Large Solar Flares Since 1976

The X-ray classification of solar flares is a most useful measure of thestrength of a flare. In this classification the most energetic flares aregiven a descriptive letter M if the X-ray power output is in the range of10^-9 to 10^-8 joules/square centimetre/second and theletter X if it is above a value of 10^-8. A multiplier number isalso attached to the description so that an X5.0 flare has a power of5*10^-8 joules/square centimetre/second.

Class M flares, particularly the less energetic ones, are likely to cause asudden ionospheric disturbance on only the lowest frequencies of the HighFrequency (HF) radio spectrum. On the other hand X class flares will cause afadeout for all HF frequencies over the entire daytime hemisphere of theEarth.

Class X flares are also more likely to be associated with a host ofinteresting effects here on Earth and in space. It is the class X flares whichare 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 firstbecame available). The list includes all flares for which the X-ray class wasX10 or greater. These flares had the most dramatic effects on HF communicationsand other systems. Note that flux values for flares above X15-17 might beestimated.

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 20flares occurred in this year. Six of the flares occurred in June 1991.
• The latest flares after solar maximum to be included in this list werethe May 1984 flare (about 4.5 years after solar max) and the September 2005flare (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 theearliest in a cycle to make the list - only 25 months after solar minimum.

Material prepared by Richard Thompson