VLF Radio Signals Research Articles

Solution uniquity of an inverse VLF problem: A case-study of the polar, ground-based, VLF radio signal disturbances caused by the ultra-energetic relativistic electron precipitations and of their southern boundaries

Here we present the results of a case study of the rare, abnormal, qualitatively specific behavior of Aldra (northern Norway) and GBR (UK) VLF transmitter signals (10–16kHz) received at Kola Peninsula. The abnormal amplitude and the phase disturbances of signals were used as a proxy for ultra-energetic relativistic (solar?) electron precipitation (URE, ∼100MeV) into the middle polar atmosphere. The disturbances have been observed under quiet or moderately disturbed geomagnetic activity. Based on bearing results, it was established that the abnormal variations of the electric conductivity of ionized middle atmosphere (of a sporadic Ds layer under the regular ionosphere D layer) were characterized by the following: (i) the time function of height h(t) of an effective spherical waveguide between the Earth surface and the sporadic Ds layer shows a minimum value equal to ∼30km and (ii) the reflection coefficient R(t) of radio wave with a grazing angle of incidence from a virtual boundary with height h(t) has a minimum value equal to ∼0.4. The southern boundaries of the ultra-energetic relativistic electron precipitations have been found as well. They turned out to be not southerly than 61 degree of magnetic latitude and similar to the ones obtained in our previous study of the events for other dates under the similar geophysical conditions although we do not know anything definite about the rigidity and density of the electron fluxes.A used calculation method of analysis is based on a necessary condition that a number n of input data should be greater than a number m of output parameter-functions. We have stated by numerical testing that a decrease of n from 6 to 4 generates a lack of uniqueness of an inverse VLF problem solution for m=2. It is important for future VLF ground-based monitoring of the URE precipitation events.

Вариации амплитуды и фазы сигналов ОНЧ-радиостанций в период солнечного затмения 20 марта 2015 г.при регистрации в Якутске и Улан-Удэ

Вариации амплитуды и фазы сигналов ОНЧ-радиостанций в период солнечного затмения 20 марта 2015 г.при регистрации в Якутске и Улан-Удэ

Precursory Anomaly in VLF/LF Recordings Prior to the Çaglayan (Erzincan-Turkey) Earthquake on July 30th, 2009

An international project network consisting of six receivers for sampling LF and VLF radio signals has been going on to record the data in Europe from different transmission stations around the World. One of them was established in Resadiye, Turkey, located just on the North Anatolian Fault Zone. The receiver works in VLF and LF bands monitoring ten frequencies (16.4, 21.75, 37.5, 45.9, 153, 180, 183, 216 and 270 kHz) with one minute sampling interval. An earthquake of Mw = 4.9 took place 225 km away from the VLF/LF station at the eastern tip of the Erzincan basin at 4 km depth on July 30, 2009. We observed some anomalies on the radio signals (37.5 and 153 kHz) that initiated about 7 days before the earthquake and disappeared soon after the earthquake. We attribute this anomaly to the Mw=4.9 earthquake as a seismo-electromagnetic precursor. The radio anomaly that appeared 7 days before the occurrence of the 2009 Caglayan (Erzincan) earthquake is in good agreement with other results indicating precursory anomalies in the project network mostly observed in seismically active countries such as Italy and Greece.

Contribution of solar hydrogen Lyα line emission in total ionization rate in ionospheric D-region during the maximum of solar X-flare

The solar Ly? line emission can be considered as the dominant source of ionization processes in the ionospheric D-region at altitudes above 70 km during unperturbed conditions. However, large sudden impacts of radiation in some other energy domains can also significantly influence the ionization rate and, in this paper, we present a study on the contribution of Ly? radiation to the ionization rate when the ionosphere is disturbed by solar X-flares. We give relevant analytical expressions and make calculations and numerical simulations for the low ionosphere using data collected by the VLF receiver located in Serbia for the VLF radio signal emitted by the DHO transmitter in Germany.

Low‐latitude ionospheric D region dependence on solar zenith angle

AbstractPhase and amplitude measurements of VLF radio signals on a short, nearly all‐sea path between two Hawaiian Islands are used to find the height and sharpness of the lower edge of the daytime tropical D region as a function of solar zenith angle (SZA). The path used was from U.S. Navy transmitter NPM (21.4 kHz) on Oahu to Keauhou, 306 km away, on the west coast of the Big Island of Hawaii, where ionospheric sensitivity was high due to the destructive interference between the ionospherically reflected wave and the ground wave, particularly around the middle of the day. The height and sharpness are thus found to vary from H′ = 69.3 ± 0.3 km and β = 0.49 ± 0.02 km−1 for SZA ~10°, at midday, to H′ > 80 km and β ~ 0.30 km−1 as the SZA approached ~70°–90°, near dawn and dusk for this tropical path. Additional values for the variations of H′ and β with solar zenith angle are also found from VLF phase and amplitude observations on other similar paths: the short path, NWC to Karratha (in NW Australia), and the long paths, NWC to Kyoto in Japan and NAU, Puerto Rico, to St. John's Canada. Significant differences in the SZA variations of H′ and β were found between low and middle latitudes resulting from the latitudinally varying interplay between Lyman α and galactic cosmic rays in forming the lower D region. Both latitude ranges showed β < 0.30 km−1 during sunrise/sunset conditions.

Variations of VLF radio signals and atmospherics during the deep earthquake with M = 8.2 occurred on 24 May 2013 near Kamchatka peninsula

To detect seismic disturbances in the lower ionosphere, we have used the signals of very-lowfrequency radio transmitters and natural radio signals—electromagnetic emission of lightning discharges—atmospherics. On earlier results of observation of atmospherics, it was obtained that the earthquake effects are displayed as weight-hourly amplitude increases on the day of event or within 3 days after them. Possible earthquake precursors are also manifested as one-day (within one to several hours) increases in the amplitude of atmospherics on average 5 - 12 days before the event. Analysis shows that seismic effects in the amplitude of atmospherics have been observed in the case of sufficiently strong (magnitude M > 4.5) and not very deep (usually no deeper than 50 km) earthquakes. The effects of the events of the earthquake with magnitude of 8.2 occurring in the Sea of Okhotsk on 24.05.13 not far from the Kamchatka Peninsula at a depth of 609 km considered in this work have shown that even deep earthquakes may have precursors in the form of disturbances in the lower ionosphere.

Tropical daytime lower D‐region dependence on sunspot number

Observed phases and amplitudes of VLF radio signals propagating on (near) tropical all‐sea paths, both short, ∼300 km, and long, ∼10 Mm, are used to find daytime parameter changes for the lowest edge of the (D‐region of the) Earth's ionosphere as the solar cycle advanced from a very low sunspot number of ∼5 up to ∼60, in the period 2009–2011. The VLF phases, relative to GPS 1‐s pulses, and amplitudes were measured ∼100 km from the transmitter, where the direct ground wave is very dominant, ∼300 km from the transmitter, near where the ionospherically reflected waves form a (modal) minimum with the ground wave, and ∼10 Mm away where the lowest order waveguide mode is fully dominant. Most of the signals came from the 19.8 kHz, 1‐MW transmitter, NWC, North West Cape, Australia, propagating ENE, mainly over the sea, to the vicinity of Karratha and Dampier on the NW coast of Australia and then on to Kauai, Hawaii, ∼10.6 Mm from NWC. Observations from the 8.1‐Mm path NPM (21.4 kHz, Hawaii) to Dunedin, NZ, are also used. The sunspot number increase from ∼5 to ∼60 was found to coincide with a decrease in the height,H′, of the midday tropical ionosphere by 0.75 ± 0.25 km (from H′ ≈ 70.5 km to H′ ≈ 69.7 km) while the sharpness, β increased by 0.025 ± 0.01 km−1 (from β ≈ 0.47 km−1 to β ≈ 0.49 km−1) where H′ and βare the traditional height and sharpness parameters used by Wait and by the U.S. Navy in their Earth‐ionosphere VLF radio waveguide programs.

Response of low latitude D-region ionosphere to the total solar eclipse of 22 July 2009 deduced from ELF/VLF analysis

Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (Geog. lat., 25.27° N; Geog. long., 82.98° E; Geomag. lat.=14° 55’ N) was investigated using ELF/VLF radio signal. Tweeks, a naturally occurring VLF signal and radio signals from various VLF navigational transmitters are first time used simultaneously to study the effect of total solar eclipse (TSE). Tweeks occurrence is a nighttime phenomena but the obscuration of solar disc during TSE in early morning leads to tweek occurrence. The changes in D-region ionospheric VLF reflection heights (h) and electron density (ne: 22.6–24.6cm−3) during eclipse have been estimated from tweek analysis. The reflection height increased from ∼89km from the first occurrence of tweek to about ∼93km at the totality and then decreased to ∼88km at the end of the eclipse, suggesting significant increase in tweek reflection height of about 5.5km during the eclipse. The reflection heights at the time of totality during TSE are found to be less by 2–3km as compared to the usual nighttime tweek reflection heights. This is due to partial nighttime condition created by TSE. A significant increase of 3dB in the strength of the amplitude of VLF signal of 22.2kHz transmitted from JJI-Japan is observed around the time of the total solar eclipse (TSE) as compared to a normal day. The modeled electron density height profile of the lower ionosphere depicts linear variation in the electron density with respect to solar radiation as observed by tweek analysis also. These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days.

Anomalies Observed in VLF and LF Radio Signals on the Occasion of the Western Turkey Earthquake (M<sub>w</sub> = 5.7) on May 19, 2011

Since 2009 a network of VLF (20 - 60 kHz) and LF (150 - 300 kHz) radio receivers is operating in Europe in order to study the disturbances produced by the earthquakes on the propagation of these signals. In 2011 the network was formed by nine receivers, of which three are located in Italy and one is in Austria, Greece, Portugal, Romania, Russia and Turkey. On May 19, 2001 an earthquake (Mw = 5.7) occurred in western Turkey, that is inside the “sensitive” area of the network. The radio data collected during April-May 2011 were studied using the Wavelet spectra, the Principal Component Analysis and the Standard Deviation trends as different methods of analysis. Evident anomalies were revealed both in the signals broadcasted by the TRT transmitter (180 kHz) located near Ankara and in a VLF signal coming from a transmitter located in Western Europe and collected by the receiver TUR of the network located in eastern Turkey. Evident precursor phases were pointed out. Some differences in the efficiency of the three analysis methods were revealed.

DaytimeDregion parameters from long-path VLF phase and amplitude

[1] 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.

Probing geospace with VLF radio signals

Andrew J Kavanagh, Michael Denton, James Denton and Hamish Harron report on a pilot study for university–school partnership that also uncovers new aspects of the behaviour of the ionosphere.

Daytime midlatitudeDregion parameters at solar minimum from short-path VLF phase and amplitude

[1] 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.

Daytime tropical D region parameters from short path VLF phase and amplitude

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.

A new approach for monitoring the 27‐day solar rotation using VLF radio signals on the Earth's surface

Typical solar parameters such as sunspot number, Lyman alpha radiation, and 10.7 cm radio flux exhibit 27‐day variations associated with the rotation of the Sun. We have discovered a robust indicator of this 27‐day rotation from measurements of VLF radio signals produced by lightning around the globe. The solar rotation signal is found only at VLF frequencies close to the Earth‐ionosphere waveguide cutoff frequency (∼2 kHz). Furthermore, the 27‐day solar rotation is detected only during daylight hours, implying a 27‐day periodicity in the daytime collision frequency between free electrons and neutral air molecules in the lower ionosphere (∼80 km). We propose that continuous monitoring of VLF radio noise at frequencies close to the waveguide cutoff could provide a new method of continuously monitoring changes in the solar rotation rate.

SNR changes of VLF radio signals detected onboard the DEMETER satellite and their possible relationship to the Wenchuan earthquake

Here we used the VLF signal data received by the DEMETER satellite, transmitted from various ground VLF transmitters which are located around China, to study the changes in the signal to noise ratio (SNR) before and after the Wenchuan earthquake, which had a magnitude of 8.0. We also found that the SNRs of different frequency signals decreased significantly over the epicenter region before the earthquake, and reverted to their original levels after the earthquake. This phenomenon may be related to the earthquake.

Disturbances in a VLF radio signal prior the <i>M</i>=4.7 offshore Anzio (central Italy) earthquake on 22 August 2005

Abstract. On 22 August 2005 an earthquake with magnitude M=4.7 occurred in the Anzio (central Italy) offshore area. From 2002, a VLF-LF radio receiver is into operation in Bari (southern Italy). The intensity and the phase of the signals transmitted by GB (f=16 kHz, United Kingdom), FR (f=20.9 kHz, France), GE (f=23.4 kHz, Germany), IC (f=37.5 kHz, Island) and IT (f=54 kHz, Sicily, Italy) has been monitored with a 5 s sampling rate. The previous epicenter is near enough to some of the radio paths and the data collected were studied in order to reveal possible seismic effects. The raw analysis revealed a clear drop in the intensity of the FR radio signal on 19 August. Then the wavelet analysis was applied to the intensity and the phase data of the different radio signals. In the mentioned day an increase in the band 60–120 min was revealed in the spectra of the FR signal. Then the principal component analysis was applied and again the 19 August stood up as an anomalous day for the FR radio signal. The path of this signal, among the paths of the five radio signals collected by the Bari receiver, is the nearest to the mentioned epicentre and the anomaly revealed on 19 August appears as a precursor of the earthquake. This result confirms the possible precursor revealed by other researchers in the air Rn content in a site located 5 miles far from the epicenter.

Anomalies in VLF radio signals related to the seismicity during November–December 2004: A comparison of ground and satellite results

A previous study of the intensity of VLF signals radiated by two ground transmitters located in Germany ( f = 16.6 kHz) and France ( f = 18.3 kHz) and received on board the French satellite DEMETER, revealed an intensity drop in the time interval November 23–December 12, 2004, probably connected with the occurrence of earthquakes ( M = 5.4–5.5) in central Europe. From 2002 a receiver has been operating at Bari (Italy) and the intensity and the phase of the VLF/LF radio signals radiated by GB ( f = 16 kHz, United Kingdom), FR ( f = 20.9 kHz, France), GE ( f = 23.4 kHz, Germany), IC ( f = 37.5 kHz, Island) and IT ( f = 54 kHz, Sicily, Italy) are being monitored with a 5 s sampling rate. In order to confirm the results obtained by the satellite measurements, both the intensity and the phase data collected by the Bari receiver from October 2004 to January 2005 have been analysed. As concerns the intensity, drops of the GB, FR, GE and IC signals were revealed before the middle of November and a drop of the IT signal appeared before December 10. The geomagnetic activity gives satisfactory justification of the November drops; on the contrary no correspondence stood up with the drop in December and seismic effects can be claimed. As concerns the phase data, at first the wavelet analysis was applied and clear anomalies were revealed on the FR and IC signals at the end of November. These anomalies appear as precursors with respect to all the mentioned seismic activity. Then, the standard deviation of the phase data sets was investigated and a clear drop was revealed on the GE signal on 23–24 November, that is just before one of the previous large earthquakes. So, not only an agreement of the ground results with those obtained by the satellite appears, but also a more precise definition of the connection between the radio anomalies and the earthquakes stands up using the ground data.

Global diagnostics of the ionospheric perturbations related to the seismic activity using the VLF radio signals collected on the DEMETER satellite

Abstract. The analysis of the VLF signals radiated by ground transmitters and received on board of the French DEMETER satellite, reveals a drop of the signals (scattering spot) connected with the occurrence of large earthquakes. The extension of the "scattering spots" zone is large enough (1000–5000 km) and, probably, it increases with the magnitude of the "relative" earthquake. A possible model to explain the phenomenology, based on the acoustic gravity waves and the ionosphere turbulence, is proposed. The method of diagnostics applied to this study has the advantage to be a global one due to the world wide location of the powerful VLF transmitters and of the satellite reception. However, a specific disadvantage exists because the method requires rather a long time period of analysis due to the large longitudinal displacements among the successive satellite orbits. At the moment, at least, one month seems to be necessary.

Continuous gradient VLF measurements: a new possibility for high resolution mapping of karst structures

The Centre of Hydrogeology Neuch‚tel (CHYN) investigates and monitors karst aquifers, which are highly vulnerable to contamination. One of our particular research tasks within this scope is the characterization and localization of the 'epikarstic zone' (0-20 m depth) (Mangin 1975; Kiraly 1995). Density, size and aperture of fractures that connect the water reservoir with the surface, are features which characterize the vulnerability of karst aquifers. Very local scale mapping of these types of high-permeability fracture zone over extensive areas is therefore of interest for karst hydrogeology. In order to investigate this zone the very low frequency (VLF, 15-30 kHz) method has been adapted at CHYN for investigation at shallow depths with high lateral resolution. Geophysical surveying using VLF radio signals began in the 1960s as a tool for prospecting conductive minerals (McNeill & Labson 1991; Paal 1965). The CHYN equipment uses VLF and low frequency (LF, 30-300 kHz) radio transmitters, which are located at several positions around the world and are used principally for navigation and submarine communication. Consequently the CHYN has developed a radio frequency-resistivity device (RF-R), also called radio magnetotelluric (RMT, 12-240 kHz), and a continuously measuring radio frequency-electromagnetic (RF-EM, 15-300 kHz) device (Turberg et al. 1994; Turberg & Barker 1996; Stiefelhagen 1998). In fissured limestone with a resistivity of several hundreds of Wm, the penetration depth for these frequencies is about 10-100 m. However, when investigating small structures at very shallow depths (0-10 m) in the epikarstic zone, these devices have limited use due to their excessive penetration depths and their poor horizontal resolution. In this paper we introduce a method that is based on measuring the vertical gradient of the horizontal magnetic field component of VLF radio signals (VLF-EM-vGRAD(Hy)). According to our knowledge, such a technique has not been previously published.

Trimpi perturbations from large ionisation enhancement patches

A number of increasingly sophisticated and realistic models have been developed in order to investigate the interaction between sub-ionospherically propagating VLF waves and regions of ionisation enhancement (LIE 1 1 The “L” in “LIE” stands for “LEP-produced” ) in the D-region caused by lightning-induced electron precipitation enhancements (LEP). This LEP-produced LIE can result in phase and amplitude perturbations on received VLF radio signals that are referred to as Trimpis or more precisely, classic Trimpis, to distinguish them from “early/fast Trimpis” or “VLF sprites” which are not caused by LEP and are not considered here. It is important, for comparison with experimentally observed Trimpi effects, that the spatial extent of the D-region electron density ( N e) perturbation is modeled accurately. Here, it is argued that most previous modeling has used patch (LIE) sizes that are typically up to 100 km in both latitudinal and longitudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also appear that maximum Δ N e values assumed have often been too large, and the patches (LIEs) have been incorrectly modelled as circular rather than elliptical in horizontal extent. Consequently, in the present work, Trimpi perturbations are determined for LIEs with smaller maximum Δ N e, larger spatial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the whole rectangular corridor linking transmitter and receiver. The Trimpi modelling program is fully 3D, and takes account of modal mixing at the LIE. The underlying theory assumes weak Born scattering, but the code calculates a non-Born skin depth attenuation function for the LIE in question. The LIE is modelled as an electron density enhancement with a Gaussian profile in all coordinates. Results for a large elliptical LIE ∼ 200 × 600 km show that significant Trimpis, ∼−0.4 dB in amplitude and ∼+4° in phase are predicted, using modest maximum Δ N e values ∼ 1.5 el/cc. Such an electron density enhancement is well within the range that would be expected to result from experimentally observed fluxes of electron precipitation following wave particle interactions with whistler-mode waves.