Sporadic E or E_s is an unusual, sporadically occurring form of enhanced radio-wave propagation.

It was discovered in the early days of VHF radio transmissions, and greatly increases the long-distance propagation of radio waves in the region of 20 - 250 MHz.

It's known that the phenomenon is caused by radio-wave reflections from 'clouds' of highly ionised air in the Earth's 'E Region' - an atmospheric layer in the Ionosphere also known as the Heaviside layer.

No conclusive theory has yet been formulated as to the origin of Sporadic E. Attempts to connect the incidence of Sporadic E with the eleven-year Sunspot cycle have provided tentative correlations. There seems to be a positive correlation between sunspot maximum and Es activity in Europe. Conversely, there seems to be a negative correlation between maximum sunspot activity and Es activity in Australasia.

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