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We study the sub-ionospheric VLF transmitter signals recorded by the Austrian Graz station in the year 2020. Those radio signals are known to propagate in the Earth-ionosphere waveguide between the ground and lower ionosphere. The Austrian Graz facility (geographic coordinates: 15.46°E, 47.03°N) can receive such sub-ionospheric transmitter signals, particularly those propagating above earthquake (EQ) regions in the southern part of Europe. We consider in this work the transmitter amplitude variations recorded a few weeks before the occurrence of two EQs in Croatia at a distance less than 200 km from Graz VLF facility. The selected EQs happened on 22 March 2020 and 29 December 2020, with magnitudes of Mw5.4 and Mw6.4, respectively, epicenters localized close to Zagreb (16.02°E, 45.87°N; 16.21°E, 45.42°N), and with focuses of depth smaller than 10 km. In our study we emphasize the anomaly fluctuations before/after the sunrise times, sunset times, and the cross-correlation of transmitter signals. We attempt to evaluate and to estimate the latitudinal and the longitudinal expansions of the ionospheric disturbances related to the seismic preparation areas.

Using the HF Doppler sounders at middle and low latitudes (Prague, Czech Republic; Tucuman, Argentina; Zhongli, Republic of China, and Sugadaira, Japan), we observed the electric fields of the geomagnetic sudden commencement (SC) propagating near-instantaneously (within 10 s) over the globe. We found that the electric fields of the preliminary impulse (PI) and main impulse (MI) of the SC are in opposite direction to each other and that the PI and MI electric fields are directed from the dusk to dawn and dawn to dusk, respectively, manifesting the nature of the curl-free potential electric field. We further found that the onset and peak of the PI electric field are simultaneous on the day and nightsides (0545, 1250, 1345 MLT) within the resolution of 10 s. With the magnetometer data, we confirmed the near-instantaneous development of the ionospheric currents from high latitudes to the equator and estimated the location of the field-aligned currents that supply the ionospheric currents. The global simultaneity of the electric and magnetic fields does not require the contribution of the magnetohydrodynamic waves in the magnetosphere nor in the F-region ionosphere. The global simultaneity and day–night asymmetry of the electric fields are explained with the ionospheric electric potentials transmitted at the speed of light by the TM0 mode waves in the Earth-ionosphere waveguide.

The impulse charge moment change (iCMC) is an important electrical property of cloud-to-ground (CG) lightning. In this paper, a new method of measuring the iCMC at distances of several hundred kilometers is proposed. The method is based on the vertical electric field below 1 kHz measured by the widely used fast electric field antenna with low frequency/very low frequency (LF/VLF) band. The impulse response of Earth-ionosphere waveguide (EIWG) is modeled using a finite difference time domain (FDTD) method considering an anisotropic ionosphere. By comparing the observed waveform with the simulated impulse response, the lightning discharge is classified into the impulsive discharge and the non-impulsive discharge. For the impulsive discharge, its iCMC is obtained directly by comparing the measured ELF waveform to the modeled impulse response at the same distance. For the non-impulsive discharge, its current moment waveform is assumed to be a sum of two Heidler’s functions, and the genetic algorithm is used to search the unknown parameters in the functions. The good agreement between the measured ELF waveform and the simulated waveform implies that the extracted current moments are reasonable. This method can be used to continuously monitor the lightning iCMC in a given time and space.

Polar ionospheric heaters have generated ULF/ELF/VLF waves by modulating the auroral electrojet at D/E region altitudes. We present theoretical/computational results indicating that modulated F‐region HF heating can generate ionospheric currents even in the absence of electrojet currents. The ELF currents are driven in a two‐step process. First, the pressure gradient associated with F‐region electron heating drives a local diamagnetic current. This acts as an antenna to inject Magneto‐Sonic (MS) waves in the ionospheric plasma. Second, the electric field of the magneto‐sonic wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that injects waves in the Earth‐Ionosphere Waveguide below and Shear Alfven waves upwards to the conjugate regions. The paper examines the scaling and limitations of the concept and suggests proof‐of‐principle experiments using the HAARP ionospheric heater.

Fully coupled lithosphere, atmosphere, and ionosphere theory has demonstrated that extremely low-frequency electromagnetic (ELF-EM) fields present a broad application prospect in deep resource exploration, but previous studies have ignored the contribution of the Earth’s curvature. This study extends the theory of ELF-EM over a stratified Earth to the case where the Earth’s curvature must be taken into account, and presents an analytical solution of the ELF-EM field excited by a grounded horizontal antenna in a spherical Earth–ionosphere model, whose theoretical approach and solution method are notably different from the flat Earth–ionosphere model. Additionally, the Earth is treated as a concentric-layered sphere rather than an ideal homogeneous sphere. We aim to investigate the effects of the Earth’s curvature on the surface field, so as to broaden the coverage of the ELF wave in resource exploration. The solution is mathematically accurate and physically reasonable, since it reflects the sphericity and radially stratified structure of the Earth. We first verify the correctness and reliability of the proposed method by comparing the results with FDTD in a full-space spherical model. Additionally, we then compared the spherical results with the conventional controlled-source electromagnetic method and flat Earth–ionosphere results. The results show that when the distance between the transmitter and the receiver is comparable to the Earth radius, the spherical model better reflects the resonance of the wave in the cavity, suggesting that the effect of the Earth’s curvature is not negligible. Then, the numerical simulations conducted to investigate the properties of the EM fields and their sensitivities to the conductivity at depth in the Earth are discussed. Finally, the EM responses of some simple electrical conductivity structures models are modeled to illustrate their prospects in future resource exploration.

Observed phases and amplitudes of VLF radio signals propagating on very long paths are used to validate electron density parameters for the lowest edge of the (D region of the) Earth's ionosphere at low latitudes and midlatitudes near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured near the transmitters (∼100–150 km away), where the direct ground wave is dominant, and also at distances of ∼8–14 Mm away, over mainly all‐sea paths. Four paths were used: NWC (19.8 kHz, North West Cape, Australia) to Seattle (∼14 Mm) and Hawaii (∼10 Mm), NPM (21.4 kHz, Hawaii) and NLK (24.8 kHz, Seattle) to Dunedin, New Zealand (∼8 Mm and ∼12 Mm). The characteristics of the bottom edge of the daytime ionosphere on these long paths were found to confirm and contextualize recently measured short‐path values of Wait's traditional height and sharpness parameters, H′ and β, respectively, after adjusting appropriately for the (small) variations of H′ and β along the paths that are due to (1) changing solar zenith angles, (2) increasing cosmic ray fluxes with latitude, and (3) latitudinal and seasonal changes in neutral atmospheric densities from the (NASA) Mass Spectrometer Incoherent Scatter‐ (MSIS‐) E‐90 neutral atmosphere model. The sensitivity of this long‐path (and hence near‐global) phase and amplitude technique is ∼ ± 0.3 km for H′ and ∼ ± 0.01 km−1 for β, thus creating the possibility of treating the height (H′ ∼70 km) as a fiduciary mark (for a specified neutral density) in the Earth's atmosphere for monitoring integrated long‐term (climate) changes below ∼70 km altitude. Key Points Ionospheric height and sharpness from long (> ∼10 Mm) VLF radio paths Height sensitivity near 70 km altitude: ∼0.3 km long term, ∼0.1 km short term Modeling: solar zenith angle, cosmic ray variations, neutral atmosphere (MSIS)

This contribution is aimed at an analysis of the dynamics of free-electron density fluctuations in the ionospheric critical plasma frequency f0F2 by using some tools from the theory of nonlinear dynamical systems. The results suggest the existence of low-dimensional attractors that point to a characterization of the free electron density fluctuations in the f0F2 as a deterministic chaotic system. The study carried out focused on the response of the ionosphere to solar activity as a function of the ascending and descending phases of the solar cycle.

Observed phases and amplitudes of VLF radio signals propagating on a short (∼360 km) path are used to find improved parameters for the lowest edge of the (D region of the) Earth's ionosphere at a geomagnetic latitude of ∼53.5° in midsummer near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured both near (∼110 km from) the transmitter, where the direct ground wave is very dominant, and at distances of ∼360 km near where the ionospherically reflected waves form a (modal) minimum with the (direct) ground wave. The signals came from the 24.0 kHz transmitter, NAA, on the coast of Maine near the U.S.‐Canada border, propagating ∼360 km E‐NE, mainly over the sea, to Saint John and Prince Edward Island. The bottom edge of the midday, midsummer, ionosphere at ∼53.5° geomagnetic latitude was thus found to be well modeled by H′ = 71.8 ± 0.6 km and β = 0.335 ± 0.025 km−1 where H′ and β are Wait's traditional height and sharpness parameters used by the U.S. Navy in their Earth‐ionosphere VLF radio waveguide programs. The variation of β with latitude is also estimated with the aid of interpolation using measured galactic cosmic ray fluxes.

Observed phases and amplitudes of VLF radio signals, propagating on a short (∼300‐km) path, are used to find improved parameters for the lowest edge of the (D region of the) Earth's ionosphere. The phases, relative to GPS 1‐s pulses, and the amplitudes were measured both near (∼100 km from) the transmitter, where the direct ground wave is very dominant, and at distances of ∼300 km near where the ionospherically reflected waves form a (modal) minimum with the (direct) ground wave. The signals came from the 19.8 kHz, 1 MW transmitter, NWC, on the North West Cape of Australia, propagating ∼300 km ENE, mainly over the sea, to the vicinity of Karratha/Dampier on the N.W. coast of Australia. The bottom edge of the mid‐day tropical/equatorial ionosphere was thus found to be well‐modeled by H′ = 70.5 ± 0.5 km and β = 0.47 ± 0.03 km−1 where H′ and β are the traditional height and sharpness parameters as used by Wait and by the U.S. Navy in their Earth‐ionosphere VLF radio waveguide programs. U.S. Navy modal waveguide code calculations are also compared with those from the wave hop code of Berry and Herman (1971). At least for the vertical electric fields on the path studied here, the resulting phase and amplitude differences (between the ∼100‐km and ∼300‐km sites) agree very well after just a small adjustment of ∼0.2 km in H′ between the two codes. Such short paths also allow more localization than the usual long paths; here this localization is to low latitudes.

Schumann resonance (SR) observations performed simultaneously by a global network consisting of three stations (Lekhta (Karelia, Russia), Moshiri (Hokkaido, Japan), and West Greenwich (Rhode Island, United States)) during almost 1 year were used for mapping world thunderstorm activity. A two‐stage inverse problem is solved for locating lightning sources distributed over the Earth's surface from the SR background signals. The first stage consists of inversions of the SR magnetic field power spectra to the distributions of lightning intensity by distance relative to an observation point. The obtained distance profiles of intensity of sources are used as tomographic projections for reconstructing a spatial distribution of sources in the second stage. We have suggested the use of source distance profiles obtained from the spectra of outputs of two orthogonal magnetic antennas operating at each observatory as separate tomographic projections. It is shown that the implementation of additional information on the azimuthal distribution of sources, provided by angular selectivity of magnetic sensors, significantly improves the quality of global lightning mapping under the condition of a limited number of observation stations. Maps of the global lightning distributions constructed by the result of inversions of SR spectra show that the most active regions vary zonally on the seasonal time scale and meridionally on the diurnal time scale being connected mainly with continental areas in the tropics.