Periodicities and spread-F associated with large-scale travelling ionospheric disturbances

Abstract

Spread-F associated with large-scale travelling ionospheric disturbances (LS-TIDS) has been discussed and examined by considering fixed-frequency phase-path information for an event on October 24/25, 1990 at Bribie Island, near Brisbane, Australia. The results suggest that radiowave reflections from the titled isoionic surfaces, which are produced by LS-TIDs with certain periodicities, are not responsible for the spread. Instead, it seems likely that the wave amplitudes of medium-scale travelling ionospheric disturbances (MS-TIDs), which are also present at these times, are enhanced when the LS-TID macroscale height changes occur. It is suggested that these wave amplitudes are sufficiently enhanced (due to lower neutral densities) to allow the significant off-vertical reflections which are responsible for the spread. Also, during the equinoctial periods of some sunspot-maximum years, it is shown using phase-path rates of change of the main trace of the F₂ layer that the LS-TIDs have preferred periodicities of 35 and 17 min.