Eruptions of activity from the Sun can cause intense streams of particles and radiation to be ejected into space. If these strike the Earth, they can cause large variations in ionospheric conditions or "ionospheric storms". There are usually geomagnetic storms in progress at the same time.
Both types of storm cause a reduction in the maximum usable frequency that the F2 layer of the ionosphere will support. There is also an increase in the amount of ionisation in the D layer where a great deal of radio frequency energy is absorbed. These two effects lead to a depression in ionospheric conditions. In extreme cases HF propagation may no longer be possible.
Such storms vary in size and can cause depressed conditions from one to several days. They tend to recur after 28 days when the same sun spot group on the Sun's surface comes back into view (the Sun has rotated once).
