Subionospheric Signals Research Articles

Investigation of Precursors in VLF Subionospheric Signals Related to Strong Earthquakes (M > 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 > 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.

Anomalous variations of VLF sub-ionospheric signal and Mesospheric Ozone prior to 2015 Gorkha Nepal Earthquake

The subject of pre-earthquake ionospheric signatures has always been contentious and debatable. Some of the previous reports have documented unforeseen and unusual variations in some of the atmospheric and ionospheric parameters well before an earthquake. Here, we analyze the ionospheric response from the Indian Subcontinent to Nepal Gorkha Earthquakes occurred between April and May 2015, which were the most powerful and disastrous natural calamities in past ~80 years over the Himalayan region left ~9000 causalities and more than ~20000 people injured with the property damage of the order of several billion dollars. In view of severe earthquakes occurrences, their prior information on the shorter time scales are warranted for mitigation of associated disasters. Here, we report for the first time, a case which shows a strong link in anomalous variations between VLF sub-ionospheric signal and mesospheric ozone prior to both April 25, 2015 (Mw = 7.8) earthquake and its biggest aftershock on May 12, 2015 (Mw = 7.3). Observations show an unusual variation in VLF signals amplitude /shift in terminator time (TT) strongly linked with positive (negative) mesospheric ozone anomaly in D-region altitudes prior to the Gorkha Nepal earthquakes. It is surmised that simultaneous continuous observations of both VLF waves and mesospheric ozone can be considered as an important tool to identify the prior earthquake signatures in the vicinity of the extremely earthquake-prone zone such as Himalayan region. In this context, the current report opens up a new dimension in lithosphere-atmosphere-ionosphere coupling during the earthquake preparation processes itself.

Criticality Analysis of the Lower Ionosphere Perturbations Prior to the 2016 Kumamoto (Japan) Earthquakes as Based on VLF Electromagnetic Wave Propagation Data Observed at Multiple Stations

The perturbations of the ionosphere which are observed prior to significant earthquakes (EQs) have long been investigated and could be considered promising for short-term EQ prediction. One way to monitor ionospheric perturbations is by studying VLF/LF electromagnetic wave propagation through the lower ionosphere between specific transmitters and receivers. For this purpose, a network of eight receivers has been deployed throughout Japan which receive subionospheric signals from different transmitters located both in the same and other countries. In this study we analyze, in terms of the recently proposed natural time analysis, the data recorded by the above-mentioned network prior to the catastrophic 2016 Kumamoto fault-type EQs, which were as huge as the former 1995 Kobe EQ. These EQs occurred within a two-day period (14 April: and , 15 April: ) at shallow depths (~10 km), while their epicenters were adjacent. Our results show that lower ionospheric perturbations present critical dynamics from two weeks up to two days before the main shock occurrence. The results are compared to those by the conventional nighttime fluctuation method obtained for the same dataset and exhibit consistency. Finally, the temporal evolutions of criticality in ionospheric parameters and those in the lithosphere as seen from the ULF electromagnetic emissions are discussed in the context of the lithosphere-atmosphere-ionosphere coupling.

Intermittency-induced criticality in the lower ionosphere prior to the 2016 Kumamoto earthquakes as embedded in the VLF propagation data observed at multiple stations

A network of 8 VLF/LF receivers has recently been established across Japan, receiving subionospheric signals from different transmitters located both in the same and other countries. The primary purpose of this network is to study disturbances in the VLF/LF propagation through the lower ionosphere in possible relation to earthquake (EQ) preparation processes. Ionospheric perturbations of possible seismic origin have long been investigated and considered very promising for short-term EQ prediction. The raw amplitude data on reception of the above-mentioned network, after being appropriately filtered, were analyzed by means of the method of critical fluctuations (MCF) in analogy to thermal critical systems. The MCF analysis of the VLF/LF propagation revealed that intermittency-induced criticality was reached in the lower ionosphere from ~1week to 3days prior to the catastrophic 2016 Kumamoto EQs. These fault-type EQs occurred within a two-day period (14 April: MW=6.2 and MW=6.0, 15 April: MW=7.0) at shallow depths (~10km) and very close epicenters, while the main event was as huge as the former 1995 Kobe EQ. MCF analysis results are compared to those by the conventional nighttime fluctuation method as well as to those by natural time analysis method obtained for the same dataset, and are found to exhibit remarkable consistency.

The 25 April 2015 Nepal Earthquake: Investigation of precursor in VLF subionospheric signal

AbstractWe present a critical analysis of the observations and interpretation of VLF subionospheric measurements related to the main Nepal Gorkha earthquake which occurred on 25 April 2015 (Mw7.8) and its major aftershock on 12 May 2015 (Mw7.3). The VLF narrowband signal used is from North West Cape (NWC) (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (latitude 25.41°N, longitude 81.93°E). Allahabad is located very close (~360 km) to these earthquake epicenters. Two widely used analysis, viz., (1) terminator time and (2) nighttime fluctuation techniques, are applied to extract seismic related effects in the NWC narrowband VLF data. The terminator time analysis yields statistically significant shifts of ~45 and ~26 min, respectively, in evening terminator time in the NWC VLF amplitude signal, 1 day before both the earthquakes. The nighttime fluctuation method shows a consistent, statistically significant, increase in three parameters 1 day before the earthquake. The observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity; other earthquakes on the signal path; and meteorological disturbances such as lightning activity, wind speed, and temperature along the transmitter‐receiver great circle path. The VLF subionospheric signal analysis results unambiguously point toward the presence of seismically excited atmospheric gravity waves during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere.

Space weather effects on the low latitude D-region ionosphere during solar minimum

The effects of the solar flares and the geomagnetic storms (disturbance storm time (Dst) < −50 nT) during December 2006 to 2008, a period during the unprecedented solar minimum of solar cycles 23 and 24, have been examined on sub-ionospheric very low frequency (VLF) signals from NWC (19.8 kHz), NPM (21.4 kHz), VTX (18.2 kHz), and NLK (24.8 kHz) transmitters monitored at Suva (18.2° S, 178.4° E), Fiji. Apart from the higher class solar flares (C to X), a solar flare of class B8.5 also produced enhancements both on the amplitude and phase. The amplitude enhancements in NLK, NPM, and NWC signals as a function of peak solar flare X-ray flux in decibel (dB; relative to 1 μW/m2) shows that the relationship curve is steeper and quite linear between the flare power levels of 0 to 15 dB; below 0 dB, the curve gets less steep and flattens towards −5 dB flare power level, while it also gets less steep above 15 dB and almost flattens above 20 dB. In general, the level of amplitude enhancement for NLK signal is higher than that for NPM and NWC signals for all solar flares. The enhancement in the amplitude and phase of VLF signals by solar flares is due to the increase in the D-region electron density by the solar flare-produced extra ionization. The modeling of VLF perturbations produced by B8.5 and C1.5 classes of solar flares on 29 January 2007 using LWPC (Long Wave Propagation Capability) V2.1 codes show that reflection height (H') was reduced by 0.6 and 1.2 km and the exponential sharpness factor (β) was raised by 0.010 and 0.005 km−1, respectively. Out of seven storms with Dst < −50 nT, only the intense storm of 14 to 16 December 2006 with a minimum Dst of −145 nT has shown a clear reduction in the signal strength of NWC and NPM sub-ionospheric signals due to storm-induced reduction in the D-region electron density.

The observation of Doppler shifts of subionospheric LF signal in possible association with earthquakes

The Doppler‐shift observation of LF (f = 60 kHz) subionospheric signal from Saga (Kyushu) (with call sign of JJY) as observed at Chofu (CHF), has been used to investigate the properties of ionospheric perturbations possibly associated with earthquakes(EQs). The period of analysis is seismo‐active half a year from January 1, 2009 to June 30, 2009, and six EQs with magnitude greater than 5.0 (in a range from 5.1 to 5.8, which took place within the wave sensitive area of the JJY‐CHF path) are dealt with. It is found from the Doppler‐shift observation at CHF that the Doppler shifts are really observed and the components in the frequency ranges of AGW (atmospheric gravity wave) and AW (acoustic wave) in the Doppler shifts are clearly enhanced, at least, before each EQ. This observational fact would lend a strong support to the important role of atmospheric oscillation channel in the lithosphere‐atmosphere‐ionosphere coupling mechanism.

Terrestrial VLF transmitter injection into the magnetosphere

Very Low Frequency (VLF, 3–30 kHz) radio waves emitted from ground sources (transmitters and lightning) strongly impact the radiation belts, driving electron precipitation via whistler‐electron gyroresonance, and contributing to the formation of the slot region. However, calculations of the global impacts of VLF waves are based on models of trans‐ionospheric propagation to calculate the VLF energy reaching the magnetosphere. Limited comparisons of these models to individual satellite passes have found that the models may significantly (by &gt;20 dB) overestimate amplitudes of ground based VLF transmitters in the magnetosphere. To form a much more complete empirical picture of VLF transmitter energy reaching the magnetosphere, we present observations of the radiation pattern from a number of ground‐based VLF transmitters by averaging six years of data from the DEMETER satellite. We divide the slice at ∼700 km altitude above a transmitter into pixels and calculate the average field for all satellite passes through each pixel. There are enough data to see 25 km features in the radiation pattern, including the modal interference of the subionospheric signal mapped upwards. Using these data, we deduce the first empirical measure of the radiated power into the magnetosphere from these transmitters, for both daytime and nighttime, and at both the overhead and geomagnetically conjugate region. We find no detectable variation of signal intensity with geomagnetic conditions at low and mid latitudes (L &lt; 2.6). We also present evidence of ionospheric heating by one VLF transmitter which modifies the trans‐ionospheric absorption of signals from other transmitters passing through the heated region.

Low frequency signal spectrum analysis for strong earthquakes

We examined changes in the spectral composition of the low frequency (LF) subionospheric signals from the NRK transmitter (37.5 kHz) in Iceland that were received in Bari (Italy) relative to the earthquake that occurred in L’Aquila on April 6, 2009. In our previous studies, we have reported the occurrence of preseismic night-time anomalies using observations from three receivers located in Bari, Graz (Austria) and Moscow (Russia). The strongest anomalies in the signals were observed in the NRK-Bari propagation path during the period 5-6 days before the L’Aquila earthquake, as well as during the series of aftershocks. During this period, similar very low frequency (VLF)/LF amplitude anomalies were also observed along several other propagation paths that crossed the L’Aquila seismogenic zone. Spectral analysis of the LF signals filtered in the frequency range 0.28 mHz to 15 mHz shows differences in the spectra for seismo-disturbed days when compared to those for either quiet or geomagnetically disturbed days. These spectral anomalies, which are only observed in the propagation path between NRK and Bari, contain signals with periods of about 10 min to 20 min. These periodic signals are absent both in the spectra of the undisturbed signals for the control paths, and in the spectra of the signals received during geomagnetic storms. The same changes in the spectral composition were observed in the analysis of LF (40 kHz) signals from the JJY transmitter in Japan that were received in Petropavlovsk-Kamchatsky (Russia) during the occurrence of three strong earthquakes with M ≥7.0. The results of this study support the theoretical prediction that the possible mechanism for energy penetration from the origin of an earthquake through the atmosphere and into the ionosphere is based on the excitation and upward propagation of internal gravity waves.

Studies on the anomalies in the behaviour of transmitted subionospheric VLF electromagnetic signals and the changes in the fourth Schumann resonance mode as signatures of two pending earthquakes

Some anomalies are observed in the subinospheric VLF electromagnetic (EM) signals at 19.8 kHz transmitted from North West Cape, Australia (lat: 21.82° S; long: 114.16° E) and 40 kHz from Fukushima, Japan (lat: 36.18° N, long: 139.85° E), recorded by VLF receivers near Kolkata (lat: 22.56° N, long: 88.5° E) during the occurrence of the two earthquakes at Andaman Island (lat:14.018° N, long: 92.92° E), India and other at South Coast of Honsu (lat: 34.78° N, long: 138.27° E), Japan. The analyses of these seismo-ionospheric VLF EM anomalies at these two frequencies during some days before and after the occurrence of earthquake on August 11, 2009 will be presented here. VLF-LF transient variations of significant magnitude in the form of spikes are observed few days prior to the day of occurrence of the earthquakes that continued several days more, then decaying gradually and finally ceased. Signals are examined to describe their connectivity with earthquakes. The enhancements of the amplitude and frequency of the fourth mode of Schumann resonance spectra have been detected during the occurrence of the two earthquakes, which will also be reported here.

Measurement of Doppler shifts of short-distance subionospheric LF transmitter signals and seismic effects

[1] The presence of ionospheric perturbations in possible association with earthquakes (EQs) and their characteristics have been extensively investigated by means of subionospheric VLF/LF propagation. As a possible generation mechanism of those perturbations, we have proposed the atmospheric gravity wave (AGW) (period = 10–100 min) channel as being most plausible on the basis of a lot of indirect evidence including the enhancement of AGW fluctuation in the subionospheric data before an EQ. In order to provide the “direct” support to this hypothesis, we have made, in this paper, the first attempt of observing Doppler shifts of short-distance subionospheric signals from a Japanese LF transmitter JJY (40 kHz) at a few stations in the Tokyo area. As a preliminary analysis, the Doppler shift data from JJY to a station, Machida (distance = 230 km) during five months in 2008 have been analyzed, which has indicated significant enhancements of Doppler shifts in the AGW range for two EQs and a further statistical correlation of Doppler shift in the AGW range with large EQs. Finally the mechanism of lithosphere-atmosphere-ionosphere coupling is discussed.

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Detection of ionospheric perturbations associated with Japanese earthquakes on the basis of reception of LF transmitter signals on the satellite DEMETER

Abstract. There have been recently reported a lot of electromagnetic phenomena associated with earthquakes (EQs). Among these, the ground-based reception of subionospheric waves from VLF/LF transmitters, is recognized as a promising tool to investigate the ionospheric perturbations associated with EQs. This paper deals with the corresponding whistler-mode signals in the upper ionosphere from those VLF/LF transmitters, which is the counterpart of subionospheric signals. The whistler-mode VLF/LF transmitter signals are detected on board the French satellite, DEMETER launched on 29 June 2004. We have chosen several large Japanese EQs including the Miyagi-oki EQ (16 August 2005; M=7.2, depth=36 km), and the target transmitter is a Japanese LF transmitter (JJY) whose transmitter frequency is 40 kHz. Due to large longitudinal separation of each satellite orbit (2500 km), we have to adopt a statistical analysis over a rather long period (such as 3 weeks or one month) to have reliable data set. By analyzing the spatial distribution of JJY signal intensity (in the form of signal to noise ratio SNR) during a period of 4 months including the Miyagi-oki EQ, we have found significant changes in the intensity; generally the SNR is significantly depleted before the EQ, which is considered to be a precursory ionospheric signature of the EQ. This abnormal effect is reasonably explained in terms of either (1) enhanced absorption of whistler-mode LF signals in the lower ionosphere due to the lowering of the lower ionosphere, or (2) nonlinear wave-wave scattering. Finally, this analysis suggests an important role of satellite observation in the study of lithosphere-atmosphere-ionosphere coupling.

A quantitative comparison of lightning‐induced electron precipitation and VLF signal perturbations

VLF signal perturbations recorded on the Holographic Array for Ionospheric/Lightning Research (HAIL) are quantitatively related to a comprehensive model of lightning‐induced electron precipitation (LEP) events. The model consists of three major components: a test‐particle model of gyroresonant whistler‐induced electron precipitation, a Monte Carlo simulation of energy deposition into the ionosphere, and a model of VLF subionospheric signal propagation. For the two representative LEP events studied, the model calculates peak VLF amplitude perturbations within a factor of three of those observed, well within the expected variability of radiation belt flux levels. The phase response of the observed VLF signal to precipitation varied dramatically over the course of the two nights and this variability in phase response is not properly reproduced by the model. The model calculates a peak in the precipitation that is poleward displaced ∼6° from the causative lightning flash, consistent with observations. The modeled precipitated energy flux (E &gt; 45 keV) peaks at ∼1 × 10−2 (ergs s−1 cm−2), resulting in a peak loss of ∼0.001% from a single flux tube at L ∼ 2.2, consistent with previous satellite measurements of LEP events. The precipitation calculated by the model is highly dependent on the near‐loss‐cone trapped radiation belt flux levels assumed, and hence our main objective is not to compare the model calculations and the VLF signal observations on an absolute basis but is rather to develop metrics with which we can characterize the VLF signal perturbations recorded on HAIL in terms of the associated precipitation flux. Metrics quantifying the ionospheric density enhancement (NILDE) and the electron precipitation (Γ) along a VLF signal path are strongly correlated with the VLF signal perturbations calculated by the model. A conversion ratio Ψ, relating VLF signal amplitude perturbations (ΔA) to the time‐integrated precipitation (100–300 keV) along the VLF path (Ψ = Γ/ΔA), of 1.2 ± 0.3 × 1010 (el m−1/dB) is suggested for precipitation events of similar location and characteristics to those examined. The total precipitation (100–300 keV) induced by one of the representative LEP events is estimated at ∼1.8 ± 0.4 × 1016 electrons, calculated directly from the conversion ratio Ψ and observations of VLF signal perturbations.

Observation evidences of atmospheric Gravity Waves induced by seismic activity from analysis of subionospheric LF signal spectra

Abstract. We analyze variations of the LF subionospheric signal amplitude and phase from JJY transmitter in Japan (F=40 kHz) received in Petropavlovsk-Kamchatsky station during seismically quiet and active periods including also periods of magnetic storms. After 20 s averaging, the frequency range of the analysis is 0.28–15 mHz that corresponds to the period range from 1 to 60 min. Changes in spectra of the LF signal perturbations are found several days before and after three large earthquakes, which happened in November 2004 (M=7.1), August 2005 (M=7.2) and November 2006 (M=8.2) inside the Fresnel zone of the Japan-Kamchatka wavepath. Comparing the perturbed and background spectra we have found the evident increase in spectral range 10–25 min that is in the compliance with theoretical estimations on lithosphere-ionosphere coupling by the Atmospheric Gravity Waves (T&gt;6 min). Similar changes are not found for the periods of magnetic storms.

Surface latent heat flux and nighttime LF anomalies prior to the M&amp;lt;sub&amp;gt;w&amp;lt;/sub&amp;gt;=8.3 Tokachi-Oki earthquake

Abstract. Surface Latent Heat Flux (SLHF) is an atmospheric parameter proportional to the evaporation from the Earth's surface. SLHF has been found to exhibit an anomalous behavior in the epicentral region prior to several coastal earthquakes. Sub-ionospheric low frequency (LF) radio sounding measurements have shown its potentiality for the short-term earthquake forecasting since the last decade. The anomalous SLHF and nighttime LF sub-ionospheric signals are found to show complementary nature associated with the large Tokachi-Oki earthquake of 25 September 2003. Such complementary nature of parameters may prove to be potential in providing early warning information about an impending earthquake.

A study of ionospheric response to regional seismic activity by VLF radio sounding

Electric field strength nighttime fluctuations in subionospheric signals of very low frequency (VLF) on Omega, Tsushima-Chofu (Tokyo) and NWC, Australia-Chofu propagation paths recorded during March–August, 1997 were studied for searching for correlation with seismic activity. Wave-like anomalies in VLF Omega signal with periods of a few hours were observed 1–3 days before or on the day of moderately strong earthquakes of magnitudes 5–6.1. Analysis of the correlation between regional seismic activity around Tokyo and dispersion of VLF fluctuations revealed that maximal correlation was observed during pre-seismic periods for shallow earthquakes (focal depth less than 40 km) that occurred at distances up to 350–400 km from receiving place. Spectral analysis of the earthquake catalogue has shown rhythmical variations of seismic activity integrated over large areas with periods of 4–14 days. Similar periodic components are observed in fluctuations of VLF signals propagating over the NWC-Chofu path. For both VLF propagation paths, a 27-day periodicity was found in fluctuations of signals that can be referred to as tidal or solar rotation influence.

Observation of Subionospheric JG2AS Signal as Precursor of the Earthquakes

Observation of Subionospheric JG2AS Signal as Precursor of the Earthquakes

VLF monitorig of atmosphere-ionosphere boundary as a tool to study planetary waves evolution and seismic influence

It is shown that sounding of the lower ionosphere boundary by subionospheric signals from powerful VLF transmitters can be a useful tool for investigation of the Planetary Waves (PW) with periods in a range of 2–30 days. A specific spring-time transition in the PW dynamic periodograms is revealed from the analysis of several years data using Tsushima, Japan VLF transmitter (10.2 kHz) along the path with length of about 1000 km. Earthquake influence in the periodograms could be sometime recognized as an appearance of specific wavelets. We discuss a possibility of PW transportation from the bottom to the upper atmosphere as modulation of shorter-scale gravity waves (GW) inside the troposphere and subsequent demodulation of the GW at the atmosphere-ionosphere boundary for explanation of the observational results.

Sferic clusters associated with early/Fast VLF events

Characteristic subionospheric signal perturbations referred to as ‘early/fast’ VLF events are found to consistently be associated with the simultaneous occurrence of unusual clusters of radio atmospherics as observed in wideband VLF data. A burst of sferic activity occurs at or before the onset of the early/fast events and lasts from several hundred milliseconds to several seconds. The cluster energy attenuates with distance more rapidly than cloud‐to‐ground (CG) lightning, suggesting that these clusters may be produced by intracloud (IC) lightning, and that the charge movement within the cloud may thus play a role in the causative mechanism of early/fast VLF events.