Daytime and nighttime mid-latitude ionospheric disturbances and their delayed occurrence after geomagnetic activity

Abstract

For a mid-latitude station the occurrence rates on ionograms of nighttime spread-F, daytime first-hop distortions (produced by medium-scale travelling ionospheric disturbances) and daytime second-hop spread are considered over a five-year period. Superposed-epoch analysis has been used to investigate the second-hop spread relative to days of enhanced activity in the other two parameters. The results show that all three parameters are closely related, with the second-hop spread being related to spread-F occurrence on the night following the daytime occurrence of this second-hop spread. Additional superposed-epoch analyses show that all three parameters are related to enhanced geomagnetic activity which is found to occur during an early-morning period which is displaced by three such periods from the days of enhanced activity for these parameters. Two peaks occur in this-early-morning period, one at 0400 local time, and the other close to local midnight. The paper concentrates particularly on this daytime second-hop phenomenon and sample ionograms are presented to illustrate some of the characteristics of the spread traces which are recorded.