VLF Signals Research Articles

Modeling of Solar Eclipse effects on the sub-ionospheric VLF/LF signals observed by multiple stations over Japan

We present the numerical modeling of field amplitude perturbations for two VLF/LF transmitters (JJI: 22.2 kHz, JJY: 40.0 kHz) signals recorded at six places (UEC-VLF network) in Japan during the Annular Solar Eclipse of 2012 (ASE-2012). We determine the characteristics of the D-region ionospheric perturbation according to the antumbra and penumbral shadow of the annular solar eclipse. A space–time dependent 2-Dimensional Elliptical Gaussian distribution Model (2DEGM) is introduced to get the effect of ASE-2012 on the Wait’s D-region parameters, such as effective VLF/LF reflection height (h′) and sharpness factor (β). The Long Wave Propagation Capability (LWPC) code is used to simulate the VLF signal amplitude profile with the help of modelled Wait’s parameters. A method of comparison of observed and LWPC simulated VLF signal amplitude across all the considered signal propagation paths is introduced to estimate final 2DEGM related parameters and hence, to model D-region electron density perturbations during ASE-2012. Using 2DEGM + LWPC, (i) we consistently simulate the observed VLF signal amplitude profiles for almost all the propagation paths of UEC-VLF network and (ii) evaluate the percentage change in D-region electron density due to ASE-2012.

Examining Solar Flare Effects on Earth's Ionosphere Using Ground-Based Measurements

This study investigated the simultaneous effects of solar flares (SFs) on both the D and E layers of Earth's ionosphere. The analysis focused on the M and X-class SFs that occurred during the 24th solar cycle as these two classes of SFs are known to produce significant effects on Earth’s environment, particularly during the daytime period. The data utilized to detect the SF events in this study were ground-based magnetometer data from the equatorial regions. Effects of the selected SF events on the E layer were investigated based on the EUEL index constructed using the geomagnetic data. Meanwhile, the changes in the strength of radio VLF signals in the D ionospheric layer during the selected SF events were monitored using Sudden Ionospheric Disturbance (SID) data. Two case studies were performed which consisted of four SF events from a total of 23 events that were detected by geomagnetic data during the period of study. Further analysis on the selected SF events showed the common effects of SFs on the D layer, which is the increment on the VLF signal measured from the SID stations although a different response was detected in the EUEL index variations. This indicates that the VLF signal always shows an increment even though a decrement in the ionization of the E layer occurs as a result of the SF events. The difference in responses could be attributed to the distinct changes in electron density of both layers during the SF occurrence. Further studies are needed to elucidate the underlying mechanism responsible for this unique response, utilizing appropriate parameters such as total electron content, as well as the electron density data to thoroughly analyze the ionospheric response during SF events.

The VLF-induced ionospheric perturbations detected by DEMETER during solar minimum

The VLF-induced ionospheric heating events from the powerful transmitter NWC are collected at extremely low solar activity years of 2008–2009, and their spatial and temporal distribution features are investigated. The spatial distribution of heating signals exhibits clear north-direction inclination of 2–3°, and covering area within 300 km radius in longitude and latitude. Plasma parameters show the obvious negative correlations between decreasing Ne and increasing Te, and positive correlations of the increasing ion density and ion temperature during the VLF heating time. The ions move upwards to cause the perturbations in ULF electric field due to mathop{V}limits^{rightharpoonup} times mathop{B}limits^{rightharpoonup}. The VLF heating events easily occur at seasons with low plasma density, which illustrates that low plasma environment plays an important role in VLF-induced ionospheric heating. Typical three enhanced VLF signals are obtained, one at 19.8 kHz with the largest amplitude, one at the broadened frequency band of 19.8 ± 300 Hz with one order of amplitude decreasing, and one between 19.5 kHz and lower hybrid frequency with at least two orders of amplitude reductions relative to that at 19.8 kHz but still one order increasing relative to non-heating region. These quasi electrostatic waves during VLF heating may not be from the VLF whistler conversion, but the lightning-whistler waves are enhanced when they pass through the plasma cavity formed during VLF heating process.

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

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

Observations of Ionospheric Disturbances Produced by a Powerful Very-Low-Frequency Radio Signal in the Magnetic Conjugated Region Respect the Transmitter

We investigate four observational cases over NWC magnetically conjugate region by DEMETER spacecraft and CSES satellite. The cases of DEMETER on 23 February 2008 and CSES on 22 March 2020 show the evident ionospheric heating effects, in which the electron density and electron temperature suggest simultaneous enhancements associated with the intense spectra of the VLF electric and magnetic field above the conjugate region. This indicates that the heating effects associated with ionospheric modification are indeed triggered by the VLF signal transmitted by the NWC transmitter. In other words, the strong disturbances induced directly above the transmitter propagate along the magnetic field lines and extend into the magnetic conjugated region. Differently, the cases on 11 February 2010 (DEMETER) and 15 February 2019 (CSES) show obvious increases in electron densities while having no significant elevation in electron temperatures. The presence of enhanced energetic electron spectra at higher L-values, rather than directly above the conjugate region, suggests precipitation events induced by the VLF transmitter.

Probing the lower ionospheric electron density modulation by the 2019 solar eclipse with VLF phase studies and ion-chemistry model

During a solar eclipse, the temporary blockage of daytime solar illumination resembles a nighttime situation up to a certain degree. This effectively modulates the lower ionosphere by reducing the ionization rate and hence electron–ion density and the VLF signal responds accordingly. This study investigates the lower ionospheric response to the Annular Solar Eclipse of December 26, 2019 (ASE19) by analyzing VLF phase modulation as observed from four different locations in India during the campaign organized by the Indian Centre for Space Physics (ICSP), Kolkata. The eclipse path crossed the four signal paths between NWC (19.8 kHz) transmitter and the receivers from time to time and produced significant deviations in the detected signal amplitudes and phases. We adopt the analysis of the phase delay mechanism to obtain the perturbed electron density fluctuations over the paths and compute changes in VLF reflection heights during the eclipse. The electron density modulation during the eclipse with an in situ theoretical chemical model using the solar obscuration at five different points over all the signal paths are also calculated and compared with the results from VLF. Both the temporal VLF responses at the receiving points and the changes in derived electron densities are found to exactly follow the obscuration profile at the point of maximum obscuration for all the paths. This article, for the first time successfully compares two distinct independent models related to lower ionosphere during a solar eclipse and demonstrates that though we lack the necessary means to directly explore the lower ionospheric dynamics, our theories are more than adequate and roughly reflect the actual scenario.

Influence of Path Illumination on the Amplitude Characteristics of VLF Signals

Influence of Path Illumination on the Amplitude Characteristics of VLF Signals

Effects of a series of large flares from a sunspot group eruption on VLF propagation

Sunspot groups are constantly evolving, causing flare eruption under the action of the solar atmospheric magnetic field. After the flare, the ejected X-rays cause the electron concentration of the Earth’s ionosphere to increase and the equivalent reflection height of the ionosphere to decrease. We studied the correlation between a series of flares erupted by sunspot groups and VLF signal propagation. Using NOAA sunspot activity area data and solar flare data released by the GOES satellite, we counted a series of flares erupted by sunspot groups in 2000. It is found that the AR9077 sunspot group continuously erupted 15 M-class and above flares and 3 X-class flares. Alpha VLF navigation station data were received in Haikou, and the phase anomalies of the VLF signal caused by a series of large flares erupted by AR9077 were observed. According to the phase anomaly, the flare is analyzed, and the relationship between X-ray flux density and ionospheric equivalent reflection height is fitted. We incorporated the change of solar zenith angle at the observation point in our calculation to correct the equivalent reflection height of the ionosphere and thus improve the accuracy of flare prediction. The calculated results are consistent with the data released by the GOES satellite. The observation results show that VLF phase anomaly correlates well with solar flares and sunspots. VLF signal can be used as a reliable scheme to predict the space environment, avoiding the impact of sunspot flares.

Observations and modeling of D-region ionospheric response of Annular Solar Eclipse on December 26, 2019, using VLF signal amplitude and phase variation

Observations and modeling of D-region ionospheric response of Annular Solar Eclipse on December 26, 2019, using VLF signal amplitude and phase variation

Comparison of VLF Signal Responses to Solar Flares along Daytime and Nighttime Propagation Paths

Solar flares have a severe impact on the near-earth space environment, during which the VLF signals observed by the ground-based instrument exhibit abnormal changes. However, the similarity and differences of VLF signal responses to solar flares over daytime and nighttime propagation paths are still unclear. Previous magnetograph measurements suggest that solar flares can also influence the dark hemisphere by the induction currents caused by the change in the ionospheric electrical conductivity of the sunlit hemisphere. To examine these effects, we have analyzed the solar flare effects on VLF propagation along two paths that are suited along the north–south direction, but with a time difference of 12 h. From late March to late May in the year of 2022, a total of 32 flare events with clear VLF responses are selected in order to analyze the similarity and differences between daytime and nighttime propagation paths. Different from the previous magnetograph measurements, it is found that the solar flare effects can only be observed from daytime VLF propagation paths. Moreover, present results show that the amplitude and phase variation of the VLF signal increases almost linearly with the magnitude of solar flares, and the stronger the solar flare, the more obvious the influence on VLF signals. However, the two paths exhibit notably different sensitivity in terms of amplitude and phase variation to solar flare class. Future studies that aim at nowcasting solar flare events using ground-based VLF receivers need to take these effects into account. The goal is to better understand the effects of solar flares on the lower ionosphere, with a view toward improving the nowcasting capability of the VLF technique for solar flares.

About the Propagation of RSDN‐20 “Alpha” Signals in the Earth‐Ionosphere Waveguide During Geomagnetic Disturbances

AbstractThe paper presents the results of the modeling of VLF radio waves propagation in horizontally inhomogeneous region of the high‐latitude Earth‐ionosphere waveguide during different types of the geomagnetic disturbances. The frequencies of the model source of signal correspond to the frequencies of the long‐distance navigation radio system RSDN‐20 “Alpha” constantly broadcasting in the VLF range in the Russian Federation. It is considered the geomagnetic storm on 24 January 2012 and geomagnetic substorm on 11 December 2015. Electron density profiles and collision rates of electrons with neutrals are taken from the measurements of EISCAT radar in Tromso and EISCAT 42m radar on Svalbard. The numerical experiments were conducted with a highly horizontally inhomogeneous electron density in the ionosphere. It is shown that the amplitudes and phases of the VLF signals at these frequencies show a noticeable reaction to changes in the ionospheric parameters during the considered geomagnetic disturbances. Calculations show that changes in amplitude of RSDN‐20 “Alpha” signals caused by the action of the Earth's magnetic field and horizontal inhomogeneities depend on the direction of signal propagation relative to the Earth's magnetic field vector and cannot be reversed if the direction of propagation changes to the opposite. The results of the numerical experiment were compared with the corresponding characteristics of the RSDN‐20 signals calculated from the observations of electromagnetic signals at the Lovozero observatory for the case on 11 December 2015.

Отклик ионосферы на прохождение тайфунов по наблюдениям методом СДВ-радиопросвечивания

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

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

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

Variation in the VLF signal noise amplitude during the period of intense seismic activity in Central Italy from 25 October to 3 November 2016

Recent research shows reductions in the VLF signal noise amplitude that begin before particular earthquakes whose epicentres are more than 100 km away from the signal propagation path. In this paper, we extend this research to studying the noise amplitude during periods of intense seismic activity in a localized area. We analyse variations in the VLF signal noise amplitude over a period of 10 days (25 October–3 November 2016) when 981 earthquakes with the minimum magnitude of 2 occurred in Central Italy. Out of these events, 31 had the magnitude equal or greater than 4, while the strongest one had the magnitude of 6.5. We observe the VLF signal emitted by the ICV transmitter located in Sardinia (Italy) and recorded in Belgrade (Serbia). Bearing in mind that the trajectory of this signal crosses the area in which the observed earthquakes occurred, we extend the existing research to study of variations in the noise amplitude of the signal propagating at short distances from the epicentres of the considered earthquakes. In addition, we analyse the impact of a large number earthquakes on characteristics of the noise amplitude and its reductions before particular events. In order to examine the localization of the recorded changes, we additionally analysed the noise amplitude of two reference signals emitted in Germany and Norway. The obtained results show the existence of the noise amplitude reduction preceding individual strong or relatively strong earthquakes, and earthquakes followed by others that occurred in a shorter time interval. However, the additional noise amplitude reductions are either not pronounced or they do not exist before the considered events in periods of the reduced noise amplitude remain from previous earthquakes. Reductions in noise amplitudes for all observed signals indicate a larger perturbed area through which they spread or its closer location to the receiver. The analysis of daily values of parameters describing the noise amplitude reveals their variations start up to 2 weeks before the seismically active period occurs.

Multi-instrumental investigation of the solar flares impact on the ionosphere on 05–06 December 2006

The sudden increase of X-radiation and EUV emission following solar flares causes additional ionization and increased absorption of electromagnetic (EM) waves in the Earth’s atmosphere. The solar flare impact on the ionosphere above Europe on 05 and 06 December 2006 was investigated using ground-based (ionosonde and VLF) and satellite-based data (Vertical Total Electron Content (VTEC) derived from GNSS observations and VLF measurements from DEMETER satellite). Based on the Kp and Dst indices, 05 December 2006 was a quiet day, while there was a geomagnetic storm on 06 December 2006. The total fade-out of the EM waves emitted by the ionosondes was experienced at all investigated stations during an X9 class flare on 05 December 2006. The variation of the fmin parameter (first echo trace observed on ionograms, it is a rough measure of the “non-deviative” absorption) and its difference between the quiet period and during the flares have been analyzed. A latitude dependent enhancement of fmin (2–9 MHz) and Δfmin (relative change of about 150%–300%) was observed at every station at the time of the X9 (on 05 December) and M6 (on 06 December) flares. Furthermore, we analyzed VTEC changes during and after the flare events with respect to the mean VTEC values of reference quiet days. During the X9 solar flare, VTEC increased depending on the latitude (2–3 TECU and 5%–20%). On 06 December 2006, the geomagnetic storm increased ionization (5–10 TECU) representing a “positive” ionospheric storm. However, an additional peak in VTEC related to the M6 flare could not be detected. We have also observed a quantifiable change in transionospheric VLF absorption of signals from ground transmitters detected in low Earth orbit associated with the X9 and M6 flare events on 05 and 06 December in the DEMETER data. Moreover, amplitude and phase of ground-based, subionospherically propagating VLF signals were measured simultaneously during the investigated flares to analyze ionosphere reaction and to evaluate the electron density profile versus altitude. For the X9 and M6 flare events we have also calculated the ionospheric parameters (sharpness, reflection height) important for the description and modelling of this medium under forced additional ionization.

A neural network based approach to classify VLF signals as rock rupture precursors

The advent of novel technologies revealed that other geophysical signals than those directly related to fault motion could be used to probe the state of deformation of the Earth's crust. Electromagnetic signals belonging to this category have been increasingly investigated in the last decade in association to natural earthquakes and laboratory rock fractures. These studies are hampered by the lack of continuous recordings and a systematic mathematical processing of large data sets. Indeed, electromagnetic signals exhibit characteristic patterns on a specific frequency band (the very low frequency, VLF) that correlate uniquely with the paroxistic rupture of rocks specimens under uniaxial laboratory tests and were also detected in the atmosphere, in association to moderate magnitude earthquakes. The similarity of laboratory and atmospheric VLF offers an unique opportunity to study the relation between VLF and rock deformation on at least two different scales and to enlarge the dataset by combining laboratory and atmospheric data. In this paper we show that the enlarged VLF dataset can be successfully used, with a neural network approach based on LSTM neural networks to investigate the potential of the VLF spectrum in classifying rock rupture precursors both in nature and in the laboratory. The proposed approach lays foundation to the automatic detection of interesting VLF patterns for monitoring deformations in the seismically active Earth’s crust.

Синхронные глобально наблюдаемые ультракороткопериодные импульсы

We have studied the properties of impulsive geomagnetic disturbances, which are observed synchronously at the network of induction magnetometers of the Institute of Solar-Terrestrial Physics (ISTP SB RAS) and Canadian stations of the CARISMA project [Mann, et al., 2008]. A feature of the pulses we detected is that their frequency range (f~5–30 Hz) lies at the junction of the ranges of two known classes of electromagnetic oscillations: ultra-low-frequency (ULF) oscillations (f<5–10 Hz), or geomagnetic pulsations, and extra-low frequency (ELF) oscillations (f~30–300 Hz); therefore, the 5–30 Hz range is poorly studied. The work is of undoubted interest for physics of processes in the magnetosphere–ionosphere–atmosphere system. Morphological analysis of the pulses detected has been carried out using data from ISTP stations. As a result, we obtained statistical characteristics of the pulses, plotted their dynamic spectra, and determined a number of unusual properties that distinguish them, on the one hand, from geomagnetic pulsations of the pulsed type (irregular pulsations of the Pi1B type), and, on the other hand, from higher frequency ELF and VLF signals (atmospherics, whistlers, etc.). On the basis of the results, we have made an assumption that a source of the pulses under study can be electrical sprites caused by powerful thunderstorms at middle and low latitudes. Using the results obtained by Wang, et al. in 2019 on spatial and temporal fixation of sprites in North China, we have confirmed that ultra-short-period pulses occur following the emergence of sprites. Thunderstorm activity, both local and global, is considered to be one of the main sources of excitation of the ionospheric Alfvén resonator (IAR), which plays an important role in coupling the ionosphere and the magnetosphere. The pulsed oscillations of interest may be one of the agents through which the energy of thunderstorms is transferred to IAR, thereby including the atmosphere in the system considered.

Synchronous globally observable ultrashort-period pulses

We have studied the properties of impulsive geomagnetic disturbances, which are observed synchronously at the network of induction magnetometers of the Institute of Solar-Terrestrial Physics (ISTP SB RAS) and Canadian stations of the CARISMA project [Mann, et al., 2008]. A feature of the pulses we detected is that their frequency range (f~5–30 Hz) lies at the junction of the ranges of two known classes of electromagnetic oscillations: ultra-low-frequency (ULF) oscillations (f<5–10 Hz), or geomagnetic pulsations, and extra-low frequency (ELF) oscillations (f~30–300 Hz); therefore, the 5–30 Hz range is poorly studied. The work is of undoubted interest for physics of processes in the magnetosphere–ionosphere–atmosphere system. Morphological analysis of the pulses detected has been carried out using data from ISTP stations. As a result, we obtained statistical characteristics of the pulses, plotted their dynamic spectra, and determined a number of unusual properties that distinguish them, on the one hand, from geomagnetic pulsations of the pulsed type (irregular pulsations of the Pi1B type), and, on the other hand, from higher frequency ELF and VLF signals (atmospherics, whistlers, etc.). On the basis of the results, we have made an assumption that a source of the pulses under study can be electrical sprites caused by powerful thunderstorms at middle and low latitudes. Using the results obtained by Wang, et al. in 2019 on spatial and temporal fixation of sprites in North China, we have confirmed that ultra-short-period pulses occur following the emergence of sprites. Thunderstorm activity, both local and global, is considered to be one of the main sources of excitation of the ionospheric Alfvén resonator (IAR), which plays an important role in coupling the ionosphere and the magnetosphere. The pulsed oscillations of interest may be one of the agents through which the energy of thunderstorms is transferred to IAR, thereby including the atmosphere in the system considered.

Numerical modelling of sub-ionospheric Very Low Frequency radio signal anomalies during the Samos (Greece) earthquake (M = 6.9) on October 30, 2020

Lower ionospheric perturbation during earthquake activity is well recognized by the anomalous behavior of Very Low or Low Frequency (VLF/LF) radio signal. In this study, we present the sub-ionospheric VLF signal modulation during the Samos earthquake that took place on October 30, 2020 (M = 6.9). We use the conventional terminator time method to check the seismogenic impression in the VLF signal during the local sunrise and sunset times. VLF signals from two transmitters ISR (Israel) and TBB (Turkey) are used for this analysis and those are recorded in a receiving station UWA in Greece. Both signals show a strong shift in the sunrise and sunset terminator times. The signal profiles are numerically simulated using the Long Wavelength Propagation Capability (LWPC) program by incorporating Wait’s two-component ionospheric parameters (β and h′). The simulation gives highly satisfactory outcomes. The outputs of LWPC are used to compute the electron density profile of the lower ionosphere. It is found that the electron density gets significantly modulated during the maximum shift in the terminator times. The electron density is found to have both increased and decreased from its normal value before the earthquake. The VLF path dependency is strongly observed in the change in the electron density profile. For ISR-UWA, nighttime reflection height has been computed using the modal conversion technique, and it is found to be similar to the LWPC outcomes. The overall condition was geomagnetically quiet, and the results have no contamination due to solar-terrestrial interaction.

A Method for Imaging Energetic Particle Precipitation with Subionospheric VLF Signals

A Method for Imaging Energetic Particle Precipitation with Subionospheric VLF Signals