A study of tidal and planetary wave periodicities present in midlatitude sporadic E layers

The diurnal and semidiurnal atmospheric tides are known to be of fundamental importance in the formation of midlatitude sporadic E layers, acting through their vertical windshear forcing of the long-living metallic ions in the lower thermosphere. Also, recent studies suggested that planetary waves play a role on sporadic E generation as well, a fact that went unnoticed in the long-going research of sporadic layers. In this paper a methodology is employed to investigate the tidal and planetary wave periodicities imprinted onto sporadic E critical frequencies foEs. In this approach, standard analysis techniques used in neutral atmospheric dynamics are applied on foEs time series obtained during summertime when sporadic E occurrence is nearly continuous. It is shown that besides the dominant and known 24-hour and 12-hour tidal periodicities in foEs, there is often a weaker terdiurnal (8-hour) oscillation present as well. In addition, there are planetary wave periodicites in foEs with periods near the normal Rossby modes, that is, 2, 5, 10, and 16 days. It is also found that the tidal oscillations in foEs undergo a strong amplitude modulation with periods comparable to the dominant planetary wave periodicities present in the data. Our results are in line with recent findings based on a single event study which suggested that sporadic E layers are affected indirectly by planetary waves through their nonlinear interaction and modulation of the atmospheric tides at lower altitudes. The close relationship between neutral wave dynamics and midlatitude sporadic E periodicities suggests that the ionosonde data can be used as an alternative means of studying tidal and planetary wave characteristics and their climatology in the lower thermosphere.