A D‐Region Ionospheric Imaging Method Using Sferic‐Based Tomography

The study of the Earth’s atmosphere and the space environment is important because of the role played by the medium in the activities that affect the Earth and its inhabitants directly or indirectly. A robust capability to monitor, model and predict the happenings in the atmospheric space through deployment of both space- and ground-based observational systems for data acquisition, is key to result-oriented scientific research in atmospheric and space sciences. In this paper, we highlight the importance of regional deployment of observational facilities for data acquisition to complement current observational tools. We briefly review the capabilities of very low frequency (VLF) radio waves (in monitoring and studying changes in the atmosphere and ionosphere), and also present data obtained from our newly deployed VLF radio waves receiver at Anchor University, Lagos (AUL). We show that the diurnal signature characterised by VLF radio signal reflected in the data of three of four propagation paths (AUL-HWU, AUL-JJI, AUL-NWC and AUL-VTX) received at AUL. This outcome shows that operational condition of the VLF radio wave receiver is good. We anticipate interesting findings as we exploit this institution-based dataset to probe ionospheric irregularities in our region for the first time. In the light of some identified ionospheric changes in the equatorial region, we anticipate that our research effort using this regional VLF dataset will yield new results that will be resourceful for better understanding and characterisation of the dynamics of the equatorial D-layer ionosphere, since our region of reception (Lagos) lies close to the equator.

Beat‐wave generation of very low frequency (VLF) waves by two HF heaters in the ionosphere is formulated theoretically and demonstrated experimentally. The heater‐induced differential thermal pressure force and ponderomotive force, which dominate separately in the D and F regions of the ionosphere, drive an electron current for the VLF emission. A comparison, applying appropriate ionospheric parameters shows that the ponderomotive force dominates in beat‐wave generation of VLF waves. Three experiments, one in the nighttime in the absence of D and E layers and two in the daytime in the presence of D and E layers, were performed. X mode HF heaters of slightly different frequencies were transmitted at CW full power. VLF waves at 10 frequencies ranging from 3.5 to 21.5 kHz were generated. The frequency dependencies of the daytime and nighttime radiation intensities are quite similar, but the nighttime radiation is much stronger than the daytime one at the same radiation frequency. The intensity ratio is as large as 9 dB at 11.5 kHz. An experiment directly comparing VLF waves generated by the beat‐wave approach and by the amplitude modulation (AM) approach was also conducted. The results rule out the likely contribution of the AM mechanism acting on the electrojet and indicate that beat‐wave in the VLF range prefers to be generated in the F region of the ionosphere through the ponderomotive nonlinearity, consistent with the theory. In the nighttime experiment, the ionosphere was underdense to the HF heaters, suggesting a likely setting for effective beat‐wave generation of VLF waves by the HF heaters.

The electromagnetic field amplitude of the subionospheric Very Low Frequency (VLF) propagation is sensitive to the lower ionospheric conditions. Accordingly, VLF waves have been proposed to study and monitor the lower ionosphere (D/E region). In this paper, the NARXNN (Nonlinear Autoregressive with Exogenous Input Neural Network) is used as a method for predicting the daily nighttime mean amplitude of VLF transmitter signals indicating the ionospheric perturbation around the transmitter-receiver path. The NARXNN has a good accuracy in predicting time series data and thus are more suitable for dynamic modeling. The NARX constructed model, which was built based on daily input variables of various physical parameters with the time interval from 1 January 2011 to 4 February 2013 such as stratospheric and mesospheric temperatures, cosmic rays, total column ozone, F10.7, Kp, AE, and Dst indices. We used the constructed model to predict high-(NLK-CHF), middle-(NPM-CHF) and low-latitude (NWC-CHF) paths. As a result, the constructed models are capable of performing reasonably good 5-day ahead predictions of the daily nighttime of VLF electric field amplitude for NPM-CHF path with the Pearson correlation coefficient (r) of 0.84 and with Root Mean Square Error (RMSE) of 3.12 dB, NLK-CHF (r = 0.80, RMSE = 3.57 dB) and NWC-CHF (r = 0.79, RMSE = 2.60 dB). We conclude that the constructed NARX NN model is capable of predicting the VLF electric field amplitude variation for different latitude paths.

The nighttime lower ionosphere behavior between 2007 to 2011 is studied estimating the nighttime reflection height (hN) from Very Low Frequency (VLF) waves propagation recorded at Brazilian Antarctic Station Comandante Ferraz (EACF). We focus on the long-term solar activity effect on hN. The results show that hN decrease while the solar activity starts to increase suggesting a control of the solar radiation on the nighttime lower ionosphere, and hence, on the electron density at night. Preliminary results show that the nighttime lower ionosphere might be influenced by the 27-days solar rotation and by long-term atmospheric oscillations as Semiannual (SAO) and Annual (AO) oscillations.

Very low frequency (VLF) radio waves have very effective long-distance propagation characteristics. Therefore, they have a wide application prospect in underwater and underground communication. To effectively suppress the interference of channel noise and improve the performance of the VLF communication system, a low-density parity check (LDPC) coded orthogonal frequency division multiplexing-minimum frequency shift keying (OFDM-MSK) communication scheme is designed according to the characteristics of the VLF channel. Considering that the harmonic interference in the VLF channel can be regarded as frequency selective fading, the OFDM technology is applied to the VLF channel. In addition, the performance of VLF communication system is mainly affected by strong impulse atmospheric noise, which can be successfully simulated by symmetrical alpha stable (SαS) distribution. To reduce the impact of SαS noise on the OFDM system and data loss in frequency selective fading channel, LDPC channel coding technology is proposed. Finally, the robustness of the LDPC coded OFDM-MSK scheme is tested and evaluated from the perspective of the bit error rate (BER). The results show that the LDPC coded OFDM-MSK communication scheme has better performance than the independent OFDM-MSK communication scheme.

When solar flares occur, as results of increases in X-rays and extreme ultraviolet (EUV) radiations the electron density in the ionosphere increases. The increase in the electron density can cause problems with satellite attitude control, satellite communication and GPS positioning accuracy. To validate the impacts of solar flares on satellites, it is necessary to quantify the increase in ionospheric plasma density caused by solar flares, although this process has not yet been precisely clarified. It is widely known that soft X-rays (0.1-0.8nm) generated during solar flares ionize the D-region ionosphere (60–90 km) and affect the propagation path of VLF (very low frequency, 3–30 kHz)/LF (low frequency, 30–300 kHz) radio waves (especially the reflection height). Lyman-alpha radiation (121.6 nm) is the major component of ultraviolet radiation, and is known to affect the D-region. However, the response of the D-region ionosphere to Lyman-alpha during solar flares remains unclear. In this study, we used the LF transmitter signals to investigate D-region ionospheric variations for the X5.89class solar flare that occurred at 01:10 UT on May 11, 2024. This was the largest solar flare among the consecutive solar flares that occurred in May 2024. The LF transmitter signals used in this study were OCTAVE/AVON data operated by our research group around the world. The LF waves were transmitted from JJY60 (60.0 kHz) in Japan and BPC (68.5 kHz) in China, and received at Pontianak (PTK), Indonesia and Rikubetsu (RKB), Japan. When a solar flare occurs the LF phase normally exhibits time variations similar to those of X-ray flux. However, during the X5.89-class solar flare, unusual oscillations in phase and amplitude were observed. Based on wavelet analysis of these oscillations, the periods were 549 s for the phase of the JJY60-PTK path and 388 s for the phase of the BPC-PTK path. To investigate the cause of the LF oscillations, we compared between the LF waves and EUV data observed by GOES-16 and SDO satellites. A part of LF oscillations shows similar time variations with that of the summation of EUV in the 20–140 nm range. A few peaks of the LF oscillation coincided with peaks in the composited EUV emissions. Coherence analysis was conducted to examine the relationship with EUV emissions at wavelengths of 117.5 nm, 133.5 nm, and 140.5 nm. The analysis revealed statistically significant coherence between the LF phase and EUV emissions within the 300–1000 s period range across all wavelengths, with particularly prominent peaks observed around 400–800 s. These results strongly suggest that the periodic variations in EUV emissions associated with the solar flare affected the electron density in the D-region ionosphere.

Ground‐based Pc5 ultralow frequency (ULF) wave power in multiple ground‐based meridians is compared to the very low frequency (VLF) wave amplitude proxy, derived from Polar Operational Environmental Satellite (POES) precipitation, for the 33 storms studied by Li et al. (2015, https://doi.org/10.1002/2015GL065342). The results reveal common L‐shell and time profiles for the ULF waves and VLF proxy for every single storm, especially at L ≤ 6, and identical discrimination between efficient and inefficient radiation belt electron acceleration. The observations imply either ULF waves play a role in driving precipitation, which is falsely interpreted as VLF wave power in the proxy, ULF waves drive VLF waves (the reverse being energetically unfeasible), or both have a common driver with nearly identical L‐shell and time dependence. Global ground‐based ULF wave power coherence implies that a small number of meridians can be used to estimate storm time radial diffusion coefficients. However, the strong correspondence between ULF wave power and VLF wave proxy complicates causative assessments of electron acceleration.

There are several sources of high energy radiations in the Universe. Each of them has a totally different emission mechanism. In this review, I concentrate on the study of the general properties of radiations emitted by matter which are accreted towards black holes and neutron stars. We also study the nature of X-ray radiation coming from the Sun in the form of energetic solar flares. These radiations are constantly bombarding the Earth’s ionosphere changing its electron content and affecting the wave propagation through the Earth and the lower-ionosphere waveguide. We monitor Very Low Frequency (VLF) radio waves and correlate our observations with the presence or absence of the high energy radiations from space. Our results during solar flares, show how the number density of the ionosphere is enhanced by these celestial sources. The terrestrial high energy phenomena such as earthquakes are believed to affect the ionospheric properties and we have monitored VLF signals during earthquakes and found possible effects of seismic events on the VLF signals.

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth’s bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 h by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2–4 h, whereas ULF waves take around 10’s of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver.

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth’s bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 hours by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2 to 4 hours, whereas ULF waves take around 10’s of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver.

Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near‐Earth space. ELF (0.3–3 kHz) and VLF (3–30 kHz) wave energy released by lightning discharges is often the agent of modification of the lower ionospheric medium that results in the conductivity changes and the excitation of optical emissions that constitute transient luminous events (TLEs). In addition, the resultant ionospheric changes are best (and often uniquely) observable as perturbations of subionospherically propagating VLF signals. In fact, some of the earliest evidence for direct disturbances of the lower ionosphere in association with lightning discharges was obtained in the course of the study of such VLF perturbations. Measurements of the detailed ELF and VLF waveforms of parent lightning discharges that produce TLEs and terrestrial gamma ray flashes (TGFs) have also been very fruitful, often revealing properties of such discharges that maximize ionospheric effects, such as generation of intense electromagnetic pulses (EMPs) or removal of large quantities of charge. In this paper, we provide a review of the development of ELF and VLF measurements, both from a historical point of view and from the point of view of their relationship to optical and other observations of ionospheric effects of lightning discharges.

Tweeks, very low frequency (VLF) electromagnetic signals generated by lightning discharges, are a valuable tool for remote sensing of the D‐region ionosphere. Propagating within the Earth‐ionosphere waveguide, they encode information about the ionosphere D‐region electron density and reflection height. This study analyzed nighttime tweek data from Suizhou Station (31.57°N, 113.32°E) collected between 2018 and 2021, estimating equivalent reflection heights and electron densities using the first‐order cut‐off frequency. The temporal and spatial variations in these parameters were examined across hourly, daily, monthly, and latitudinal scales, offering new insights into D‐region dynamics. Nighttime electron densities were found to range from 20 to 32 el/cm3 in the altitude range of 85–95 km. Reflection height and electron density exhibit clear temporal and spatial patterns. Hourly variations are primarily influenced by residual solar radiation, while daily variations are less consistent, reflecting contributions from multiple ionization sources. Monthly trends display strong seasonality, with reflection heights peaking in summer and decreasing during transitional months. Electron density increases steadily from February to August and declines toward the following February. Reflection heights decrease by approximately 2 km from 5°N to 15°N and rise by 3 km toward 45°N, while electron density increases with magnetic latitude, ranging from 21 el/cm3 at low magnetic latitudes to 31 el/cm3 at higher magnetic latitudes. These findings demonstrate the effectiveness of tweeks as a method for nighttime D‐region ionospheric remote sensing, providing valuable insights into D‐region variation patterns and characteristics. The results hold practical significance for communication, navigation, positioning, and timing applications.

<p>The seasonal variation of the daytime lower ionosphere, monitored using the propagation of Very Low Frequency (VLF) radio waves, shows an asymmetry when comparing the spring and autumn transitions. Considering the solar zenith angle variation, it can explain the spring transition but not the autumn one. The climatological variation exposes that the maximum of the VLF deviation is around the beginning of October. Thus, the deviation is called “the October effect”. This study aims to understand the possible atmospheric phenomena behind this effect. We use VLF signals transmitted from USA (NAA, f = 24 kHz), UK (GQD, f=19.6 kHz) and Iceland (NRK, f = 37.5 kHz) recorded in Northern Finland from 2011 to 2019. We compare our results with the Whole Atmosphere Community Climate Model with the thermosphere-ionosphere eXtension (WACCM-X) data. The October effect is separated into climatological earliest and latest effect according to WACCM-X climatological earliest and latest transitions from eastward to westward mean zonal winds</p>

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.

Ball lightning caused by a semi-relativistic runaway electron avalanche