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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.

WOS:000610265200001 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.

Systematic analysis of ionosopheric parameters measured at mid- and low-latitudes was performed to study the ionospheric response to solar flares. The lowest recorded ionosonde echo, the mimimum frequency (fmin, a qualitative proxy for the nondeviative radio wave absorption occurring in the D-layer), furthermore the dfmin parameter (difference between the value of the fmin and the mean fmin for reference days) have been investigated. The time series of the fmin and dfmin parameters recorded at meridionally-distributed ionosonde stations in Europe and South Africa were analyzed during eight X and M class solar flares during solar cycle 23. The solar zenith angles of the observation sites at the time of the selected flares have been also taken into account. Total and partial radio fade-out was experienced at every ionospheric stations during intense solar flares (> M6). 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 fmin (2–9 MHz) and dfmin (1–8 MHz) parameters was observed at almost every stations. The fmin and dfmin parameters show an increasing trend with the enhancement of the X-ray flux. Based on the results, the dfmin 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&thinsp

Schumann resonances (SR) are the electromagnetic oscillations of the spherical cavity bounded by the electrically conductive Earth and the conductive but dissipative lower ionosphere (Schumann in Z Naturforsch A 7:6627–6628, 1952). Energetic emissions from the Sun can exert a varied influence on the various parameters of the Earth’s SR: modal frequencies, amplitudes and dissipation parameters. The SR response at multiple receiving stations is considered for two extraordinary solar events from Solar Cycle 23: the Bastille Day event (July 14, 2000) and the Halloween event (October/November 2003). Distinct differences are noted in the ionospheric depths of penetration for X-radiation and solar protons with correspondingly distinct signs of the frequency response. The preferential impact of the protons in the magnetically unshielded polar regions leads to a marked anisotropic frequency response in the two magnetic field components. The general immunity of SR amplitudes to these extreme external perturbations serves to remind us that the amplitude parameter is largely controlled by lightning activity within the Earth–ionosphere cavity.

Superposed epoch analysis (SEA) was used to study possibly statistically significant variations of the critical frequency (foEs) and virtual height (h’Es) of the sporadic E layer (Es) related to thunderstorm activity generated in the troposphere. The reference time for the SEA was the time of lightning strokes measured by the World Wide Lightning Location Network at the ionosonde station of Rome (41.9 $$^\circ $$ N, 12.5 $$^\circ $$ E) during the year 2009. The results obtained reveal that: (a) a statistically significant decrease of foEs after the time of lightnings has been found for time windows of $$\pm $$ 100 h; (b) the effects of thunderstorms on the ionosphere is larger when the thunderstorm approaches from the opposite direction to the mean neutral stratosphere–mesosphere wind flow; (c). a statistically significant decrease of foEs related to thunderstorms during nighttime was observed. No significant changes in foEs and hEs over the seasonal time scale as well as in the latter parameter in the three (a–c) cases related to thunderstorms.

Possible thunderstorm – sporadic E (Es) layer coupling effects are investigated during two measurement periods, one in 2013 and one in 2014. The analysis was based on ionospheric observations obtained from a Digisonde at Pruhonice, the Czech Republic, an ionosonde at Nagycenk, Hungary, and a 3.59 MHz five-point continuous HF Doppler system located in the western part of the Czech Republic. The latter is capable of detecting ionospheric wave-like variations caused by neutral atmospheric waves generated by thunderstorms. The present study searches for possible impacts on Es layers caused by the presence of two active thunderstorms: one passing across the Czech Republic on June 20, 2013 (19:00–01:00 LT), and one through Hungary on July 30, 2014 (11:00-01:00 LT). During these two time periods, presence and parameters of Es layer were inferred from ionograms, recorded every minute at Pruhonice and every two minutes at Nagycenk, whereas concurrent lightning activity was monitored by the LINET detection network. In addition, transient luminous events (TLEs) were also observed during both nights from Sopron, Hungary and from Nýdek, the Czech Republic. A noticeable fact was the reduction and disappearance of the ongoing Es layer activity during part of the time in both of the traversing thunderstorms. The analysis indicated that the critical frequency foEs dropped below ionosonde detection levels in both cases, possibly because of thunderstorm activity effects. This option, however, needs more case studies in order to be further substantiated.

Meteorological processes (cold fronts, mesoscale convective complexes, thunderstorms) in the troposphere can generate upward propagating waves in the neutral atmosphere affecting the behaviour of the ionosphere. One type of these waves are the internal atmospheric gravity waves (AGWs) which are often generated by thunderstorms. Davis & Johnson (2005) found in low pressure systems that a localized intensification of the sporadic E layer (Es) can be attributed to lightnings. To confirm this result, we have performed two different statistical analysis using the time series of the critical frequency (foEs), the virtual height of the sporadic E layer (h'Es), and meteorological observations (lightnings, Infrared maps) over the ionospheric station of Rome (41.9° N, 12.5° E). In the first statistical analysis, we separated the days of 2009 into two groups: stormy days and fair-weather days, then we studied the occurrence and the properties of the Es separately for the two different groups. No significant differences have been found. In the second case, a superposed epoch analysis (SEA) was used to study the behaviour of the critical frequency and virtual height 100 hours before and after the lightnings. The SEA shows a statistically significant decrease in the critical frequency after the time of the lightnings, which indicates a sudden decrease in the electron density of the sporadic E layer associated with lightnings. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper gives a resume of the papers written in English which (a) describe some of the recording instruments in use at the Nagycenk Observatory (NCK) since the International Geophysical Year (IGY 1957–1958) and up to the present time, (b) summarise the most important and different types of observations associated with thunderstorms which have been made there, and (c) discuss their various geophysical interpretations. The paper describes the main results which have been obtained in four areas of thunderstorm associated atmospheric and geospace science within the context of Earth system science. These relate to the following parameters of atmospheric electricity: the vertical electric potential gradient just above the Earth’s surface and the air–Earth current as well as the point discharge current, Schumann resonance (SR) signals of the Earth-ionosphere cavity at 8, 14 and 20 Hz, transient luminous events (TLEs), and some aspects of the behaviour of the ionosphere. Deductions from these data sets are concerned with the global lightning activity and the conductivity of the air, with diurnal, seasonal, annual and long-term variations of the SR amplitudes and resonant frequencies in terms of migrating thunderstorm centres, with transient SR excitations and with sprites and other TLEs, and with ionospheric disturbances. The paper closes with some thoughts on future research directions based on the observations at NCK and Sopron and the results achieved since the IGY.