Short-Term Delays in the Occurrence of Mid-Latitude Ionospheric Disturbances Following Other Geophysical and Solar Events.

Abstract

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 shortterm forecasting of mid-latitude disturbance conditions.

Short‐term delays (hours) of ionospheric spread F occurrence at a range of latitudes, following geomagnetic activity

“…However, it can now be reported that similar inverse relationships exist for PM spread F in Rz-minimum years and AM spread F in Rzmaximum years. Direct relationships were also found and reported [Bowman, 1995], connecting geomagnetic activity during the hours around midnight with AM spread F. This paper will report on results from low-and middle-latitude stations, which are essentially the same as those found for equatorial stations, with both inverse and direct relationships with geomagnetic activity recorded. The paper will also consider nights when the level of ESF occurrence changes from high to zero for a day or two and then changes back to a high level, an aspect of ESF which does not seem to have been reported previously.…”

“…Using controls representing high and low levels of ESF occurrence, Bowman [1995] found an inverse relationship with geomagnetic activity which occurred between 1000 and 1800 local time (LT), but particularly around the times 1200 and 18013 LT. These analyses involved only PM occurrence in Rz-maximum years and AM occurrence in R•-minimum years, sunspot-cycle periods when PM and AM spread F occurrence (respectively) was a maximum.…”

“…Except for periods of very high Rz the annual and solar-cycle variations of AM spread F are similar to those found for midlatitudes. Bowman [1995] has suggested that even in equatorial regions, as seems likely for midlatitudes [Bowman, 1990], the spread F traces on ionograms result primarily Copyright 1998 by the American Geophysical Union.…”

“…In considering spread F occurrence in equatorial regions (ESF), as recorded by ionograms, Bowman [1993aBowman [ , 1995 found it appropriate to subdivide the ESF into premidnight (PM) and after-midnight (AM) periods. The PM period is dominated by the large postsunset height rise, its occurrence and the related spread F being , directly related to the sunspot number (Rz), while the AM period is inversely related.…”

“…The analyses have investigated the short-term responses of spread F occurrence (as identified on ionograms) at low-and middle-latitude stations to geomagnetic activity changes so that comparisons can be made with results from equatorial stations reported earlier [Bowman, 1995]. Using superposed-epoch methods, it is found that the AE index is either enhanced or depressed a few hours prior to spread F occurrence depending on whether or not low or high (respectively) spread F controls are used.…”