Sudden Ionospheric Disturbance (SID)

In solar-terrestrial terms, HF propagation anomalies due to ionospheric changes resulting from solar flares, proton events and geomagnetic storms.

An abnormally high ionization density in the D region caused by an occasional sudden solar flare, i.e., outburst of ultraviolet light from the Sun. Note: 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.