Terdiurnal Signature of the Critical Frequency of the Sporadic E Layer in Mid-Latitude Regions Based on Ionosonde Observations

Ionospheric sporadic E layers are very thin, but with a much higher electron density than normal E regions that occur at altitudes of about 90–130 km. Vertical wind shear is considered the main source of mid-latitude sporadic E layer formation, which leads to periodicity, such as 24-h, 12-h, and so on. In this paper, a time series analysis of the critical frequency of the sporadic E layer (foEs) observed by an ionosonde is performed at seven stations (spanning about 37° N–51° N and 29° S–67° S) to investigate the terdiurnal signature in it. Except for the already known 24-h and 12-h periodicities features which are related to diurnal and semidiurnal tides, new findings are also obtained. The 8-h periodicity is a regular and repeatable feature at high mid-latitude regions of both hemispheres. The 8-h periodicity is more prominent at mid-latitudes (~50° N and ~60° S) during the winter and spring months of the hemisphere, which agrees with the terdiurnal tide features. It also shows that the amplitude of the 8-h periodicity is equivalent to the 12-h periodicity component in summer and autumn and almost the same as the 24-h periodicity component in winter under certain circumstances. This indicates that the 8-h periodicity should be taken into consideration for sporadic E layer modeling.