The observed compression and expansion of the F2 ionosphere as a major component of ionospheric variability

Abstract

AbstractThis paper examines a number of sources of ionospheric variability and demonstrates that they have relationships in common which are currently not recognized. The paper initially deals with medium to large‐scale traveling ionospheric disturbances. Following sections deal with nontraveling ionospheric disturbance (TID) ionospheric variations which are often repetitious from day to day. The latter includes the temporary rise in F2 height associated with sunset in equatorial latitudes resulting from strong upward drift in ionization driven by an E × B force. The following fall in height is often referred to as the premidnight collapse and is accompanied by a temporary increase in foF2 as a result of ionospheric compression. An entirely different repetitious phenomenon reported recently from middle latitudes in the Southern Hemisphere consists of strong morning and afternoon peaks in foF2 which define a midday bite‐out and occur at the equinoxes. This behavior has been speculated to be tidal in origin. All the sources of ionospheric variability listed above exhibit similar relationships associated with a temporary expansion and upward lift of the ionospheric profile and a fall involving a compression of the ionospheric profile producing a peak in foF2 at the time of maximum compression. Such ionospheric compression/decompression is followed by a period in which the ionospheric profile recovers. Such relationships in traveling ionospheric disturbances (TIDs) have been noted previously. The present paper establishes for the first time that relationships hitherto seen as occurring only with TIDs are also present in association with other drivers of ionospheric variability.