Height and Intensity Variations of Mid-Latitude Sporadic E-Layers Observed by FORMOSAT-3/COSMIC and Ionosondes

Abstract

Although the climatology of sporadic E (Es) layers has been studied with Radio Occultation (RO) and ionosonde techniques, detailed comparisons between the findings of these two different methods remain limited. The present study uses 11 years of COSMIC mission and ionosonde observations to investigate diurnal and annual variations in the intensity and height of midlatitude Es layers, providing a comprehensive comparative analysis. Radio Occultation observations show regular diurnal and annual variations in Es layer intensity and height. The ${S4}_{\max }$ index, which has been identified with the Es intensity, takes maximal values before local noon and later in the day near sunset. In addition, the RO Es layer heights (${hEs}_{RO}$) are found to be higher near sunrise and late afternoon hours. The ionosonde critical frequencies ($foEs$) exhibit a broad maximum at prenoon, but not a second peak later in the day. The virtual layer heights ($h^{\prime }Es$) maximize at the same time as it happens in RO observations, taking, however, values that are distinctly higher than the corresponding ${hEs}_{RO}$ ones. Given that $foEs$ and $h^{\prime }Es$ overestimate the layer intensities and heights during daytime, recent studies proposed methods to correct these ionosonde-measured parameters and thus replace them with the layer critical metal ion reflection frequency ($fo\mu Es$) and the real height ($hEs$). By adopting these methodologies in the present analysis, the agreement between the diurnal mean variations, obtained separately from RO and ionosonde measurements, was improved significantly. In addition, the yearly RO observations showed that Es layers intensify enough to become detectable at higher altitudes at sunrise and maximize during daytime. This supports a recent proposition where metal atom photoionization (MAP) plays a key role in shaping the sporadic E diurnal cycle.