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The sporadic E (Es) layer virtual height h′Es${h}^{\prime }Es$and the ordinary wave critical frequency foEs$foEs$are routinely measured ionogram parameters, used to characterize the Es altitude occurrence and intensity. It has become common practice to take the real height hEs$hEs$to be about equal to h′Es${h}^{\prime }Es$by assuming that signal propagation delays in the E region plasma below the layer are small and can be neglected. Although this applies for nighttime, during daytime it may overestimate hEs$hEs$significantly. The present paper relies on true height analysis theory to devise a simplified method and propose an algorithm that can estimate Es real heights reasonably well. The method relies on h′Es${h}^{\prime }Es$and foEs$foEs$ionosonde measurements and E region electron density profiles obtained from the International Reference Ionosphere model. The algorithm is applied to a typical set of Digisonde observations to compute hEs$hEs$and examine real height variations and functional dependencies. Whereas hEs$hEs$≃ h′Es${h}^{\prime }Es$at nighttime, during daytime there are notable h′Es${h}^{\prime }Es$− hEs$hEs$differences taking values less than 10 km for most of the observed layers. During the early morning and early afternoon hours, however, when weak layers appear at upper heights, the virtual to real height differences become larger reaching 20–25 km. The method proposed here for the estimation of hEs$hEs$can be easily applied to improve the accuracy of the results of sporadic E layer studies. Sporadic E (Es) refers to layers of enhanced electron density in the E region ionosphere between ∼90 and 120 km. They have been studied with ionosondes extensively. The ionogram key parameters related to sporadic E are the ordinary wave critical frequency foEs$foEs$and the layer virtual height h′Es${h}^{\prime }Es$. The virtual height is half the elapsed time from the ionosonde to the reflecting layer and back, multiplied by the speed of light. Since the true Es height hEs$hEs$is not measured, it has become common practice to set hEs$hEs$≃ h′Es${h}^{\prime }Es$by assuming that the ionosonde signal suffers negligible retardation in the lower E region below the layer. This approximation is valid during nighttime but not during daytime, especially for weak layers in upper altitudes. This problem has been recognized for a long time but was downplayed. The present paper provides a method and an algorithm that can estimate sporadic E real heights hEs$hEs$. It relies on theoretical principles and uses the ionosonde‐measured Es parameters and E region electron density profiles from the International Reference Ionosphere (IRI) model. The results suggest that the proposed hEs$hEs$algorithm is a useful tool that improves the accuracy of the ionosonde‐based Es studies. Ionosondes measure the sporadic E layer virtual height h′Es$Es$but not the real layer height hEs$hEs$which needs to be estimatedA method and an algorithm are proposed to compute hEs$hEs$from the ionosonde‐measured h′Es$Es$and foEs$foEs$parameters, and IRI‐modeled Ne${N}_{e}$(h) profilesThe method is applied to ionosonde measurements to compute hEs$hEs$and search for diurnal variations and functional dependences on h′Es$Es$and foEs$foEs$ Ionosondes measure the sporadic E layer virtual height h′Es$Es$but not the real layer height hEs$hEs$which needs to be estimated A method and an algorithm are proposed to compute hEs$hEs$from the ionosonde‐measured h′Es$Es$and foEs$foEs$parameters, and IRI‐modeled Ne${N}_{e}$(h) profiles The method is applied to ionosonde measurements to compute hEs$hEs$and search for diurnal variations and functional dependences on h′Es$Es$and foEs$foEs$

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In this work, we investigate various types of ionospheric disturbances observed over Europe following the earthquake that occurred in Turkey on 6 February 2023. By combining observations from Doppler sounding systems, ionosondes, and GNSS receivers, we are able to discern different types of disturbances, propagating with different velocities and through different mechanisms. We can detect co‐seismic ionospheric disturbances close to the epicenter, as well as ionospheric signatures of acoustic waves propagating as a consequence of propagating seismic waves. Unlike the vast majority of past ionospheric co‐seismic disturbance studies that are primarily based on Total Electron Content variations, reflecting disturbances propagating around the F‐region peak, the focus of the present study is the manifestation of disturbances at different ionospheric altitudes by exploiting complementary ionospheric remote sensing techniques. This is particularly highlighted through ionospheric earthquake‐related signatures established as specific ionogram deformations known as multiple‐cusp signatures which appear as additional cusps at the base of the F‐region attributed to electron density irregularities generated by Rayleigh surface waves that generate acoustic waves propagating up to the ionosphere. Therefore this study underlines the advantage that multi‐instrument investigations offer in identifying the propagation of earthquake‐related ionospheric disturbances at different ionospheric altitudes and distances from the earthquake epicenter. The 2023 Turkey earthquake induced a spectrum of ionospheric disturbances as shown by networks of GNSS receivers, ionosondes and HF DopplerDisturbances appeared as shock acoustic wave‐induced traveling ionospheric disturbances in the near field and Rayleigh wave‐induced ionogram multiple‐cusp signature at longer distancesMulti‐instrument remote sensing techniques can detect ionospheric disturbances at a range of altitudes‐distances from the epicenter The 2023 Turkey earthquake induced a spectrum of ionospheric disturbances as shown by networks of GNSS receivers, ionosondes and HF Doppler Disturbances appeared as shock acoustic wave‐induced traveling ionospheric disturbances in the near field and Rayleigh wave‐induced ionogram multiple‐cusp signature at longer distances Multi‐instrument remote sensing techniques can detect ionospheric disturbances at a range of altitudes‐distances from the epicenter

We present an analysis of the evolution of the ionospheric Total Electron Content (TEC) and its variability index Vσ during the Starlink launch disaster on 3–8 February 2022 two‐phase geomagnetic storm. JPL GIM‐TEC global maps with increased spatial (1° in latitude and longitude) and temporal (15 min cadence) resolution as well as SWARM satellite data have been used in the analysis. Comparison of data for five time periods including Starlink launches with geomagnetic Dst index demonstrates thermospheric mass densities increasing by about 50% during the development of the storm on 3–5 February. A spectrum periodogram analysis is applied on TEC at sunrise and sunset solar terminator (STh) at an altitude h= 300 km. Periodicities in the solar terminator TEC generated waves are estimated as 24 hr, 12 hr, 1 hr, 30 min, 20 min, and 15 min. A significant morning−evening asymmetry resulted in the density anomalies concentrated in a certain—evening—sector of terminator when plasma is shifted to the dusk side forming a global asymmetry. Neutral and electron plasma density at the dawn−dusk terminator increased globally, during the storm on 3–5 February 2022Sunrise‐sunset Total Electron Content (TEC) oscillation was observed in all cases with asymmetry of morning−evening values with dominant evening magnitude maximizing during the stormSunrise‐sunset TEC exhibited oscillation with periods of 24 hr, 12 hr, 1 hr, 30 min, 20 min, and 15 min as extracted from JPL GIM‐TEC global maps Neutral and electron plasma density at the dawn−dusk terminator increased globally, during the storm on 3–5 February 2022 Sunrise‐sunset Total Electron Content (TEC) oscillation was observed in all cases with asymmetry of morning−evening values with dominant evening magnitude maximizing during the storm Sunrise‐sunset TEC exhibited oscillation with periods of 24 hr, 12 hr, 1 hr, 30 min, 20 min, and 15 min as extracted from JPL GIM‐TEC global maps

The bottomside thickness and shape (B0 and B1) are the critical key elements for depicting a realistic electron density profile in the International Reference Ionosphere (IRI) model. We investigated their longitudinal variability using a large database of FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) electron density profiles covering the period 2006–2015 from which these parameters are derived using a simple analytic Chapman least squares fitting. The parameters are then compared with the IRI‐2012 submodels, that is, Bil‐2000, Gul‐1987, and ABT‐2009 during different seasons and solar activity. Besides apparent adherence of B0 to the equatorial region, the opposite occurrence pattern of maximum B0 to that of NmF2 is realized, manifesting semiannual distribution and hemispheric asymmetry. Unlike NmF2, the presence of single anomaly crest in B0 in all seasons except March equinox and their leading local time stamps are among the key observations. In spite of regular representation of IRI submodels during equinox, the performance of Gul‐1987 seems to be inferior during solstice with respect to geomagnetic field and equatorial electrodynamics, thereby depicting an erroneous distribution of B0. The striking wave‐like feature in B0 longitudinal variability is also revealed indicating its possible connection with the nonmigrating diurnal/semidiurnal tides. Moreover, the solar activity effect on B0 appears to be relatively higher over the Asian and Pacific longitudes (60–150°E). Although speculations are made over B1 variability, its complete longitudinal characteristics are indistinguishable in our results, except the subordinate magnitude as compared to IRI estimations. The important findings from this study may reinforce the understanding of bottomside variability and future improvements in IRI. Longitudinal variability of B0 and B1 is investigated from FORMOSAT‐3/COSMIC occultation profilesLongitudinal wave‐like feature in B0 indicates nonmigrating tidal influences with latitudinal anticorrelation connection with NmF2Discrepancy among the bottomside submodels in IRI is noticed with the relatively improved performance in ABT‐2009

The largest geomagnetic storm in solar cycle 24 occurred during 17–18 March 2015 where the main phase of the storm commenced from 07:00 UT of 17 March 2015 and reached the Dstnegative minimum at 22:00 UT. The present paper reports observations of total electron content (TEC), amplitude, and phase scintillations from different GPS stations of India during the storm of 17 March and highlights its effects on GPS. It also presents the global equatorial spread F(ESF) occurrence during the storm using total ion density drift measurements from Communication and Navigation Outage Forecast System (C/NOFS) satellite. TEC enhancements were noted from stations along 77°E meridian around 10:00 UT on 17 March compared to 16 and 18 March indicating positive storm effects arising out of equatorward neutral wind in the local morning to noon sector of the main phase. Intense scintillation observations from Calcutta were most extensive during 15:00–16:00 UT, 17 March, and the receiver recorded a longitude deviation of 5.2 m during this time. Cycle slips of the order of 8 s could be observed during periods of intense phase scintillations on the same night. Intense scintillation observation from Palampur is an exceptional phenomenon attributed to the dramatic enhancement of the electric field due to prompt penetrating (undershielded) electric leading to a very high upward ion velocity over the magnetic equator as recorded by C/NOFS. The total ion density measured globally by C/NOFS reveals two distinct longitude regions of ESF occurrence during the storm: (i) East Pacific sector and (ii) Indian longitude during the storm. The time and longitude of ESF occurrence could be predicted using the time of southward turning of interplanetary magnetic field Bz. TEC enhancement observed associated with 17‐18 March 2015 geomagnetic stormIntense scintillation observations beyond the northern crest of EIACycle slips and position deviations were recorded by GPS from Calcutta during the St. Patrick's Day storm

Midlatitude ionosonde observations show that there is a sporadic E (Es) diurnal cycle that starts in higher altitudes at sunrise. This property is labeled as the “Sporadic E sunrise effect.” Given that sporadic E layers are composed of metal ions, the sunrise effect implies that metal atom solar photoionization could play a role in the diurnal variability of sporadic E occurrence. This possibility is endorsed by Arecibo's incoherent scatter radar observations, showing that weak ion layers at lowest E and uppermost D region heights appear at sunrise to live during the daytime, apparently coming out of nighttime metal atom mesospheric layers. The solar photoionization of metal atoms increases the abundance of metal ions available for Es layer generation during the daytime, whereas this effect is absent at nighttime. This can explain why sporadic E layers start or intensify at sunrise all year round and why Es activity maximizes during sunlit hours, as has been reported in many ionosonde and satellite radio occultation studies. The significance of metal atom solar photoionization on the regular diurnal variation of Es went unnoticed, despite existing evidence for a long time. The present paper provides a base for a better physical understanding of the pronounced 24‐hr periodicity in Es layer intensity and places a step toward the improvement of simulation models for the predictions of sporadic E layer characteristics. “Sporadic E” (Es) is a term introduced in the ionosonde studies to describe layers of enhanced ionization that form in the midlatitude E region. The sporadic E formation mechanism involves long‐living metal ions of meteoric origin which converge vertically in a wind shear under the action of the geomagnetic field Lorentz force driven by neutral winds. In recent years, there has been evidence to suggest that sporadic E is not as “sporadic” as the name implies, but a regularly occurring phenomenon that is subject to a great deal of variability at various temporal and spatial scales. Among the Es variations, a prominent one is its diurnal variation, established from ionosonde and satellite radio occultation studies. The present research deals with the diurnal variation of sporadic E, aiming to provide an integrated physical understanding, which so far remained incomplete. Besides the tidal wind control of the diurnal variability and altitude descent of sporadic E layers, a new physical mechanism is introduced that involves metal atom solar photoionization. This elucidates why the sporadic E diurnal cycle starts or intensifies at sunrise and why sporadic E activity maximizes during daytime, therefore it explains the strong 24‐hr periodicity in sporadic E layer occurrence and intensity. The midlatitude sporadic E (Es) layer activity follows a diurnal cycle that starts in higher altitudes at sunrise and maximizes during daytimeMetal atom photoionization acts during sunlit hours to increase metal ion densities available for Es layer formation and intensificationMetal atom photoionization explains why sporadic E layers appear at sunrise and is likely the reason for Es activity to maximize in daytime The midlatitude sporadic E (Es) layer activity follows a diurnal cycle that starts in higher altitudes at sunrise and maximizes during daytime Metal atom photoionization acts during sunlit hours to increase metal ion densities available for Es layer formation and intensification Metal atom photoionization explains why sporadic E layers appear at sunrise and is likely the reason for Es activity to maximize in daytime

For the first time, we reconstructed global distribution of both the total electron content disturbance W index and TEC values for eight extreme storms (Dst < −250 nT) occurred before the epoch of GNSS observations in solar cycle 22. We created a model based on superposed epoch analysis of the training set of GIM‐W maps of nine SC23 extreme storms. Global GIM‐W index maps are calculated from 15‐min UPC GIM‐TEC (UQRG) as the logarithmic deviation of instantaneous TEC from the monthly median GIM‐MTEC empirical model. We introduced the storm phase metrics for main and recovery phases of the positive ionosphere disturbance (the WU‐index), the negative disturbance (the WL‐index) and the ring current (the Dst‐index). The probabilistic forecasting model (Pmodel) for SC22 GIM‐Wxmaps is developed based on GIM‐W maps of the SC23 training set. The storm phase distribution Φxfor the eight SC22 extreme storms is calculated from the proxy time shift (lag) of peak WUmaxand WLminrelative to Dstmin. Proxy GIM‐TECxmaps are calculated by adjusting the GIM‐MTEC median to the GIM‐Wxprediction. Validation of the technique based on data of UPC and JPL for four intense ionospheric storms showed a root‐mean‐square error less than 3 TECU. The proposed technique can be applied for both the past and future forecasting of GIM‐W index and GIM‐TEC maps. Probabilistic Pmodel is developed to produce proxy GIM‐Wxand GIM‐TECxfor eight extreme storms of SC22 before the epoch of GNSS observationsThe first storm phase metrics is introduced for the ionospheric positive storm WU, negative storm WL and Dst ring current storm indicesValidation of GIM‐TECxwith JPL and UPC maps for four extreme storms of SC23 and SC24 yields RMSE less than three TECU Probabilistic Pmodel is developed to produce proxy GIM‐Wxand GIM‐TECxfor eight extreme storms of SC22 before the epoch of GNSS observations The first storm phase metrics is introduced for the ionospheric positive storm WU, negative storm WL and Dst ring current storm indices Validation of GIM‐TECxwith JPL and UPC maps for four extreme storms of SC23 and SC24 yields RMSE less than three TECU

We examine systematic differences between topside electron density measurements and different topside model formulations including α‐Chapman, NeQuick, and an improved NeQuick (NeQuick‐corr) topside formulation, recently proposed. The global topside electron density data set used, was extracted from Radio Occultation (RO) topside electron density profiles on board Low‐Earth Orbit satellites from the COSMIC/FORMOSAT‐3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite) mission. By using RO topside electron density measurements collocated with digisonde stations, we ensure that our investigation is based on two independent data sets (RO and digisondes). A subset of these profiles, with matched (within 5%) peak RO‐digisonde characteristics (foF2 and hmF2) is also exploited. This subset is exploited to extend the investigation on the basis of a higher quality validation data set. The comparison demonstrates that α‐Chapman and NeQuick‐corr underestimate, whereas NeQuick overestimates COSMIC topside electron density observations. The main outcome of this study is the significant NeQuick topside representation improvement that can be achieved near the F region peak, if the key parameter g, which controls the change of scale height with respect to altitude, is optimized to a value of 0.15 (compared to a currently adopted value of 0.125). The NeQuick‐corr topside formulation using the optimized gvalue of 0.15 outperforms all other topside formulations. α‐Chapman and NeQuick‐corr underestimate, whereas NeQuick overestimates COSMIC topside electron density measurementsNeQuick‐corr provides a superior topside representation compared to the other topside formulationsAn optimized global value of g= 0.15 further improves NeQuick‐corr topside formulation α‐Chapman and NeQuick‐corr underestimate, whereas NeQuick overestimates COSMIC topside electron density measurements NeQuick‐corr provides a superior topside representation compared to the other topside formulations An optimized global value of g= 0.15 further improves NeQuick‐corr topside formulation