Geo Magnetic Storms
Among the events most disruptive to HF communications are geomagnetic storms.which are caused by major bursts of radiation and particles from the sun. When these bursts arrive at the earth they disturb our magnetic field which results in increased noise and changes to the composition of the ionosphere.
The folowing graph was published in the Pacificon QRP Forum Oct 17 1998. The comprehensive article is credited to Paul Harden NA5N