We have investigated the solar flare effects on ionospheric absorption with the systematic analysis of ionograms measured at midlatitude and low-latitude ionosonde stations under different solar zenith angles. The lowest recorded ionosonde echo, the minimum frequency ( f _min , a qualitative proxy for the “nondeviative” radio wave absorption occurring in the D-layer), and the d f _min parameter (difference between the value of the f _min and the mean f _min for reference days) have been considered. Data were provided by meridionally distributed ionosonde stations in Europe and South Africa during eight X - and M -class solar flares in solar cycle 23. Total and partial radio fade-out was experienced at every ionospheric station during intense solar flares (> M 6). The duration of the total radio fade-out varied between 15 and 150 min and it was highly dependent on the solar zenith angle of the ionospheric stations. Furthermore, a solar-zenith-angle-dependent enhancement of the f _min (2–9 MHz) and d f _min (1–8 MHz) parameters was observed at almost every station. The f _min and d f _min parameters show an increasing trend with the enhancement of the X-ray flux. Based on our results, the d f _min parameter is a good qualitative measure for the relative variation of the “nondeviative” absorption, especially in the case of the less intense solar flares, which do not cause total radio fade-out in the ionosphere (class < M 6).

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 purpose of the present study is to investigate simultaneously pre-earthquake ionospheric and atmospheric disturbances by the application of different methodologies, with the ultimate aim to detect their possible link with the impending seismic event. Three large earthquakes in Mexico are selected (8.2 Mw, 7.1 Mw and 6.6 Mw during 8 and 19 September 2017 and 21 January 2016 respectively), while ionospheric variations during the entire year 2017 prior to 37 earthquakes are also examined. In particular, Total Electron Content (TEC) retrieved from Global Navigation Satellite System (GNSS) networks and Atmospheric Chemical Potential (ACP) variations extracted from an atmospheric model are analyzed by performing statistical and spectral analysis on TEC measurements with the aid of Global Ionospheric Maps (GIMs), Ionospheric Precursor Mask (IPM) methodology and time series and regional maps of ACP. It is found that both large and short scale ionospheric anomalies occurring from few hours to a few days prior to the seismic events may be linked to the forthcoming events and most of them are nearly concurrent with atmospheric anomalies happening during the same day. This analysis also highlights that even in low-latitude areas it is possible to discern pre-earthquake ionospheric disturbances possibly linked with the imminent seismic events.

The sudden increase of X-radiation and EUV emission following solar flares causes additional ionization and increased absorption of electromagnetic (EM) waves in the Earth’s atmosphere. The solar flare impact on the ionosphere above Europe on 05 and 06 December 2006 was investigated using ground-based (ionosonde and VLF) and satellite-based data (Vertical Total Electron Content (VTEC) derived from GNSS observations and VLF measurements from DEMETER satellite). Based on the Kp and Dst indices, 05 December 2006 was a quiet day, while there was a geomagnetic storm on 06 December 2006. The total fade-out of the EM waves emitted by the ionosondes was experienced at all investigated stations during an X9 class flare on 05 December 2006. The variation of the fmin parameter (first echo trace observed on ionograms, it is a rough measure of the “non-deviative” absorption) and its difference between the quiet period and during the flares have been analyzed. A latitude dependent enhancement of fmin (2–9 MHz) and Δfmin (relative change of about 150%–300%) was observed at every station at the time of the X9 (on 05 December) and M6 (on 06 December) flares. Furthermore, we analyzed VTEC changes during and after the flare events with respect to the mean VTEC values of reference quiet days. During the X9 solar flare, VTEC increased depending on the latitude (2–3 TECU and 5%–20%). On 06 December 2006, the geomagnetic storm increased ionization (5–10 TECU) representing a “positive” ionospheric storm. However, an additional peak in VTEC related to the M6 flare could not be detected. We have also observed a quantifiable change in transionospheric VLF absorption of signals from ground transmitters detected in low Earth orbit associated with the X9 and M6 flare events on 05 and 06 December in the DEMETER data. Moreover, amplitude and phase of ground-based, subionospherically propagating VLF signals were measured simultaneously during the investigated flares to analyze ionosphere reaction and to evaluate the electron density profile versus altitude. For the X9 and M6 flare events we have also calculated the ionospheric parameters (sharpness, reflection height) important for the description and modelling of this medium under forced additional ionization.

The Széchenyi István Geophysical Observatory, also known as the Nagycenk Geophysical Observatory (NCK), was established in 1957. It has been the only measurement site in Hungary where observations of various parameters of the atmospheric global electric circuit are made in the framework of organized research under the umbrella of the Hungarian Academy of Sciences (MTA). Measurements of the atmospheric electrical potential gradient (PG) and Schumann resonances (SRs) running quasi-continuously in the observatory for decades provide an invaluable source of information for geophysical research. This paper gives an overview on the history of the observatory and particularly on various atmospheric electricity (AE) measurements on-site to commemorate the efforts and excellence of the people who served atmospheric sciences by dedicating their lives to obtaining high-quality, reliable data and scientific achievements at the highest possible level.

Large-Scale Travelling Ionospheric Disturbances (LSTIDs) are wave-like ionospheric fluctuations, generally triggered by geomagnetic storms, which play a critical role in space weather dynamics. In this work, we present a machine learning model able to forecast the occurrence of LSTIDs over the European continent up to three hours in advance. The model is based on CatBoost, a gradient boosting framework. It is trained on a human-validated LSTID catalogue with the various physical drivers, including ionogram information, geomagnetic, and solar activity indices. There are three forecasting modes depending on the demanded scenarios with varying relative costs of false positives and false negatives. It is crucial to make the model predictions explainable, so that the output contribution of each physical factor input is visualised through the game-theoretic SHapley Additive exPlanation (SHAP) formalism. The validation procedure consists of a global-level evaluation and interpretation step, firstly, followed by an event-level validation against independent detection methods, which highlights the model’s predictive robustness and suggests its potential for real-time space weather forecasting. Depending on the operating mode, we report an improvement ranging from +72% to +93% over the performance of a rule-based benchmark. Our study concludes with a comprehensive analysis of future research directions and actions to be taken towards full operability. We discuss probabilistic forecasting approaches from a cost-sensitive learning perspective, along with performance-centric model monitoring. Finally, through the lens of the conformal prediction framework, we further comment on the uncertainty quantification for end-user risk management and mitigation.