Radio occultation measurements performed by the satellites CHAMP, GRACE and FORMOSAT-3/COSMIC provide a huge data set for atmospheric and ionospheric investigations on a global scale. The data sets are used to extract information on sporadic E layers appearing in the lower ionospheric E region. This is done by analyzing signal amplitude variations of the GPS L₁ signal. Sporadic E altitudes in northern midlatitudes found from radio occultation measurements are compared with ground-based ionosonde soundings. A large correlation of sporadic E altitudes from these two techniques is found.

Scintillations in the signal-to-noise (SNR) profiles of GNSS radio occultation (RO) measurements at lower ionospheric altitudes are caused by compact layers of high ionization also known as sporadic E (Es) layers. It is widely accepted that Es layers are formed by the wind shear mechanism at northern and southern midlatitudes. However, approaching the equatorial regions, electric fields also control the formation and dispersal of these layers. In this paper we concentrate, in particular, on the occurrence rate and on the altitudes of Es layers appearing in a narrow band along the Earth’s magnetic equator. We analyzed several million of RO profiles concerning sporadic E occurrence as well as altitude and observed a high daily, seasonal and longitudinal variability which is controlled partly by zonal winds and electric fields. Especially Es layers at higher altitudes show a clear anticorrelation with the zonal electric field intensity measured by the Swarm satellites. Further, we solve the existing contradiction of Es layer signatures being present in equatorial ionosonde measurements while they are only rarely seen in RO recordings. Graphical Abstract

The development of sporadic-E (Es) layers over five Digisonde stations in the American sector is analyzed. This work aims to investigate the dynamic of such layers during the days around the geomagnetic storm that occurred on 8 September 2017. Therefore, a numerical model (MIRE) and Radio Occultation (RO) technique are used to analyze the E layer dynamics. The results show a downward movement in low-middle latitudes due to the wind components that had no significant changes before, during, and after the geomagnetic storm. In fact, our data and simulations showed weak Es layers over Boulder, Cachoeira Paulista, and Santa Maria, even though the winds were not low. However, the RO data show the terdiurnal and quarterdiurnal influence in the Es layer formation, which can explain this behavior. In addition, we observed an atypical Es layer type, slant Es layer (Es_s), during the main phase of the magnetic storm over Boulder. The possible cause of the Es_s layers was gravity waves. Another interesting point is the spreading Es layer occurrence associated with the Kelvin–Helmholtz Instability (KHI). Finally, it is confirmed that the disturbed electric field only influenced the Es layer dynamics in regions near the magnetic equator.

Sporadic E (Es) layers are a well-known ionospheric phenomenon, whose occurrence can cause anomalous propagation of radio waves utilized for communication and broadcast. Es layers are very thin layers of metallic ions such as Fe+, Mg+, and Ca+ formed mainly by atmospheric tidal wind shears. Their peak density exceeds the E-layer peak density and can even exceed the F-layer peak density. Because of their effect on radio waves a user needs to know when there is a high probability of Es occurrence. It is the goal of this study to develop a global model of Es occurrence probability and to make it publicly available through inclusion in the International Reference Ionosphere (IRI).We use Es data obtained by Arras et al. (2022) from GNSS radio occultation observations from various satellites (CHAMP, Spire, KOMPSAT-5, COSMIC1 & 2, TANDEM-X, and TerraSAR-X) for the time period 2001-2022. We will briefly discuss the variation of the Es occurrence rate with latitude, longitude, local time, season, solar activity, and magnetic activity. as observed with this data base. We found that the strongest dependencies are with local time, latitude and season, while weaker ones exist with longitude and solar activity. We use spherical harmonics to describe the global expansion of the strongest influences establishing a core model. Second order influences, e.g., solar activity variations, are modelled as a perturbation on the core model. First results obtained with the new model will be presented.ReferencesArras, C., Resende, L.C.A., Kepkar, A. et al. Sporadic E layer characteristics at equatorial latitudes as observed by GNSS radio occultation measurements. Earth Planets Space 74, 163 (2022). https://doi.org/10.1186/s40623-022-01718-y