A complex combination of sudden changes in the condition of the ionosphere, and the effects of these changes. A sudden ionospheric disturbance usually occurs in association with a solar flare and is seen only on the sunlit side of the earth. The return of the ionosphere to its “normal” condition following a pronounced sudden ionospheric disturbance usually takes from half an hour to an hour. The following are the most important effects accompanying a sudden ionospheric disturbance: 1) shortwave fadeout, a condition in which there is a marked and abrupt increase in absorption in the D-region for high-frequency (HF) radio waves, and a consequent loss of long-distance radio reception in this range of frequencies; 2) magnetic crotchet, a sudden change in the horizontal component of the earth's magnetic field due to an increase in the conductivity of the lower ionosphere, the changes being in the nature of an augmentation of the normal quiet-day magnetic change; 3) sudden enhancements of long-wave atmospherics recorded in the frequency range between 10 and 100 kHz, due to the improved reflectivity at oblique incidence of the D-region for such low-frequency radio waves; 4) sudden phase anomalies of discrete low-frequency radio waves (10–100 kHz), due to a lowering of the D-region; and 5) sudden field-strength anomalies of distant low-frequency radio signals (10–100 kHz), due to interference between the ground wave and the sky wave.