Much apparent day-to-day variability in the H component of the Sq variation in low and middle latitudes is not due to a variability in ionospheric currents, but arises from contamination by often unrecognized disturbance field variations associated with distant agnetospheric currents. There appears to be little, if any, day-to-day variability in the shape, type or phase of the pure Sqvariation. There is, however, a substantial day-to-day variability in the daily range of Sq and this has a worldwide rather than a local cause. No conclusive evidence exists for motion of the Sq foci. In low and middle latitudes the Sq variation is essentially a daytime phenomenon.
Production of ionospheric 1-m irregularities in the auroral zone E region by high power ionospheric heating has been observed by means of the STARE auroral radar. Irregularities were produced at night, on eight occasions by using an O mode heating wave and on two occasions by using an X mode heating wave. The growth time of the irregularities is found to be approximately 30 s. Several physical possibilities to explain these findings are discussed, and a mechanism is suggested that is closely related to Sato's mechanism of quiet auroral arc formation.
The storm‐time variations of the ionosphere in middle latitudes during ionospheric storms are described. Usually the total electron content is increased during the first 24 hours of the storm. The total content then falls to below its quiet‐day value and recovers after one or two days. The peak electron concentration in the F region is similarly increased at the beginning of the storm and is later depressed. The fall in peak electron concentration commences some hours earlier than the fall in total electron content. The equivalent slab thickness is increased above the quiet‐day value throughout the storm, corresponding to an increase in the electron‐ion temperature. In weak magnetic storms the ionospheric disturbance may not last longer than one day, and only the initial elevation of total electron content occurs.
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