Explore the vast range of titles available.

Resistivity Profiling and Very Low Frequency (VLF) electromagnetic methods were introduced to study fracture zones detection in Zaouia Jdida locality, within the Errachidia basin. The Horizontal Profiling was conducted in Wenner- array, with = 300 m and profile lines oriented NW–SE and NE–SW. The resistivity measurements were taken using MAE advanced geophysics instruments. The VLF profiles were implanted with the length reaches 1000 m and profile lines oriented in NE–SW direction. The VLF measurements were collected using T-VLF iris instrument and the data filtering was done using KHFFILT software. Two filters, Karous-Hjelt and Fraser, were applied to the real component of the secondary electromagnetic field. The qualitative interpretation of resistivity results, showed the presence of subsurface targets; fracture zones were detected at 70m, 240m and 450m positions along the profile P1, at 180m, 340m and 450m positions from the profile P2. The semi-quantitative interpretation of VLF results revealed the presence of two principal fracture zones at L3 and L5 locations, oriented NW–SE, at a depth range of 30 m to 60 m. The VLF anomaly observed at L3 location is confirmed by the resistivity measurements from the profile P1 (at 70m station). The identified fractures represent the potential zones for groundwater supply and then will have an implication on storage and movement of groundwater in the prospect area.

The signals received by very low-frequency/extremely low-frequency nonlinear receivers are frequently affected by intense atmospheric pulse noise stemming from thunderstorms and global lightning activity. Current noise processing algorithms designed for nonlinear channels within these frequency ranges, which are predicated on fractional p-order moment alpha stable distribution criteria (where 0 < p < α < 2, and p and α denote distinct characteristic indices of alpha stable distribution noise), are constrained by their reliance on limited p-order moment statistics. As a result, the performance of low-frequency nonlinear channel receivers experiences significant degradation when confronted with robust pulse noise interference (0 < p < α < 2). To tackle this challenge, the present study introduces a novel variable step robust mixed norm (RMN) adaptive filtering algorithm, designated as SVS-RMN, which is based on the Sigmoid function. Leveraging the nonlinearity of the Sigmoid function and building upon the power function Hammerstein nonlinear channel model, the algorithm aims to enhance the RMN algorithm by deriving new cost functions and adaptive iteration formulas. The performance of the proposed algorithm is evaluated in comparison to conventional RMN algorithms based on fractional low-order moment (FLOM) criteria (0 < p < 2), as well as other algorithms employing variable step sizes and either FLOM or radial basis function (RBF) criteria, across various intensities of pulse noise and mixed signal-to-noise ratios. The experimental results reveal the following: (1) The proposed algorithm effectively mitigates strong pulse noise interference and significantly enhances the tracking performance of the RMN algorithm compared to conventional RMN algorithms based on FLOM criteria. (2) In terms of computational efficiency, simplicity of structure, convergence speed, and stability, the proposed algorithm surpasses other algorithms based on FLOM or RBF criteria.

On 31 August 2010, more than 100 transient luminous events were observed to occur over Typhoon Lionrock when it passed at ∼210 km to the southwest of the NCKU site in Taiwan. Among them, 14 negative gigantic jets (GJs) with clear recognizable morphologies and radio frequency signals are analyzed. These GJs are all found to have negative discharge polarity and thus are type I GJs. Morphologically, they are grouped into three forms: tree‐like, carrot‐like, and a new intermediate type called tree‐carrot‐like GJs. The ULF and ELF/VLF band signals of these events contain clear signatures associated with GJ development stages, including the initiating lightning, the leading jet, the fully developed jet, and the trailing jet. Though the radio waveform for each group of GJs always contains a fast descending pulse linked with the surge current upon the GJ‐ionosphere contact, the detailed waveforms actually vary substantially. Cross analysis of the optical and radio frequency signals for these GJs indicates that a large surge current moment (CM) (>60 kA‐km) appears to be essentially associated with the tree‐like GJs. In contrast, the carrot‐like and the tree‐carrot‐like GJs are both related to a surge CM less than 36 kA‐km, and a continuing CM less than 27 kA‐km further separates the carrot‐like GJs from the tree‐carrot‐like GJs. Furthermore, on the peak CM versus charge moment change diagram for the initiating lightning, different groups of GJs seem to exhibit different trends. This feature suggests that the eventual forms of negative GJs may have been determined at the initiating lightning stage. Key Points From the morphology, the negative gigantic jets can be grouped into three forms The radio signals of the GJs contain clear signatures in various GJ development From the derived physical parameters, the formations of these GJs are explained

We developed a model of short‐term slow slip events (SSEs) on the 3‐D subduction interface beneath Shikoku, southwest Japan, considering a rate‐ and state‐dependent friction law with a small cutoff velocity for the evolution effect. We assume low effective normal stress and small critical displacement at the SSE zone. On the basis of the hypocentral distribution of low‐frequency tremors, we set three SSE generation segments: a large segment beneath western Shikoku and two smaller segments beneath central and eastern Shikoku. Using this model, we reproduce events beneath western Shikoku with longer lengths in the along‐strike direction and with longer recurrence times compared with events beneath central and eastern Shikoku. The numerical results are consistent with observations in that the events at longer segments have longer recurrence intervals. The activity of SSEs is determined by nonuniform frictional properties at the transition zone. We also attempt to model the very low frequency (VLF) earthquakes that accompany short‐term SSEs, on a 2‐D thrust fault. We consider a local patch in which the friction parameters are varied. In the case that critical displacement is very small at the patch, fast multiple slips occur at the patch. In the case that the effective normal stress is high at the patch, the patch acts as a barrier to SSEs; when it ruptures, however, rapid slip occurs. Because the source time functions of these cases are somewhat different, it would be possible in the future to assess if either case is an appropriate model for VLF earthquakes.

An improved method for identifying nighttime tweek signals in WHU VLF measurements was developed by redesigning the extraction process and validated through comparison with World-Wide Lightning Location Network (WWLLN) data. Using the enhanced method, 1,728,032 tweek signals were identified from four years (2018–2021) of VLF data, forming the most comprehensive tweek dataset for the mid–low latitude region in China. Statistical analysis reveals distinct nighttime variations in tweek occurrence rates, which increase from 18:00 LT to 20:00 LT, remain high until 04:00 LT, and gradually decrease towards sunrise. Seasonal differences in propagation distance are evident, ranging from ~2000 km in summer to ~4000 km in winter, corresponding to the seasonal shift of lightning activity. The cutoff frequency showed apparent daily and seasonal fluctuations, and the trends of daily variation are opposite between winter and summer. The annual variation in cutoff frequency presents a pattern different from previous cognition, with a minimum of 1.62 kHz in summer and a maximum of 1.68 kHz in winter, influenced by the magnetic cyclotron frequency at ionospheric reflection points. These findings improve the understanding of nighttime tweek characteristics and ionospheric dynamics in East Asia, offering valuable insights for ionospheric research and VLF communication systems.

The structural changes and very-low-frequency (VLF) nonlinear dielectric responses are measured to evaluate the aging state of cross-linked polyethylene (XLPE) in power cables under various thermal aging conditions. For this purpose, the accelerated thermal aging experiments were performed on XLPE insulation materials at 90 °C, 120 °C and 150 °C with different durations of 240 h, 480 h and 720 h, respectively. The Fourier transform infrared spectrum (FTIR) characterization and differential scanning calorimeter (DSC) were tested to analyze the influence of different aging on physicochemical properties of XLPE insulation. Besides, the VLF dielectric spectra show that the permittivity and dielectric loss change significantly in the VLF range from 1 mHz to 0.2 Hz. A voltage–current (U–I) hysteresis curve referring to a standard sinusoidal voltage and the response current were introduced to characterize the nonlinear dielectric properties of XLPE insulation caused by thermal aging.

Measurements of Very-Low-Frequency (VLF) transmitter signals have been widely used to investigate the effects of various space weather events on the D-region ionosphere, including nowcasting solar flares. Previous studies have established a method to nowcast solar flares using VLF measurements, but only using measurements from dayside propagation paths, and there remains limited focus on day–night mixed paths, which are important for method applicability. Between March and May of 2022, the Sun erupted a total of 56 M-class and 6 X-class solar flares, all of which were well captured by our VLF receiver in Antarctica. Using these VLF measurements, we reexamine the capability of the VLF technique to nowcast solar flares by including day–night mixed propagation paths and expanding the path coverage in longitude compared to that in previous studies. The amplitude and phase maximum changes are generally positively correlated with X-ray fluxes, whereas the time delay is negatively correlated. The curve-fitting parameters that we obtain for the X-ray fluxes and VLF signal maximum changes are consistent with those in previous studies for dayside paths, even though different instruments are used, supporting the flare-nowcasting method. Moreover, the present results show that, for day–night mixed paths, the amplitude and phase maximum changes also scale linearly with the logarithm of the flare X-ray fluxes, but the level of change is notably different from that for dayside paths. The coefficients used in the flare-nowcasting method need to be updated for mixed propagation paths.

Tweek signals are a special type of VLF (very low frequency) pulse, originally produced by lightning discharge, which becomes dispersive after repetitive bounces within the waveguide between the Earth’s surface and lower ionosphere. As such, tweek signals carry critical information about the region near the reflection height of the VLF waves, namely the D-region ionosphere. Although tweek measurements have been widely utilized in studies of the D-region ionosphere and lightning discharge, few statistical studies have been conducted, mainly due to the difficulty of manually identifying tweek signals from the enormous amount of VLF data with heavy noise. Considering the importance of tweek signals and the lack of a high-precision detection model, in this study, we propose a method to automatically and accurately pick out tweek signals from VLF measurements. This method is explicitly developed based on the you only look once (YOLO) model and a post-tracing process. Using a total of 2495 randomly selected VLF spectrogram images as the testing set, we evaluated the performance of this method. The precision and recall are found to be 92.0% and 89.2% for the first-order mode, and 97.5% and 86.7% for the first-two-order mode tweek, respectively. The time needed to process 10-s VLF measurements with a cadence of 4 μs is only 6.5 s, allowing for identifying the tweek signals from continuous VLF measurements in real time. Therefore, this method represents a reliable means to automatically detect tweek signals and enables the opportunity to statistically investigate the D-region ionosphere and lightning discharge via these signals.

Earlier studies have recorded seismic effects in sub-ionospheric very low frequency (VLF) signals, predominantly examine a single VLF propagation path associated with a specific earthquake. This research expands on these findings by taking a more advanced setup. Using Japan’s VLF network with eight stations nationwide and a single transmitter signal from JJI (22.2 kHz), our study examines four earthquakes, each with a magnitude over 6, happening near VLF network locations. The chosen earthquakes are Iwaki on July 11, 2014, Aomori prefecture on August 10, 2014, Ogasawara on May 30, 2015, and Makurazaki on November 13, 2015. 3 are shallow and close to land, but deliberately included are one deep oceanic quake (Ogasawara) at 664 km depth and one occurring under pre-geomagnetic storm conditions 6 days before (Makurazaki). We examine nighttime VLF signal distortion during earthquake events and investigate the presence of atmospheric gravity waves. This study supports that deep oceanic earthquakes can perturb the ionosphere at considerable distances from their epicenter. Furthermore, we investigate the simultaneous ionospheric impact due to geomagnetic storms and the earthquake. By examining the seismic effects preceding the storm occurrence, we find the likelihood of seismogenic effects that are not contaminated by other factors. The response of the initial seismogenic perturbation varies at different distances from the epicenter. Surprisingly, nearby receivers do not experience early disturbances. This indicates how challenging it is to forecast the location of the earthquake.

Solar eclipse is a unique phenomenon that produces an orderly disturbance in the ionosphere within a specific time frame. It provides us an opportunity to understand the ionospheric response due to its systematic variation during an eclipse. The amplitude and phase of a Very Low Frequency (VLF) signal carry the response of lower ionospheric perturbation due to the varying solar radiation impinging on Earth. During the Annular Solar Eclipse on December 26, 2019, (ASE2019), Indian Centre for Space Physics (ICSP), Kolkata, India conducted a nationwide VLF radio signal monitoring campaign to obtain the amplitude and phase variations of propagating VLF signal from fourteen different locations across India. The signal amplitude and phase profile exhibit unique profiles at these locations. These profiles in the VLF signal during ASE2019 are explained using the Long Wavelength Propagation Capability (LWPC) code and the modeled variation of solar disk obscuration by the moon. Furthermore, the lower ionospheric electron density (Ne) computed from the model is in agreement with the observed lower ionospheric conditions.