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A sudden ionospheric disturbance (SID) is an abnormally high ionization density in the D region of the ionosphere caused by a solar flare. The SID results in a sudden increase in radio-wave absorption that is most severe in the upper medium frequency (MF) and lower high frequency (HF) ranges. (Adapted from Federal Standard 1037C and from MIL-STD-188.)
From the American Association of Variable Star Observers: "Earth's ionosphere reacts to the intense X-ray and ultraviolet radiation released during a solar flare. The ionospheric disturbance enhances VLF radio propagation. By monitoring the signal strength of a distant VLF transmitter, sudden ionospheric disturbances (SIDs) are recorded and indicate a recent solar flare event."[1]
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