VLF Electric Field Research Articles

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.

Effect of VLF electric field changes associated with major shallow earthquakes (M = 5.6–5.9) occurred in Indian subcontinent, on the atmosphere

Effect of VLF electric field changes associated with major shallow earthquakes (M = 5.6–5.9) occurred in Indian subcontinent, on the atmosphere

Anomalous subsurface VLF electric field emissions related to Nepal earthquakes (M = 7.8, M = 7.3), and their generation and propagation mechanisms

Anomalous subsurface VLF electric field emissions related to Nepal earthquakes (M = 7.8, M = 7.3), and their generation and propagation mechanisms

On the propagation of VLF electric field emissions associated with earthquakes in the middle layer of the earth crust

An ensemble of elementary radiators is generated on the basement rock because of the applied stresses in the preparation zone of the earthquakes in the earth crust. Considering such an ‘ensemble’ as the source of electromagnetic signals, the strength of the electric field is estimated at different distances and frequencies lying in range (3 – 27 kHz) at three different conductivities of the crustal layers (10-8, 10-9, 10-10 S/m). The results of the computation are presented in this paper. Moreover, propagation distances for the seismogenic VLF emissions have also been calculated in the frequency band (3 – 27 kHz) at the conductivities lying in the range (10-8 - 10-10 S/m) within the limit of detectability of measuring instruments (10-7 V/m). It is observed that these distances increase with the decrease of conductivity of the middle layer of crust. Furthermore, theoretical results of comptations are verified from the experimental observations of the seismic event that occurred at the distance of 698 km from the observing station at Chaumuhan Mathura (Geographic lat. 27.490N, long. 77.670E). In addition to this, generation and propagation mechanisms of seismo-electromagnetic radiations have also been discussed briefly.

Excitation of VLF perturbations in the F-region of the ionosphere by beating HF O-mode waves

Action of two HF O-mode pump waves with different frequencies on the ionosphere is suggested as a source of artificial quasielectrostatic VLF perturbations in the F-layer. Such perturbations with the beat frequency are excited in the vicinity of small-scale artificial plasma irregularities (striations) by upper hybrid resonance (UHR) oscillations. While UHR oscillations are trapped inside striations, the generated VLF perturbations are not trapped in such small-scale plasma depletions. These VLF oscillations propagate at small angles with respect to the background magnetic field. Estimates show that the excited VLF electric fields are strong enough, while magnetic perturbations are much weaker. The scattering of quasielectrostatic VLF perturbations on small-scale plasma irregularities in the F-layer provides the generation of the whistler mode waves that can be detected on board satellites.

A multi-experiment approach to ascertain electromagnetic precursors of Nepal earthquakes

The experimental data obtained from three different techniques such as (i) subsurface VLF electric field measured with the help of a borehole antenna (ii) total electron content (TEC) of the ionosphere measured with a GPS receiver and (iii) VLF amplitude of subionospheric fixed frequency (f = 19.8 kHz) NWC (Naval Communication Station) transmitter signals monitored with SoftPAL receiver are analysed in relation to two major Nepal earthquakes occurred on 25 April (M = 7.8) and 12 May (M = 7.3), 2015. The VLF electric field data and fixed frequency VLF transmitter signal data are taken from Mathura and Agra stations in India whereas TEC data are taken from three IGS stations, one of which Lucknow (Lck3) is located in India and others Lhasa (Lhaz) and Wuhan (Wuhan) are located in China, all within 800 km from epicenters of two earthquakes except Wuhan which is located at about 2865 km. The results of the analysis indicate that while borehole and TEC data show anomalies in normal variations within 8–15 days prior to the earthquake of 25 April, the same occur within 5–7 days prior to the earthquake of 12 May. The subionospheric variations of VLF amplitude also show an anomaly 4 days before the later earthquake. Occurrence of these anomalies is examined critically in the light of local lightning and magnetic storms. However, these factors are not found to influence the data. A brief description of generation and propagation mechanisms of seismogenic emissions is also presented.

Line Charge Densities and Currents of Downward Negative Leaders Estimated From VHF Images and VLF Electric Fields Observed at Close Distances

In this paper, a method for retrieving the line charge density and the current of a downward leader based on ground observations of lightning images and electric field changes was proposed and examined. The method was then applied to two downward negative leaders in a rocket-triggered lightning discharge with measurements of broadband interferometric images and electric field changes at close distance. Only the descending channel portion between about 900–400 m high of the two leaders were analyzed, where the channel was in free air rather than the trace of triggering wire. The line charge density for both leaders showed a general increasing trend with a big impulsive change around 500 m high, as the leaders moved downward, which was in good agreement with the prediction of existing downward leader models. The range of the line charge density was similar for the two leaders (−0.03 to −0.32 mC/m and −0.02 to −0.33 mC/m, respectively). The impulsive change of the line charge density was corresponding to a sharp turn of the leader channel around 500 m high, which may imply that the sharp turn of the leader channel was probably associated with a pre-existing local positive space charge pocket. The estimated leader currents for two leaders ( −20 to −315 A and −77 to −541 A, respectively) were also in good agreement with independent estimations in the literature.

Fine spatial evolution of leaders and M-components in rocket-triggered lightning observed with a broadband interferometer

Based on measurements of VHF radiation sources and VLF electric fields with a broadband interferometer system, the spatial evolution of leader processes and K-breakdowns and M-components involved in a classically-triggered negative lightning discharge have been analyzed. While a normal classically-triggered negative discharge usually starts with a positive leader initiates from the tip of the ascending triggering-wire and moves upward, there was no such an initial upward positive leader (UPL) being observed for the present discharge, probably due to low resolution and sensitivity of the measurements. Instead, there was a downward negative leader (PDL) at the preliminary stage of the discharge being observed, followed by a 173-ms-long lasting M-component-wise process and two leader/return-stroke processes. The PDL was most likely a leader process along the channel trace possibly built by the undetected UPL, as its speed which ranged from 3.7 × 106 m/s to 0.3 × 106 m/s is similar to that of a dart leader in literature. The long lasting M-component-wise process consisted of a slow negative-going change stage (Ma), followed by a fast negative-going change stage (Mb) and then a slow positive-going change stage (Mc). Ma was found to be intra-cloud negative breakdowns moving towards overhead position of the PDL trace. Mb would be considered as a common M-component (channel brightening), which starts with a K breakdown in cloud (Mb1) moving horizontally towards overhead position of the previous PDL, followed by an event (Mb2) moving up from ground to cloud along PDL trace. As Mb2 reaching the cloud, more new K breakdowns (Mc) appeared in cloud around extremities of the pre-built channels by Ma and Mb. The leader preceding the first return stroke (L1) started inside the cloud and propagated downward to the triggering-wire trace, but with a different channel to that of PDL. As the leader touched the triggering wire trace, it appeared to propagate upward along the same channel of PDL. The upward portion of L1 might be interpreted as a reflection of L1 at top of the triggering-wire trace towards the PDL trace due to the difference in conductivity and potential between the PDL trace, the triggering wire trace and the L1 channel, which was optically invisible but bright in VHF. The speed of the downward portion of L1 decreased from 2.32 to 0.32 × 106 m/s as it descended, while that of the upward portion of L1 increased from 0.85 to 2.7 × 106 m/s as it ascended. The leader preceding the second return stroke (L2) behaved similarly to L1 but with higher speeds.

IONOSPHERIC PERTURBATIONS ASSOCIATED WITH THE M8.6 SUMATRA EARTHQUAKE ON 28 MARCH 2005

During 15 days in a relative quiet geomagnetic time around the M8.6 Sumatra earthquake on 28 March 2005, ionospheric perturbations occurred before this event on March 20, 22, 23, and 28,2005 in many parameters such as the electric field anti plasma parameters in the equatorial area. Since it was a quiet magnetic time, those four days referred above may be related to the preparation for this strong earthquake. The electron density showed two types of anomalies, one being a monotone increase in the single peak values with amplitudes exceeding 1 sigma, such as on March 20 and 28 in 2005; the other one changing the normal single peak to double crests and a trough in the equatorial area, which occurred on March 22 and 23, 2005. Both the anomalies are distributed over a very large area of 100 degrees longitude and 30 degrees latitude, presenting a long duration and global feature. The study of VLF electric field spectrums demonstrated that those anomalies were not presented in the geomagnetic conjugate feature. Electric field perturbations were concentrated in a relative smaller area than those of N(e) occurring only in the nearest orbit to the epicentre with only monotone increasing N(e) anomalies. It is thought that it is difficult for an electric field generated close to the seismic preparation area in the equatorial area to penetrate into the ionosphere directly, but it may occur through the coupling mechanism of lithosphere-atmosphere-ionosphere. Combined with the analysis of ion composition and varying shape, this shows that prior to this earthquake the variation in oxygen density reversed to that of hydrogen and helium density, similarly to other ionospheric precursors before other earthquakes. It is concluded that conjoint analysis among different parameters should he applied in order to distinguish the precursors related to the earthquakes.

Ant Colony Optimization detects anomalous aerosol variations associated with the Chile earthquake of 27 February 2010

This study attempts to acknowledge AOD (Aerosol Optical Depth) seismo-atmospheric anomalies around the time of the Chile earthquake of 27 February 2010. Since AOD precursor alone might not be useful as an accurate and stand alone criteria for the earthquake anomalies detection, therefore it would be more appropriate to use and integrate a variety of other precursors to reduce the uncertainty of potential detected seismic anomalies. To achieve this aim, eight other precursors including GPS-TEC (Total Electron Content), H+, He+, O+ densities (cm−3) and total ion density (cm−3) from IAP experiment, electron density (cm−3) and electron temperature (K) from ISL experiment and VLF electric field from ICE experiment have been surveyed to detect unusual variations around the time and location of the Chile earthquake. Moreover, three methods including Interquartile, ANN (Artificial Neural Network) and ACO (Ant Colony Optimization) have been implemented to observe the discord patterns in time series of the AOD precursor. All of the methods indicate a clear abnormal increase in time series of AOD data, 2days prior to event. Also a striking anomaly is observed in time series of TEC data, 6days preceding the earthquake. Using the analysis of ICE data, a prominent anomaly is detected in the VLF electric field measurement, 1day before the earthquake. The time series of H+, He+, O+ densities (cm−3) and total ion density (cm−3) from IAP and also electron density (cm−3) and electron temperature (K) from ISL, illustrate the abnormal behaviors, 3days before the event. It should be noted that the acknowledgment of the different lead times in outcomes of the implemented precursors strictly depend on the proper understanding of Lithosphere–Atmosphere–Ionosphere (LAI) coupling mechanism during seismic activities. It means that these different anomalies dates between LAI precursors can be a hint of truthfulness of multi-precursors analysis.

Anomalous subsurface VLF electric field changes related to India-Nepal border earthquake (M=5.3) of 4 April 2011 and their lithosphere – atmosphere coupling observed at Mathura

Anomalous subsurface VLF electric field changes related to India-Nepal border earthquake (M=5.3) of 4 April 2011 and their lithosphere – atmosphere coupling observed at Mathura

Variation of the first cut‐off frequency of the Earth‐ionosphere waveguide observed by DEMETER

More than four years of VLF electric field data recorded by DEMETER have been analyzed, in order to monitor the first cut‐off frequency (QTM1) of the Earth‐ionosphere waveguide, at around 1.6–1.8 kHz. Since losses in a waveguide are maximized right at the cut‐off frequency, DEMETER (∼700 km orbit) can detect the minimum of energy of the leaking fields coming from the waveguide. This measurement permits to draw a global map of its value (f1), which is directly related to the effective height of the ionosphere (h) by the relation f1 = c/2h (c is the speed of light). It enables the remote sensing of the D region, which is one of the less known layers of the ionosphere, because it is too low for satellites to orbit inside it and too high for balloons to reach it. The effective height depends mainly on the electron density (Ne) and neutral density (Nn) profiles, which determine the plasma frequency and the electron mobility. The effective height shifts downward 5–10 km in southern warm season in the South Pacific Ocean. Another effect is observed in the Indian and Atlantic Oceans; the effective height decreases its value twice a year, in the area of roughly ±15° from the geomagnetic equator. The main causes for the changes on the effective reflection height are the solar radiation and the thunderstorm activity. However, the observed shifts are more prominent over the oceans, and a possible explanation for this difference could be attributed to i) less polluted conditions above the oceans (aerosols change the atmospheric conductivity and then the global atmospheric electric circuit), ii) the effect of the current associated to the thunderclouds on the bottom of the ionosphere because thunderstorms are much more numerous above land, or iii) ionization by elves because their occurrence is larger above oceans.

Strong magnetic field fluctuations within filamentary auroral density cavities interpreted as VLF saucer sources

The Geoelectrodynamics and Electro‐Optical Detection of Electron and Suprathermal Ion Currents (GEODESIC) sounding rocket encountered more than 100 filamentary density cavities associated with enhanced plasma waves at ELF (<3 kHz) and VLF (3–10 kHz) frequencies and at altitudes of 800–990 km during an auroral substorm. These cavities were similar in size (∼20 m diameter in most cases) to so‐called lower‐hybrid cavities (LHCs) observed by previous sounding rockets and satellites; however, in contrast, many of the GEODESIC cavities exhibited up to tenfold enhancements in magnetic wave power throughout the VLF band. GEODESIC also observed enhancements of ELF and VLF electric fields both parallel and perpendicular to the geomagnetic fieldB0within cavities, though the VLF E field increases were often not as large proportionally as seen in the magnetic fields. This behavior is opposite to that predicted by previously published theories of LHCs based on passive scattering of externally incident auroral hiss. We argue that the GEODESIC cavities are active wave generation sites capable of radiating VLF waves into the surrounding plasma and producing VLF saucers, with energy supplied by cold, upward flowing electron beams composing the auroral return current. This interpretation is supported by the observation that the most intense waves, both inside and outside cavities, occurred in regions where energetic electron precipitation was largely inhibited or absent altogether. We suggest that the wave‐enhanced cavities encountered by GEODESIC were qualitatively different from those observed by earlier spacecraft because of the fortuitous timing of the GEODESIC launch, which placed the payload at apogee within a substorm‐related return current during its most intense phase, lasting only a few minutes.

Reply to comment by R. Grard et al. on “An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe”

Reply to comment by R. Grard et al. on “An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe”

An analysis of VLF electric field spectra measured in Titan's atmosphere by the Huygens probe

A numerical simulation of Titan's electromagnetic cavity in the VLF band is carried out using the Transmission Line Matrix (TLM) method, with the aim of assessing the VLF electric field spectra sent by the Huygens probe. In an Earth‐like model, successive peaks would be expected in the spectra, associated with multiple reflections of the electromagnetic wave on the external surfaces of Titan's electromagnetic cavity, formed by the ionosphere and a conductive ground or underground surface. However, owing to high losses conferred by the electrical conductivity to Titan's atmosphere, the direct numerical and experimental spectra are decreasing functions of the frequency without resonances or other special forms. The weak atmospheric resonances are extracted from the data measured by the Huygens's sensors after a procedure based on analysis of the late time response is applied to the experimental spectra. The new late time spectra present the appearance expected, with successive peaks and a mean between adjacent resonances of 0.67 kHz, in agreement with the late time TLM results when the Borucki et al. (2006) conductivity profile with aerosols is introduced in the numerical model. This gap in frequency between successive peaks gives an effective optical distance between the ground and the conductive ionosphere of approximately 220 km, considerably higher than the terrestrial one where this distance is around 75 km.

Identification of earthquake sources responsible for subsurface VLF electric field emissions observed at Agra

Employing USGS earthquake data for Indian region and very low frequency (VLF, 3 kHz) subsurface electric field data obtained by using a borehole antenna at Agra in India, statistical analysis has been carried out to identify the earthquake sources responsible for VLF data. The correlation coefficient between occurrence number of VLF noise bursts and earthquakes are calculated and level of null hypothesis tested. The results show that seismic activities occurring close to the observing station and the main boundary fault located at the southern base of Himalaya are the main sources of VLF emissions recorded at the station.

Lightning sferics and stroke-delayed pulses measured in the stratosphere: Implications for mesospheric currents

[1] During the Brazil Sprite Balloon Campaign 2002–03, the vector ELF to VLF (25 Hz–8 kHz) electric and magnetic fields driven by cloud-to-ground (CG) lightning strokes at horizontal distances of 75–600 km were measured at altitudes of 30–35 km. Electric field changes were measured for each of the 2467 CG strokes detected by the Brazilian Integrated Lightning Network. ELF pulses that occur 4–12 ms after the retarded time of the lightning sferic, which have been previously attributed to sprites, were found for 1.4% of 934 strokes examined. Thus, lightning-driven electric and magnetic field changes were common and stroke-delayed ELF pulses were rare, which disagrees with results from the Sprites99 Balloon Campaign.

VLF electromagnetic noise bursts observed in a borehole and their relation with low-latitude hiss

In an attempt to monitor subsurface VLF electric field changes associated with earthquakes, a borehole antenna has been installed at Bichpuri, Agra (Geograph. Lat. 27.2°N, Geograph. Long. 78°E, Geomag. Lat. 17.1° N, L=1.1 ) in India and observations have been taken since February, 1998 using an analog system. The electric field changes have been found to occur in the form of noise bursts of varying amplitudes and durations. A thorough analysis of the data of the year 1999 has shown occasional occurrence of large amplitudes, long duration (∼5– 6 h or more) noise bursts which show seasonal, stormtime, and diurnal variations similar to those of low latitude magnetospheric hiss observed on the ground. Similar noise bursts have also been recorded in the vertical terrestrial antenna operated in conjunction with the borehole antenna. This result indicates penetration of ionospheric/magnetospheric VLF signals to large depths in the crustal region and cautions for careful identification of potential seismogenic signals in VLF data.

Stratospheric balloon measurements of electric fields associated with thunderstorms and lightning in Brazil

Measurements of electric fields associated with thunderstorms and lightning were obtained during two balloon flights carrying double‐probe electric field detectors launched from Cachoeira Paulista (22°44′S, 44°56′W), Brazil, on January 26, 1994, and March 23, 1995. From data obtained in 1994, a linear relationship between the quasi‐dc vertical electric field peak amplitude and the decay time constant of lightning signatures was found for negative flashes. The results are compared to similar data for intracloud flashes. Based on electric field data obtained in 1995 and on the present knowledge about the differences between positive cloud‐to‐ground and intracloud flashes, two methods to distinguish them at balloon altitudes are presented: The first is based on an estimate of the destroyed charge in the event; the second is based on the peak amplitude ratio between the vertical quasi‐dc and the VLF electric field. The behavior of the vertical quasi‐dc electric field before and after large cloud‐to‐ground lightning flashes is discussed and attributed to the existence of a shielding layer around the thunderstorm. This shielding layer is associated with a threefold or greater decrease in the conductivity inside the cloud. An abrupt variation observed in the quasi‐dc electric field possibly associated with the occurrence of positive flashes was observed and attributed to the formation of a transient shielding layer just above the thunderstorm, which could be produced by the near‐breakdown field inside the cloud.

Confinement of nonthermal continuum radiation to low latitudes

Applying a wave intensity map technique to 4 years of plasma wave observations from the VLF electric field instrument on the Hawkeye spacecraft reveals the latitudinal distribution of the Earth's nonthermal continuum radiation. The wave intensity maps at frequencies below (23.7 kHz), near (31.1 kHz), and above (56 kHz) the average magnetopause plasma frequency clearly identified a source region and propagation extent of the nonthermal continuum. The Hawkeye observations show that the nonthermal continuum radiation is primarily observed at relatively low latitudes, is rarely observed over the high‐altitude polar cap, and is consistent with previous observations as being beamed outward from the dawn source region. The observed angular distribution of the nonthermal continuum radiation is modeled by ray‐tracing calculations which show that the trapped and escaping components of the nonthermal continuum can exist simultaneously at the same frequency but with very different spatial distributions in the magnetosphere. In addition, the observations and the ray‐tracing calculations show that the trapped continuum radiation component does not uniformly illuminate the magneto spheric cavity as some have previously believed but is confined to mostly low latitudes.