VLF Signals Research Articles

Lightning evolution and VLF perturbations associated with category 5 TC Yasa in the South Pacific Region

In this paper, we present the D-region ionospheric response during the lifespan (10–19 December 2020) of a severe category 5 tropical cyclone (TC) Yasa in the South Pacific by using the very low frequency (VLF, 3–30 kHz) signals from NPM, NLK, and JJI transmitters recorded at Suva, Fiji. Results indicate enhanced lightning and convective activity in all three regions (eyewall, inner rainbands, and outer rainbands) during the TC Yasa that are also linked to the wave-sensitive zones of these transmitter–receiver great circle paths. Of the three regions, the outer rainbands showed the maximum lightning occurrence; hence convective activity. Prominent eyewall lightning was observed just before the TC started to weaken following its peak intensity. Analysis of VLF signals amplitude showed both negative and positive perturbations (amplitudes exceeding ± 3σ mark) lasting for more than 2 h with maximum change in the daytime and nighttime signal amplitudes of − 4.9 dB (NPM) and − 19.8 dB (NLK), respectively. The signal perturbations were wave-like, exhibiting periods of oscillations between ~ 2.2 and 5.5 h as revealed by the Morlet wavelet analysis. Additionally, the LWPC modeling of the signal perturbations indicated a 10 km increase in the daytime D-region reference height, H′, and a 12 km decrease in the nighttime D-region H′ during TC Yasa. The D-region density gradients (sharpness), β, showed small perturbations of 0.01–0.14 km−1 from its normal values. We suggest that the observed changes to the D-region parameters are due to the enhanced convection during TC Yasa which excites atmospheric gravity waves producing traveling ionospheric disturbances to the D-region.Graphical

Earthquakes associated subionospheric VLF anomalies recorded at two low latitude stations in the South Pacific region

The JJI VLF (22.2 kHz) transmitter signal received at two low-latitude stations, one in Port Vila (geog. coord., 17.73°S, 168.33°E), Vanuatu and other in Suva (18.14°S, 178.44°E), Fiji, was analyzed for any VLF changes due to 16 Earthquakes (EQs) with magnitudes 5.5 to 7.7, during 2018 (JJI-Vanuatu path, 6.8 Mm) and 2007 to 2018 (JJI-Suva path, 7.5 Mm). The VLF signal amplitude analysis included terminator time (TT), average daytime and nighttime amplitude variation, nighttime fluctuation, and mother Morlet wavelet methods. Out of 16 EQs only eleven EQs have shown subionospheric VLF changes including the decrease in the amplitude for about 2–8 h on the EQ day, unusual shifts in the TT of up to 5–9 min, and the decrease in the average daytime and nighttime signal amplitude of about 1–1.5 dB and 1–5 dB, respectively, on the mainshock day of the EQs. The dA(t) < 0 condition was observed about 4–5 days before the EQ which stabilized after 3–4 days from the EQ day. A decrease in the non-normalized and normalized trend of below -2σ (standard deviation) mark was found on the EQ day and an increase in the non-normalized and normalized NF and dispersion of above +2σ mark on the day of seismic activity was found. Mother wavelet analysis of EQ associated changes in the signal amplitude showed a strong and enhanced presence of short frequency (∼0.05–0.10 mHz) wave-like signatures, a few days prior, on the day of EQ, and after the EQ day as compared to normal days.

Response of the Earth’s Lower Ionosphere to Solar Flares and Lightning-Induced Electron Precipitation Events by Analysis of VLF Signals: Similarities and Differences

The lower ionosphere influences the propagation of electromagnetic (EM) waves, satellite and also terrestrial (anthropic) signals at the time of intense perturbations and disturbances. Therefore, data and modelling of the perturbed lower ionosphere are crucial in various technological areas. An analysis of the lower ionospheric response induced by sudden events during daytime-solar flares and during night-time-lightning-induced electron precipitation was carried out. A case study of the solar flare event recorded on 7 September 2017 and lightning-induced electron precipitation event recorded on 16 November 2004 were used in this work. Sudden events induced changes in the ionosphere and, consequently, the electron density height profile. All data are recorded by Belgrade (BEL) radio station system and the model computation is used to obtain the ionospheric parameters induced by these sudden events. According to perturbed conditions, variation of estimated parameters, sharpness and reflection height differ for analysed cases. Data and results are useful for Earth observation, telecommunication and other applications in modern society.

Analysis of the Solar Terminator Shift Determined from the VLF Signal Amplitude in the Period Around the Intense Seismic Activity in Central Italy from 25 October to 3 November 2016

Analysis of the Solar Terminator Shift Determined from the VLF Signal Amplitude in the Period Around the Intense Seismic Activity in Central Italy from 25 October to 3 November 2016

Responses of the Very Low Frequency Transmitter Signals During the Solar Eclipse on December 26, 2019 Over a North–South Propagation Path

During the solar eclipse, the perturbation of ionospheric D layer changes the characteristics of the Earth’s Ionospheric Waveguide (EIWG) on which the very low frequency (VLF, 3–30 kHz) wave propagation depends. Therefore, the amplitude and phase of the VLF signal transmitted through the waveguide will be abnormal. In this article, based on the VLF transmitter signals observed in Suizhou (31.57°N, 113.32°E) during the total solar eclipse on December 26, 2019 and the days before and after, the variation characteristics of VLF transmitter signals along the north–south propagation path are analyzed in detail. Responses of the amplitude and phase of the signal during the solar eclipse are closely related to the solar obscuring rate. There is a positive correlation between the signal fluctuation and the solar obscuring rate, and the peak time of the two has a delay of ~5 min. By adopting the amplitude and phase of the observed signals and performing the Long Wavelength Propagation Capability (LWPC) propagation simulations, the electron density of the ionosphere over the propagation path is calculated. The results show that the electron density profile above the path during the solar eclipse changes significantly. The electron density decreases with a maximum drop of ~53.5% at the 70 km height, and the reflection height of the signal increases correspondingly. The obtained results are useful to better understand the propagation characteristics of VLF transmitter waves and the corresponding response features of the ionospheric D layer to solar radiation flux variations, especially during the solar eclipse.

Determine the physical mechanism and source region of beat wave modulation by changing the frequency of high-frequency waves

This paper introduces a new approach for the determination of the source region of beat wave (BW) modulation. This type of modulation is achieved by transmitting high-frequency (HF) continuous waves with a frequency difference f, where f is the frequency of modulated ELF/VLF (extremely low frequency/very low frequency) waves from two sub-arrays of a high power HF transmitter. Despite the advantages of BW modulation in terms of generating more stable ELF/VLF signal and high modulation efficiency, there exists a controversy on the physical mechanism of BW and its source region. In this paper, the two controversial theories, i.e., BW based on D-E region thermal nonlinearity and BW based on F region ponderomotive nonlinearity are examined for cases where each of these two theories exists exclusively or both of them exist simultaneously. According to the analysis and simulation results presented in this paper, it is found that the generated VLF signal amplitude exhibits significant variation as a function of HF frequency in different source regions. Therefore, this characteristic can be utilized as a potential new approach to determine the physical mechanism and source location of BW.

A New Four‐Parameter D‐Region Ionospheric Model: Inferences From Lightning‐Emitted VLF Signals

AbstractWe present a new four‐parameter model of the D‐region (60–90 km) ionospheric electron density, useful in very low frequency (VLF, 3–30 kHz) remote sensing. VLF waves have a long history of use to indirectly infer D‐region conditions, as they reflect efficiently and thus are sensitive to small changes in the electron density. Most historical efforts use VLF observations along with a forward model of the D‐region and VLF propagation. The ionospheric assumptions in the forward model are altered until the output matches the observation. The most common D‐region model, known as the Wait‐Spies ionosphere, takes the electron density as exponentially increasing with altitude and specifies a height and steepness. This model was designed to capture the VLF propagation variations evident at a single frequency. The real D‐region is likely more complex. The limited number of D‐region rocket passes that have previously been compiled tend to show the existence of a “ledge” somewhere between 70 and 90 km. Broadband VLF signals emitted from lightning allows a more sophisticated parametrization. Using carefully averaged amplitudes and phases of VLF sferics, we formulate a more general four‐parameter D‐region model that includes a ledge discontinuity. Using lightning‐emitted VLF observations along with a theoretical model, we find that this model better describes the ionosphere during the daytime. During the ambient nighttime and during a solar flare the two‐parameter ionosphere may be sufficient, at least for the purposes of calculating broadband VLF propagation, since the ledge either weakens or moves outside the altitude range of VLF sensitivity.

STUDY THE BIOPOTENTIAL PARAMETER FOR DETECTION OF SEISMIC AND ENVIORNMENTAL CHANGES IN INDIAN REGION

The changes in the magnetic flux generated (electric, magnetic and electromagnetic waves) on the surface of earth due to sudden changes is a matter of discussion. These emissions occur along the fault line generated due to geological and tectonic processes. When sudden changes occur in the environment due to seismic and atmospheric variations, these sensing was observed by creatures and human bodies because the animals and trees adopt the abnormal signals and change the behavior. We have analyzed the changing behavior of recorded signal by live sensors (i.e. banyan tree). So we use the deep rooted and long age’s banyan tree. Banyan tree (long ages) in which root has been working as a live sensor and record the geological and environmental changes. We record the low frequency signals which propagated through solar-terrestrial environment affect directly the root system of the banyan tree and changes has been observed by live sensors. Then, VLF signal ma(y propagate to the earth-ionosphere waveguide. We have also analyzed the different parameters of live cells which is inbuilt in latex of the tree, so we record the dielectric parameters of green stem latex and found some parameters i.e., dielectric constant (ε) and dielectric loss (ε’) of various trees to verify these natural hazards and found good correlation. Therefore, we can say by regularly monitoring the bio-potential signal and dielectric properties of banyan tree we can able to find the precursory signature of seismic hazards and environmental changes.

The Estimation of D‐Region Electron Densities From Trans‐Ionospheric Very Low Frequency Signals

AbstractDue to the relatively low ionization levels present in the D‐region of the ionosphere (60–90 km), few reliable methods exist for estimating electron densities at this altitude. Very Low Frequency (VLF, 3–30 kHz) radio waves, unlike higher frequency waves used for similar remote sensing applications, are sensitive to these low ionization levels. Much past work has addressed the estimation of D‐region electron densities using ground‐based measurements of signals originating from VLF transmitters or atmospheric lightning. We introduce a new technique for the estimation of ionospheric D‐region electron densities using satellite observations of trans‐ionospheric VLF signals originating from high‐powered VLF transmitters. Although most of the energy radiated from these transmitters is confined to sub‐ionospheric propagation modes, a small fraction penetrates the ionosphere in a right‐hand circularly polarized “whistler mode” which has been observed by many satellites. We show that the modal interference pattern previously observed enables the estimation of the electron density profile in the D‐region from a single satellite pass. Using data from the DEMETER experiment, each collection is compared with results from a validated theoretical model to select a best‐fit D‐region electron density profile. Results show expected seasonal variations and are consistent with rocket observations. Although the D‐region electron density estimates derived with this technique are shown to exhibit long‐term seasonal variations consistent with rocket and ground‐based observations, it is important to note that validation of these results remains difficult due to the lack of other techniques for continuously sensing the D‐region.

Long-lasting disturbances in the mid-latitude sub-ionospheric VLF radio signals due to the super geomagnetic storm of 17 March 2015

• Effect of super geomagnetic storm on the sub-ionospheric VLF radio signals. • Long-lasting VLF signal disturbances observed in the mid-latitude ionosphere. • West-east and east-west propagation paths showed different disturbance patterns. • Enhanced electron density in the D-region follows the precipitated electron flux. • Wave-like oscillations with periods in the range of 15–80 min have been observed. This paper reports disturbance in the mid-latitude sub-ionospheric VLF radio signals due to the super geomagnetic storm which began on 17 March 2015. Narrow-band signals from the NAA transmitter are studied for the storm period recorded at eight mid-latitude receiving stations spread over the Europe and USA. Daytime signals amplitude at all places showed a disturbing pattern after 17 March. Fluctuation in the nighttime signals significantly increased in the succeeding nights. As a primary effect of the storm, the entire diurnal signals in the transoceanic west to east long propagation paths enhanced by 3–5 dB, which gradually decreased over the period of ~ 10 days following the storm recovery. A different behavior was observed in the east to west short propagation paths over the landmass, where during the peak storm the daily variations of the VLF amplitude reduced to 20–25% of a normal day and, after ~ 10 days the signals returned to the pre-storm condition. Modeling of the radio waves in the west to east paths shows that the D-region electron density was increased by ~ 8-fold and varied up to 10 days. Electron density variations in the D-region closely follows the variations of precipitated electron flux as observed by the POES satellite over the region. The elevated electron density in the D-region ionosphere caused by the extension of the auroral precipitation to the mid-latitudes along with interference among the various waveguide modes in the earth-ionosphere waveguide during the storm is suggested for the cause of observed VLF signals behaviors.

Ionospheric Plasma Fluctuations Induced by the NWC Very Low Frequency Signal Transmitter

AbstractThe Australian NWC (North West Cape) signal transmitter is known to strongly interfere with the topside ionosphere. We analyze 456 conjunctions between Swarm A, B and NWC, in addition to 58 conjunctions between NorSat‐1 and NWC. The in‐situ measurements provided by these satellites include the 16 Hz Swarm Advanced Plasma Density data set, and the novel 1,000 Hz plasma density measurements from the m‐NLP system aboard NorSat‐1. We subject the data to a detailed PSD analysis and subsequent superposed epoch analysis. This allows us to present comprehensive statistics of the NWC‐induced plasma fluctuations, both their scale‐dependency, and their climatology. The result should be seen in the context of VLF signal transmitter‐induced plasma density fluctuations, where we find counter‐evidence for the existence of turbulent structuring induced by the NWC transmitter.

Amplitude Variations of VLF Signal of NWC Transmitter (19.8 khz) Propagating Over the Earthquake Epicentre

Lower ionosphere perturbations accompanying earthquakes can be investigated by VLF signals of radio stations. Amplitude variations of VLF signal of NWC transmitter (19.8 kHz) are considered during registration in Yakutsk from August 21 to September 15, 2018 to detect lower ionosphere disturbances as a result of the earthquake (magnitude is 6.6, depth is 35 km) on Hokkaido Island (42.686 ° N 141.929 ° E) on September 5, 2018. On the day of the earthquake the effect manifested as the amplitude increase of the radio signal.

Strong Amplification of ELF/VLF Signals in Space Using Neutral Gas Injections From a Satellite Rocket Engine

AbstractThe first demonstration of rocket exhaust driven amplification (REDA) of whistler mode waves occurred on May 26, 2020 by transferring energy from pickup ions in a rocket exhaust plume to EM waves. The source of coherent VLF waves was the Navy NML Transmitter at 25.2 kHz located in LaMoure, North Dakota. The topside ionosphere at 480 km altitude became an amplifying medium with a 60 s firing of the Cygnus BT‐4 engine. The rocket engine injected exhaust as a neutral cloud moving perpendicular to field lines that connected the NML transmitter to the VLF Radio Receiver Instrument (RRI) on e‐POP/SWARM‐E. Charge exchange between the ambient O+ ions and the hypersonic water molecules in the exhaust produced H2O+ ions in a ring‐beam velocity distribution. The 25.2 kHz VLF signal from NML was amplified by 30 dB for a period 77 s as observed by the RRI. Simultaneously, preexisting coherent ELF waves at 300 Hz were amplified by 50 dB during and after the Cygnus burn. Extremely strong coherent emissions and quasiperiodic bursts in the 300–310 Hz frequency range lasted for 200 s after the release. The excitation of an ELF whistler cavity may have lasted even longer, but the orbit of the SWARM‐E/e‐POP moved the RRI sensor away from the wave emission region. The amplified 300 Hz ELF waves may have gained even more energy by cyclotron resonance with radiation belt electrons as they were ducted between geomagnetic‐conjugate hemispheres.

Variation of some planetary seismic hazard indices on the occasion of Lefkada, Greece, earthquake of 17 November, 2015

In this paper we investigate the variation of the tidal triggering effect efficiency, by means of the tidal seismicity compliance parameter p, and the lower Ionosphere variations, by means of the variation of the High-Frequency limit, fo, of the ionospheric turbulence content with the time and space proximity to the site of the earthquake occurrence as well as by the intensity variations of VLF signals transmitted over the seismic area. It is shown that the “Earth tides-seismicity compliance parameter” p may be used as a medium time earthquake warning while the frequency content of the ionospheric turbulence over the earthquake epicenter, deduced directly from GPS network TEC observations or indirectly through the VLF transmission network, may be used for the short time earthquake forecasting, deserving the special attention of the authorities, the scientists and the society.

Investigation of Precursors in VLF Subionospheric Signals Related to Strong Earthquakes (M &gt; 7) in Western China and Possible Explanations

Earthquakes may disturb the lower ionosphere through various coupling mechanisms during their seismogenic and coseismic periods. The VLF signal radiated from ground-based transmitters is affected when it passes near the disturbed region above the seismogenic area, and this anomaly can be recorded by ground-based VLF receivers. In this paper, the seismic anomalies before two strong earthquakes (M &gt; 7) that occurred in western China were detected using the ground-based observation of VLF signal; the possible reasons for the anomalies were discussed using full wave simulation. The amplitude of the VLF signals observed by the link between NOV, KHA transmitter, and VLF receivers at Ya’an and Tonghai show obvious anomaly by nighttime fluctuation analysis. The simulated results demonstrate that the anomalies could have been induced by ascending/descending of the bottom height of the ionosphere, caused by depletion/increase in D region electron density. The simulated result also illustrates that terminator time shift could have been induced by descending of the bottom boundary of the ionosphere, which is due to modal interference between different wave modes.

ELF/VLF Communication Experiment by Modulated Heating of Ionospheric Auroral Electrojet at EISCAT

Amplitude modulation high-frequency (HF) radio waves at extremely low frequencies (ELF, 0.3-3 kHz) or very low frequencies (VLF, 3-30 kHz), heating the ionosphere, can nonlinearly modulate the auroral electrojet to produce ELF/VLF waves for long-distance communication. This article presents the results of the first ELF/VLF communication experiments being conducted at Tromsø, where the European Incoherent Scatter Scientific Association (EISCAT) HF facility is located. The ELF/VLF waves from the ionosphere are modulated by quaternary phase shift keying (QPSK) and used for communication. The ELF signals of 2017 Hz at 20 and 100 b/s, as well as VLF signals of 3517 Hz at 20, 100, and 400 b/s, received by a ground-based receiver (15 km away from the heater) were decoded, albeit with a high bit error rate (BER) for 3517 Hz at 20 b/s. With the decoding principle of QPSK, we attempted to find a solution to reduce the BER for future communication experiments. Based on the results of the ground experiment, the transmitted signals of 2017 Hz at 20 b/s were selected in the China Seismo-Electromagnetic Satellite (CSES) reception experiments. In a first, we demonstrate that ELF communication signals generated by the amplitude modulation heating of the ionosphere have been successfully received by the satellite and the ground-based receiver. These experimental results confirm that the ELF/VLF waves radiated by amplitude modulation heating of the auroral electrojet can be used for long-distance communication in future.

Daytime VLF Emissions during the Magnetic Storm Recovery Phase: the Event of January 5, 2015

This paper discusses the spectral and temporal features of the 7-h daytime VLF noise emission burst in the frequency band of 1‒10 kHz observed on the Earth’s surface during the late recovery phase of a moderate magnetic storm (Kр = 5 and Dst ~ –80 nT) under nearly quiet space weather. These VLF emissions were recorded simultaneously at two auroral stations located at a geomagnetic latitude of ~64° MLAT and at a longitude distance of ~400 km from each other: Finnish Kannuslehto Station (KAN) and Russian Lovozero Observatory (LOZ). The dynamic VLF wave spectrum is considered at different time scales. During the development of the discussed daytime VLF burst, the noise emissions at below 3.5 kHz gradually turn into periodic (QP) emissions with a quasi-repetition of ~3 s. Having compared the directions of short isolated high-frequency (above 5 kHz) VLF signals (so-called “birds”) arriving at KAN and LOZ, we draw conclusions on the possible location and spatial dynamics of the exit area of emission from the ionosphere.

Low‐Latitude VLF Radio Signal Disturbances Due to the Extremely Severe Cyclone Fani of May 2019 and Associated Mesospheric Response

AbstractWe present new results of lower ionospheric disturbances due to the extremely severe cyclonic storm Fani over northeastern part of the Indian Ocean. Very low frequency radio signal received at Kolkata, India, and mesospheric temperature and ozone concentration data from the NASA's TIMED satellite are used for this purpose. Significant wave‐like oscillations and strong amplitude anomalies in daytime and nighttime VLF signal were observed when the cyclone center was within ∼700 km from the receiver. Both the mesospheric ozone concentration and temperature showed maximum anomalies beyond 3σ during the cyclone period. Maximum ozone anomaly and maximum VLF anomaly occurred on the same day when cyclone intensity was maximum, while the maximum temperature anomaly occurred on the next day during landfall. Mesospheric temperature enhancement around VLF reflection heights indicates changes in the chemical composition and electron‐neutral balance in the D region ionosphere. Simulation of VLF signal showed that the D region reflection height decreased by 7.9 km and the D region electron density increased by ∼20 times compared to precyclone midnight values at the maximum perturbed area along the propagation path. Wavelet analysis confirmed significant enhancement of atmospheric gravity waves spectrum with periods 10 min to 2 hr around landfall. Further, a strong anticorrelation between total wavelet power in the wave band of periods 10–30 min and cyclone pressure suggests a possibility of monitoring cyclone intensity from mesospheric gravity waves using VLF radio measurements.

A two year survey for VLF emission from fireballs

Here we report on a two year continuous survey to examine possible VLF signals associated with meteors brighter than magnitude −5. Our survey allowed both calibrated temporal and spatial correlations between VLF signals and fireball light curves. We used continuous observations from the Atmospheric Weather Electromagnetic System for Observation Modeling and Education (AWESOME) VLF receiver system (Cohen et al., 2010) deployed at the Elginfield Observatory near London, Ontario, Canada (43N, 81W) to monitor VLF radio signals and correlate with all-sky video recordings of fireballs. This survey from May 2017 to March 2019 was cued using fireballs detected by the Southern Ontario Meteor Network (Brown et al., 2010; Weryk et al., 2008). The GPS conditioned timing of the AWESOME system was continuously synchronized with video recordings directly in the video stream to ensure sub-frame VLF-optical time calibration. The AWESOME system has two orthogonal VLF antennas which permits directional calculation of incoming VLF signals, which were compared to the apparent optically measured locations of simultaneously detected fireballs. VLF events potentially linked to fireballs were checked against the National Lightning Detection Network (NLDN) database to remove false positive association with lightning. During the two year survey interval, over 80 bright meteors (apparent magnitude brighter than −5, brightest recorded event −7.8) were detected and compared to VLF signals detected by the AWESOME system. No definitive evidence was found for VLF emission from meteors up to a limiting magnitude of −7.8.

Variation in natural short-period ionospheric noise, and acoustic and gravity waves revealed by the amplitude analysis of a VLF radio signal on the occasion of the Kraljevo earthquake (Mw = 5.4)

We analyse the lower ionosphere disturbances in the time period around the Mw 5.4 Kraljevo earthquake (EQ), which occurred on 3 November 2010 in Serbia. The results presented herein are based on analysis of the amplitudes of three VLF signals emitted in Italy, UK, and Germany and recorded in Serbia whose variations primarily result from changes in the electrical properties of the lower ionosphere at a distance more than 120 km from the epicentre of the EQ. The primary goals of this study are to reveal specific variations as possible EQ precursors as well as disturbances following the EQ event recorded by the observational equipment, and to investigate whether better time resolution data can affect the analysis of the lower ionosphere disturbances possibly connected to the EQ. We process two sets of data with sampling periods of 1 min and 0.1 s. As the first analysis indicates the absence of long-term disturbances, which can clearly be connected to the Kraljevo EQ, our attention is focused on the study of short-period noise amplitude and the excitation and attenuation of acoustic and gravity waves in the lower ionosphere. Processing of the amplitudes of three VLF signals during the nights of the four EQs with magnitude greater than 4 that occurred in Serbia, as well as EQs of all magnitudes during the three days of 3, 4, and 9 November, indicates that the detected ICV radio signal noise amplitude reduction starting before the Kaljevo EQ is also observed for 13 additional EQ events near the signal propagation path, and occurred during all three days (for all EQs with magnitude greater than 4 and several less intensive events). Excitation and attenuation of acoustic waves are also found for all these EQ events with a magnitude greater than 4.