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Sporadic E
Short-lived patches of ionization that open VHF over long distances.
Short-lived patches of ionization that open VHF over long distances.
Sporadic E (written Es) is a thin, dense patch of ionization that appears in the E region, roughly 100 km up, for minutes to hours. It is dense enough to reflect frequencies far above what the E layer normally handles, so it can open the 10 m, 6 m and occasionally 2 m bands to stations far beyond the horizon. "Sporadic" is the honest word: it comes and goes, and no one can promise it.
The leading explanation is that winds at different heights in the E region shear against each other and squeeze long-lived metal ions, left behind by burning meteors, into thin, dense sheets. A sheet of electrons that dense reflects VHF as a mirror would. The details are still debated, which is partly why forecasting it is hard.
Geometry then sets the distance. The layer is so low that a signal leaving along the horizon meets it at a shallow angle, and the ground distance for one hop is limited to roughly 2,200 km. Most hops are shorter. Two hops, with a bounce off the ground between them, reach roughly twice as far, which is how Es can occasionally link far-apart regions.
In the northern hemisphere Es is most likely from late spring through summer, with a smaller peak around midwinter, and during the daytime and early evening. The southern hemisphere shows the same pattern shifted by six months. It is largely independent of the solar cycle, so it can liven up 10 m and 6 m even when the sunspot count is low.
Openings can start suddenly and end the same way, and the clouds drift, so signals often swell and fade within minutes. A band may be loud while the one above it is silent, because each cloud has its own ceiling. Several clouds can open different directions at once, so a beacon on the band tells you more than a forecast.