SHORT-TERM DELAYS IN THE OCCURRENCE OF MIDLATITUDE IONOSPHERIC DISTURBANCES FOLLOWING OTHER GEOPHYSICAL AND SOLAR EVENTS

Five years of ionograms recorded at a mid-latitude station, Moggill (near Brisbane, Australia) have been examined to determine days of high and days of low activity for 3 ionospheric disturbance parameters. These are nighttime spread-F, daytime first-hop distortions and daytime second-hop spread. Superposed-epoch analyses using these days as controls show that statistically above-average occurrence levels for these parameters can be expected for low geomagnetic activity a few hours before the recording periods for these parameters. Conversely when geomagnetic activity is high a few hours earlier the occurrence levels of these parameters are suppressed. Also, a 3 day delay after enhanced geomagnetic activity at specific local times (reported earlier) is also illustrated and discussed. When mid-latitude spread-F occurrence levels are considered on a global scale it is found that changes in the levels of occurrence are in antiphase with the 10.7 cm solar flux, in particular the 27-day variation due to the solar rotation. It seems likely that these relationships with geomagnetic activity and the 10.7 cm solar flux can be explained in terms of the upper-atmosphere neutral-particle density (UA-NPD) changes which are associated with these geophysical parameters. It is argued that the UA-NPD changes modulate the wave amplitudes of the medium-scale travelling ionospheric disturbances in such a way that large wave amplitudes (needed for significant disturbance conditions) result from lower levels of the UA-NPD. The results suggest that further work on these short term relationships may in the future facilitate the short-term forecasting of mid-latitude disturbance conditions.