Sporadic E propagation results from nodes of plasma (ionised gas) set up in the E region of the ionosphere. There, existing ions of helium and hydrogen are augmented by metal ions from ablated atoms from meteors.
When cooked by the sun in the upper regions, the metal ions form nodes of ionisation. Then they descend. As they enter the E region at about 100km above the Earth, they add to plasma already there, comprising ions of hydrogen and helium. These E region high-density nodes are stable in the short term and support refraction of waves in the VHF bands from about 28MHz to 144MHz.
These nodes are often referred to as clouds, or more romantically, patches. ‘Patches’ is very apt since they are of all differing sizes and thicknesses – typically randomly from about 0.5km to 5km thick and randomly from 10 to 1,000 square kilometres in area. In the summer in the Northern Hemisphere the patches are largest and thickest towards the Tropic of Cancer at 23°’N and smallest and thinnest towards the Arctic Circle at 66°N.
The diagram below shows this idea, though the patch size has been enlarged and stylised.
The descending nodes of plasma enter the E region at three important times of day – early morning, mid-day and early afternoon, and after midnight over the mid-path. The highest density nodes occur after mid-day. This is illustrated by the diagram below.
The result of this plasma is enhanced VHF propagation occurring over anything from a few minutes to several hours each day, predominantly in the summer months in the Northern Hemisphere and to a lesser extent in January.
Occurrence of patches is sporadic.
When favourable, contacts are possible over distances from about 500km to about 6,000km. Greatest distance is available to those using data modes like FT8 (see elsewhere on this site for discussion on system values for SSB and FT8).
Other discussions under this topic deal with how the nodes come to form. A composite model is proposed.