Small-scale ionospheric troughs detected over a range of mid-latitude locations

Spread-F structures at three spaced mid-latitude stations (Canberra, 45° CGlat; Bribie Island, 36° CGlat; Townsville, 28° CGlat) obtained by N(h) analyses reveal modulated ionospheric height rises, electron-density depletions with spread-F recorded during the recovery stage of these ionospheric structure changes. At Bribie Island fixed-frequency phase-path measurements (at 1.98 MHz) reveal a travelling ionospheric disturbance (TID) wavetrain of several cycles (periodicity 25 m) before and during the height rise associated with the spread-F event. When these mid-latitude spread-F characteristics (including speeds of movement) are compared with those related to the daytime trough in high-latitude regions the two phenomena are found to be similar. However, the magnitudes of the high-latitude changes are much greater than for the mid-latitude spread-F events. Because of these similarities the spread-F electron-density depletions reported here have been called small-scale troughs. It is suggested that both phenomena may result from the breaking of atmospheric gravity waves