Large Solar Flares Since 1976
The X-ray classification of solar flares is a most useful measure of the strength of a flare. In this classification the most energetic flares are given a descriptive letter M if the X-ray power output is in the range of 10-9 to 10-8 joules/square centimetre/second and the letter X if it is above a value of 10-8. A multiplier number is also attached to the description so that an X5.0 flare has a power of 5*10-8 joules/square centimetre/second.
Class M flares, particularly the less energetic ones, are likely to cause a sudden ionospheric disturbance on only the lowest frequencies of the High Frequency (HF) radio spectrum. On the other hand X class flares will cause a fadeout for all HF frequencies over the entire daytime hemisphere of the Earth.
Class X flares are also more likely to be associated with a host of interesting effects here on Earth and in space. It is the class X flares which are of greatest interest.
The following table lists, in order of importance defined by X-ray class, the most significant solar flares from 1976 (when regular X-ray data first became available). The list includes all flares for which the X-ray class was X10 or greater. These flares had the most dramatic effects on HF communications and other systems. Note that flux values for flares above X15-17 might be estimated.
Some interesting aspects of the table are:
- Only 2 of the 20 flares occurred prior to the maximum of the cycle (December 1979 for Cycle 21 and July 1989 for Cycle 22).
- The year of 1991 was the worst year for large solar flares - 7 of the 20 flares occurred in this year. Six of the flares occurred in June 1991.
- The latest flares after solar maximum to be included in this list were the May 1984 flare (about 4.5 years after solar max) and the September 2005 flare (about 5.5 years after solar max - although the Cycle 23 peak to minimum length was quite long, about 8.5 years). The July 1978 flare was the earliest in a cycle to make the list - only 25 months after solar minimum.
Material prepared by Richard Thompson