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 0.01 to 0.1 ergs/square centimeter/second and the letter X
if it is above a value of 0.1. A multiplier number is also attached to the
description so that an X5.0 flare has a power of
0.5 ergs/square centimeter/second.
Class M flares, particularly the less energetic ones, are likely to
cause a fadeout 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 day light 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 of greatest interest to those affected by the sun.
The table lists, in order of importance defined by X-Ray class,
the most significant solar flares since 1976 at which time
regular X-Ray data first became available. The list
includes all flares for which the X-Ray class was equal to, or above,
X10.0. All of these flares
had the most dramatic effects on HF communications and other
systems.
Educational 
MAG: Normal