Earth-ionosphere Waveguide Research Articles

Specularly reflected whistler: A low-latitude channel to couple lightning energy to the magnetosphere.

Lightning-generated whistlers profoundly affect the energetic particle population in Earth's radiation belts, influencing space weather and endangering astronauts and satellites. We report the discovery of specularly reflected (SR) whistler in which the lightning energy injected into the ionosphere at low latitudes reaches the magnetosphere after undergoing a specular reflection in the conjugate ionosphere, contradicting previous claims that lightning energy injected at low latitudes cannot escape the ionosphere. SR whistlers provide a low-latitude channel to transport lightning energy to the magnetosphere. We calculate the relative contributions of SR, magnetospherically reflected, subprotonospheric, and ducted whistlers to the lightning energy reaching the magnetosphere. When SR whistlers are considered, the global lightning energy contribution to the magnetosphere doubles, implying that the previous estimates of the impact of lightning energy on radiation belts may need substantial revisions. Whistler dispersion and intensity analyses quantitatively confirm our results and suggest new remote-sensing methods of the magnetosphere, ionosphere, Earth-ionosphere waveguide, and lightning flashes.

Analysis of Pre-Seismic Ionospheric Disturbances Prior to 2020 Croatian Earthquakes

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.

Prediction the Diurnal Variation of VLF Waves in Earth-Ionosphere Waveguide Based on BPNN-TL Method

Prediction the Diurnal Variation of VLF Waves in Earth-Ionosphere Waveguide Based on BPNN-TL Method

HF radio channel modeling by a waveguide approach

We present a modified method of HF radio channel modeling based on a waveguide approach. The waveguide approach represents the electromagnetic field of radiation inside the Earth—ionosphere waveguide as an eigenfunction series of a radial boundary problem with impedance conditions on the Earth surface and radiation conditions at infinity. The transfer function of the radio channel is represented as a series of products of angular-operator Green functions, excitation coefficients, and coefficients for receiving individual normal modes. We have obtained a solution of the boundary value problem of determining the eigenfunctions and eigenvalues of the radial operator. The solution can be applied to the frequency range below the ionospheric F-layer critical frequency. We examine algorithms for calculating distance-frequency, frequency-angular, and amplitude characteristics of signals by analyzing and numerically summarizing the series in terms of strongly damped normal modes.

Моделирование КВ-радиоканала на основе волноводного подхода

We present a modified method of HF radio channel modeling based on a waveguide approach. The waveguide approach represents the electromagnetic field of radiation inside the Earth—ionosphere waveguide as an eigenfunction series of a radial boundary problem with impedance conditions on the Earth surface and radiation conditions at infinity. The transfer function of the radio channel is represented as a series of products of angular-operator Green functions, excitation coefficients, and coefficients for receiving individual normal modes. We have obtained a solution of the boundary value problem of determining the eigenfunctions and eigenvalues of the radial operator. The solution can be applied to the frequency range below the ionospheric F-layer critical frequency. We examine algorithms for calculating distance-frequency, frequency-angular, and amplitude characteristics of signals by analyzing and numerically summarizing the series in terms of strongly damped normal modes.

Spatial and Temporal Distribution of Northwest Cape Transmitter (19.8 kHz) Radio Signals Using Data Collected by the China Seismo-Electromagnetic Satellite

Very Low Frequency (VLF) waves radiated from ground-based transmitters are crucial for long-distance communication and underwater navigation. These waves can reflect between the Earth’s surface and the ionosphere for Earth–ionosphere waveguide propagation. Additionally, they can penetrate not only the ionosphere but also the magnetosphere, where they interact with high-energy particles in the radiation belt. Therefore, studying the spatial and temporal distribution of VLF radio signals holds significant importance. Such research enables us to understand the propagation characteristics of VLF signals, their interaction with radiation belt particles, and their response to space weather and lithospheric activity events. In this paper, we investigate the seasonal variations in the intensity of the Northwest Cape (NWC) transmitter (19.8 kHz) radio signals at satellite altitude and the displacement of the electric field’s peak center. Our analysis is based on the nightly China Seismo-Electromagnetic Satellite (CSES) data from 2019 to 2021. The results reveal the following: (1) There is no significant seasonal variation in the electric field strength within a small area (2.5° radius) around the NWC transmitter. However, a clear seasonal variation in the electric field strength is observed within a larger area (15° radius), with higher strength during winter compared with summer. (2) The power spectral density of the electric field remains constant within the peak central area (approximately 1~2° radius), but it decays with distance outside this region, showing a north–south asymmetry. Moreover, the decay rate of the radiation electric field is slower in the northern direction than in the southern direction. (3) The center of the electric field moves northward from summer to winter and southward from winter to summer. (4) In winter, VLF waves radiated by the NWC transmitter may predominantly propagate by being ducted toward the conjugate hemisphere.

Performance Analysis of Artificial Intelligence Approaches for LEMP Classification

Lightning Electromagnetic Pulses, or LEMPs, propagate in the Earth–ionosphere waveguide and can be detected remotely by ground-based lightning electric field sensors. LEMPs produced by different types of lightning processes have different signatures. A single thunderstorm can produce thousands of LEMPs, which makes their classification virtually impossible to carry out manually. The lightning classification is important to distinguish the types of thunderstorms and to know their severity. Lightning type is also related to aerosol concentration and can reveal wildfires. Artificial Intelligence (AI) is a good approach to recognizing patterns and dealing with huge datasets. AI is the general denomination for different Machine Learning Algorithms (MLAs) including deep learning and others. The constant improvements in the AI field show us that most of the Lightning Location Systems (LLS) will soon incorporate those techniques to improve their performance in the lightning-type classification task. In this study, we assess the performance of different MLAs, including a SVM (Support Vector Machine), MLP (Multi-Layer Perceptron), FCN (Fully Convolutional Network), and Residual Neural Network (ResNet) in the task of LEMP classification. We also address different aspects of the dataset that can interfere with the classification problem, including data balance, noise level, and LEMP recorded length.

Influence of strong solar proton events on propagation of radio signals in the VLF range in a high-latitude region

In this paper, we examine the features of RSDN-20 signal propagation in a high-latitude Earth–ionosphere waveguide during solar proton events, using computational experiment methods. We have analyzed two proton ground-level enhancement (GLE) events of December 13, 2006 (GLE70) and September 10, 2017 (GLE72). Electron density profiles were constructed using the Global Dynamic Model of Ionosphere (GDMI) and the RUSCOSMICS model, developed at PGI. We present estimated phase and amplitude changes in RSDN-20 signals during precipitation of high-energy protons in the high-latitude region of the Earth–ionosphere waveguide. From the results of computational experiments and the analysis of the electromagnetic signal attenuation based on analytical Maxwell’s equation system solution in magnetized ionospheric plasma, we have found a pattern in the signal attenuation frequency dependence associated simultaneously with the signal reflection height, electron density profiles, and the collision frequency of electrons with neutral particles and ions. We discuss limitations of the computational experiment method and compare simulation results with data from Lovozero and Tuloma observatories.

Влияние сильных солнечных протонных событий на распространение радиосигналов в диапазоне ОНЧ в области высоких широт

In this paper, we examine the features of RSDN-20 signal propagation in a high-latitude Earth–ionosphere waveguide during solar proton events, using computational experiment methods. We have analyzed two proton ground-level enhancement (GLE) events of December 13, 2006 (GLE70) and September 10, 2017 (GLE72). Electron density profiles were constructed using the Global Dynamic Model of Ionosphere (GDMI) and the RUSCOSMICS model, developed at PGI. We present estimated phase and amplitude changes in RSDN-20 signals during precipitation of high-energy protons in the high-latitude region of the Earth–ionosphere waveguide. From the results of computational experiments and the analysis of the electromagnetic signal attenuation based on analytical Maxwell’s equation system solution in magnetized ionospheric plasma, we have found a pattern in the signal attenuation frequency dependence associated simultaneously with the signal reflection height, electron density profiles, and the collision frequency of electrons with neutral particles and ions. We discuss limitations of the computational experiment method and compare simulation results with data from Lovozero and Tuloma observatories.

Features of Radio Signal Propagation in the VLF Range at High Latitudes during Solar Proton Events

In this paper, we study the amplitude and phase characteristics of VLF signals of an anthropogenicorigin during solar proton events using the methods of a computational experiment. We consider the events ofOctober 30, 2003 and January 23, 2012. Electron density profiles are plotted using data from the VHF EISCATincoherent scatter radar located in Tromsø, Norway. Based on the processed data of computational experiments,that under the conditions of solar proton events, mainly amplitude distortions of VLF signals wereshown to be observed while there is a frequency dependence of the magnitude of distortions of the signals ofthe RSDN-20 long-range navigation radio system. The signal phases of the RSDN-20 system are less affectedby weak solar proton events. The effect of the lower boundary of the Earth-ionosphere waveguide in the casesof propagation of signals from the RSDN-20 system over the surface of land and ocean during a solar protonevent was studied.

Empirical Parameterization of Broadband VLF Attenuation in the Earth‐Ionosphere Waveguide

AbstractThe Earth‐ionosphere waveguide (EIW) determines the propagation of Very Low Frequency (VLF; ∼3−30 kHz) waves. Characterizing the waveguide is a longstanding challenge due to its large spatial scale and the complex variability of the lower ionosphere. Here we apply a novel linear basis function regression technique to characterize attenuation in the EIW using broadband measurements of lightning‐generated radio waves. The process begins with defining a basis function set, which ideally encompasses a feature set that can predict the variability seen in VLF attenuation properties. With this basis set defined, a system of linear equations is then constructed using sensor pair observations to eliminate the dependency on source amplitude in each observation. Using this formalism, an empirical attenuation model for broadband signals from lightning is constructed and the dependence on attenuation properties with the boundary conditions is explored. The empirically derived results show attenuation rates over ice that are 12 dB/Mm higher compared to paths over saltwater. Well‐known east/west attenuation rate asymmetry stemming from anisotropic reflection coefficients of the ionosphere is also demonstrated and investigated. For example, under a daytime ionosphere, westward‐propagating waves suffer up to 2.8 dB/Mm greater attenuation compared to eastward‐propagating waves. The trained model is used for propagation corrections in a global lightning locating system (LLS), but this technique can be expanded to further study VLF attenuation rates by employing different sets of basis functions.

Excitation of an Earth–Ionosphere Waveguide at Frequencies above That of the First Transverse Resonance

Excitation of an Earth–Ionosphere Waveguide at Frequencies above That of the First Transverse Resonance

Peculiarities of excitation of the Earth–ionosphere waveguide at frequencies above the frequency of the first transverse resonance

Experimental data on the registration of signals from tweek-atmospherics and an ionospheric source formed because of heating of the ionosphere at frequencies higher than the frequency of the first transverse resonance of the Earth–ionosphere waveguide (~1.8 kHz) are considered. It is shown that, far from the source, at frequencies near the resonance, almost complete circular left-hand polarization of the horizontal magnetic field is observed, and the angles of incidence are small and range from 10° to 35°. A joint analysis of observational data and simulation results showed that it is possible to calculate the effective reflection height from estimates of the incidence angle, which can be used to diagnose the lower ionosphere. But, in the case of an ionospheric source, it is important to take into account the inhomogeneity of its radiation pattern.

Automatic recognition of tweek atmospherics and plasma diagnostics in the lower ionosphere with the machine learning method

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide over long distances. In this paper, we developed an automatic method to recognize tweek atmospherics and diagnose the lower ionosphere based on the machine learning method. The differences (automatic-manual) of each ionosphere parameter between the automatic method and the manual method are 0.07  2.73 km, 0.03  0.92 cm-3, and 91  1068 km for h, Ne, and d, respectively. Moreover, the automatic method is capable of recognizing higher harmonic tweek sferics as well. These evaluation results of the model suggest that the automatic method is a powerful tool to investigate the long-term variations of the lower ionosphere.

Modeling of Diurnal Variation Characteristics of VLF Wave Propagation in Earth-Ionosphere Waveguide With FDTD Method

In this letter, Wait's exponential (EXP) model and international reference ionosphere (IRI) model are combined to study the diurnal variation characteristics of very low frequency (VLF) wave propagation. The EXP model is used below the IRI model to compensate for the subionosphere electron density lack of the IRI. The major EXP parameters are reconstructed from the lower IRI electron density for different times of the day. Being capable of characterizing the subionosphere details, the combined IRIEXP model gets higher accuracy than the pure IRI model for VLF diurnal variation prediction. The measurement result is replicated by using the finite-difference time-domain method to validate the presented model.

НАПРАВЛЕНИЯ СОВЕРШЕНСТВОВАНИЯ ПАССИВНЫХ РАДИОТЕХНИЧЕСКИХ СРЕДСТВ МОНИТОРИНГА ГРОЗОВОЙ АКТИВНОСТИ

The options for the development of passive radio engineering facilities for monitoring thunderstorm activity are presented. They allow reducing errors in determining the coordinates of lightning discharges by large-base systems by taking into account the transformation of atmospheric forms during the propagation in the Earth-ionosphere waveguide channel and improving the visual representation of lightning activity data using clustering algorithms for more detailed analysis aimed at improving reliability of determination of the current stage of the thunderstorm cloud development, with probabilistic assessment of a degree of its thunderstorm hazard and short-term forecast of subsequent propagation.

VLF/LF Lightning Location Based on LWPC and IRI Models: A Quantitative Study

The group velocity of lightning electromagnetic signals plays an important role in lightning location systems using the time difference of arrival (TDOA) method. Accurate estimation of group velocity is difficult due to the space- and time-varying properties of the Earth-ionosphere waveguide. Besides, the analytical solution of the group velocity is difficult to obtain from the classic mode theory, especially when higher-order modes, anisotropic geomagnetic background, diffuse ionosphere profile, and propagation path segmentation are all taken into consideration. To overcome these challenges, a novel numerical method is proposed in this paper to estimate the group velocity of the lightning signal during ionospheric quiet periods. The well-known Long Wavelength Propagation Capability (LWPC) code is used to model the propagation of VLF/LF radio waves. Since LWPC uses a simplified ionospheric model which is unable to describe the subtle variations of ionospheric parameters over time and space, the IRI-2016 model is incorporated into the numerical modeling process to provide more accurate ionosphere parameters. Experimental results of a VLF/LF lightning location network are demonstrated and analyzed to show the effectiveness of our method. The proposed method is also applicable when there is a sudden ionospheric disturbance as long as the parameters of the ionosphere are obtained in real time by remote sensing methods.

Estimation of the Fade Duration of Low-Frequency Radio Waves in the Earth–Ionosphere Waveguide under the Action of a Strong Local Perturbation in the Atmosphere

Estimation of the Fade Duration of Low-Frequency Radio Waves in the Earth–Ionosphere Waveguide under the Action of a Strong Local Perturbation in the Atmosphere

Отклик ионосферы на прохождение тайфунов по наблюдениям методом СДВ-радиопросвечивания

The response of the lower ionosphere to the passage of several dozen typhoons has been studied using a regional network of VLF stations in the Russian Far East. The experimental data presented in all cases clearly demonstrates wavelike disturbances of the subionospheric VLF signal amplitude and phase during the active stage of typhoons crossing radio paths. With the exception of magnetoactive and seismoactive days, this means that the disturbances generated by a typhoon, when propagating into the upper ionosphere, pass through the lower ionosphere, causing corresponding disturbances in the amplitude and phase of the VLF signal. Spectral analysis shows that the range of the wave disturbances detected corresponds to the periods of atmospheric internal gravity waves (IGW). A mechanism of the action of IGWs on the lower ionosphere is proposed which allows us to interpret the VLF signal phase variations observed. According to this mechanism, the action of IGW on the lower ionosphere is caused by polarization fields arising during the wave motion of plasma in the lower part of the F layer. These fields projected along geomagnetic field lines into the lower ionosphere cause the upper wall of the Earth—ionosphere waveguide to rise or fall.

Ionospheric response to the passage of typhoons observed by subionospheric VLF radio signals

The response of the lower ionosphere to the passage of several dozen typhoons has been studied using a regional network of VLF stations in the Russian Far East. The experimental data presented in all cases clearly demonstrates wavelike disturbances of the subionospheric VLF signal amplitude and phase during the active stage of typhoons crossing radio paths. With the exception of magnetoactive and seismoactive days, this means that the disturbances generated by a typhoon, when propagating into the upper ionosphere, pass through the lower ionosphere, causing corresponding disturbances in the amplitude and phase of the VLF signal. Spectral analysis shows that the range of the wave disturbances detected corresponds to the periods of atmospheric internal gravity waves (IGW). A mechanism of the action of IGWs on the lower ionosphere is proposed which allows us to interpret the VLF signal phase variations observed. According to this mechanism, the action of IGW on the lower ionosphere is caused by polarization fields arising during the wave motion of plasma in the lower part of the F layer. These fields projected along geomagnetic field lines into the lower ionosphere cause the upper wall of the Earth—ionosphere waveguide to rise or fall.