[2] The effect was discovered by John Howard Dellinger around 1935 and also described by the German physicist Hans Mögel (1900-1944) in 1930.
When a solar flare occurs on the Sun a blast of intense ultraviolet (UV) and x-ray (sometimes even gamma ray) radiation hits the dayside of the Earth after a propagation time of about 8 minutes.
This high energy radiation is absorbed by atmospheric particles, raising them to excited states and knocking electrons free in the process of photoionization.
These fadeouts last for a few minutes to a few hours and are most severe in the equatorial regions where the Sun is most directly overhead.
SIDs are observed and recorded by monitoring the signal strength of a distant VLF transmitter.