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date: 18 January 2018

Evershed effect

A Dictionary of Astronomy

Ian Ridpath

Evershed effect 

An outward flow of gas in the penumbra of a sunspot, starting from the boundary with the umbra and moving radially out through the penumbra, and sometimes a little beyond it. The effect can be observed spectroscopically, particularly for spots near the solar limb, but may also be visible as outward-moving facular points, forming a so-called moat. The maximum outflow velocity is about 2 km/s. There is an inverse Evershed flow at higher altitudes, in which the flow (up to 20 km/s) is directed both inwards and downwards towards the sunspot. The effect is named after the English astronomer John Evershed (1864–1956), who discovered it in 1909.