground-based telescopes, while newer ones use data from spacecraft.

The most commonly used classification at present measures the maximum flux of X-rays (of wavelength 0.1 to 0.8 nanometer) produced by the flare. The approximate flux is indicated by a letter A, B, C, M, or X , with A being the weakest flares and X the strongest. (The letters C, M, and X stand for Common, Medium, and Xtreme.) Numbers after the letter, as in C6.2 or X14, measure the flux more precisely according to the scheme shown in the table below.

Flare Class

Maximum X-Ray Flux
watts per square meter

Maximum X-Ray Flux
ergs per square centimeter per second


n x 10-8

n x 10-5


n x 10-7

n x 10-4


n x 10-6

n x 10-3


n x 10-5

n x 10-2


n x 10-4

n x 10-1

Examples: M4.2 = 4.2 x 10-5 W/m2; X16 =16 x 10-4 W/m2 = 1.6 x 10-3 W/m2. For A, B, C, and M flares the value of n is between 1.0 and 9.9, but for X scales the value of n can be as large as needed.


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