Flare classification and Reporting

Flares are classified on a rather curious scale, running A, B, C, M and X. It is logarithmic, and is calibrated as follows:

Class Energy

X 10-4..10-3 W/m2 (measured at 0.1nm to 0.8nm wavelength)
M 10-5..10-4 W/m2
C 10-6..10-5 W/m2
B 10-7..10-6 W/m2
A 10-8..10-7 W/m2

As it is logarithmic (base 10), each class can be subdivided 1.0 to 9.9, leading to a flare being quoted as C5.6 or M1.2. The background level from a ‘quiet’ sun is often within A or B-class, with most flares being of B-class or C-class. More energetic active regions produce M-class or X-class flares. At the extreme, a flare can exceed X9.9, and produce X17 or X20 flares. Flares of X-class pose a threat to orbiting satellites, as well as human space travellers. C-class flares are easily detected as sudden ionospheric disturbances, while some larger B-class events can also be recorded. X-class events produce spectacular SID’s, as the ionosphere slowly recovers.

When recording SID’s, start, peak and end times are required. ‘Start’ is the time at which the event is first recorded, ‘Peak’ is the time at which maximum or minimum signal strength is recorded, and ‘End’ is the time at which the signal strength returns to its previous diurnal trend. Start and peak times are easily read, while the end time often requires a little guesswork to identify.

The amplitude of the disturbance usually correlates with the flare class, but will depend on the state of the ionosphere at the time. Since it is an indirect observation of solar activity, the amplitude is not recorded in VLF reports. The length (duration) of the SID recorded does not always correlate with flare class, but can be recorded as the ‘importance’ of the event on the Earth. This has traditionally been recorded as follows:


Duration (minutes)

Importance Scale



19 .. 25


26 .. 32


33 .. 45


46 .. 85


86 .. 125


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