Sporadic E or Es layers are thin, transient, and highly ionized plasma layers in the ionospheric E region, that exhibit significant spatial and temporal variability. Their behavior is governed by a complex interplay of atmospheric and ionospheric processes. They play a critical role in radio wave propagation, affecting communication and navigation systems, and also serve as indicators of the atmosphere–ionosphere coupling. Despite their importance, many aspects of Es layer formation and variability are still poorly understood, particularly regarding their modulation by atmospheric tides, planetary waves, and ionospheric processes. This thesis aims to address these knowledge gaps by investigating Es variability using satellite-based observations. Several years of global radio occultation data are used to investigate Es layer variability, with a focus on mid- and low-latitude regions. A comprehensive spectral analysis of Es occurrence rates reveals the dominant role of atmospheric tides, including both solar and lunar components. Migrating and non-migrating tidal modes are identified as key drivers of vertical wind shears, which, according to the wind shear theory, facilitate metallic ion convergence into thin, dense layers. Seasonal and latitudinal variations of diurnal and semidiurnal tidal components are quantified, offering new insights into the modulation of Es layers by these atmospheric oscillations. This thesis also examines the role of planetary waves, and ultra-fast Kelvin waves, in Es variability. These waves contribute to Es variability through horizontal ion convergence and wave–driven modulation of the background wind field. These findings are supported by a detailed spectral analysis of Es intensity and further validated by geopotential height data, providing robust observational evidence of the relationship between planetary waves activity and Es layer formation. Additionally, a comparative analysis of ionosonde and radio occultation detection methods is conducted, highlighting systematic differences arising from their distinct measurement principles. While ionosondes offer high temporal resolution at specific locations, satellite techniques are more effective in capturing the global Es variability. These findings emphasize the complementarity of the two methods for capturing Es variability across different spatial and temporal scales. This thesis advances the understanding of sporadic E layer modulation through atmospheric waves. The results not only deepen our understanding of Es variability but also provide insights into atmosphere-ionosphere coupling processes. These findings have a direct impact on space weather forecasting, ionospheric modeling, and the optimization of communication and navigation systems that rely on ionospheric stability. The results of this study establish a basis for future research into atmosphere-ionosphere interactions and the development of predictive capabilities for Es layer behavior.
Sahar Sobhkhiz-Miandehi (Thu,) studied this question.