Sometimes, especially during periods of high solar activity, the magnetic lines in the Sun
can become so twisted that they eventually release a large amount of energy in the form
of a solar flare. These flares produce a large amount of x-ray radiation. This radiation
can strongly ionize the D layer in a short amount of time, a minute or less, resulting in a
large amount of attenuation of radio waves passing through the affected region (that is,
the side of the Earth facing the Sun). Large flares can cause ionization levels
sufficient to completely stop all shortwave propagation, causing the infamous "HF Blackouts".
Several satellites in orbit around the Earth continuously monitor the x-ray levels from the
Sun, and can detect solar flares as they occur.
Solar flares are categorized according to their strength, using letters and numbers.
The letters range are A,B,C,M and X, and a number follows the letter, such as C4.5 or
M8.2, etc. A flares are the weakest, and X flares the strongest. There is no upper limit.
The strongest flare ever recorded was an X28, in November of 2003. It was possibly higher,
some think as high as X40, but the sensors used to measure flares cannot go that high.
gives you the current solar x-ray conditions, as well as graphs showing the past several
days readings, and alerts you to solar flares immediately when they occur.
In addition to major solar flares, there is a constant background of x-ray radiation from
the Sun. In general, the lower this background level, the better radio propagation conditions
Large solar flares can also eject bursts of charged particles, called Coronal Mass Ejections.
These CMEs can strike the Earth if the flare occurs on a region of the Sun facing the
Earth. If so, they can disrupt the Earth's magnetic fields, causing auroras (northern and
southern lights) which can affect shortwave radio propagation. They can also cause generally
higher noise levels, and weaker signals.