Sporadic E is a form of radio propagation widely used by radio hams. It is one of the most interesting forms of propagation - as the name indicates it occurs sporadically and many of the mechanisms behind it are not well understood.
Sporadic E arises when clouds of intense ionisation occur in the E region of the ionosphere. Initially these will affect the lower frequency bands, and it can produce openings on 10 metres and as the frequency rises, 6 metres, 4 metres and occasionally 2 metres may be affected. When 2 metres is affected, openings are often quite short - an hour or two is long, but openings of a few minutes only are known.
During the course of an opening the direction of the propagation can change significantly as the ionisation clouds are blown about by the winds in the upper atmosphere.
Find out more about this mysterious form of radio propagation that can be sued to good effect on many of the HF and VHF amateur bands: