Showing papers on "Very low frequency published in 2014"

Universality of slow earthquakes in the very low frequency band

Abstract: Deep tectonic tremors have been observed together with signals in the very low frequency (VLF) band from 0.02 to 0.05 Hz, which have been identified as VLF events in limited regions of subduction zones. By stacking broadband seismograms relative to the timing of tremors, we have detected similar signals in all regions where tremors occur in western Japan. These signals are inverted to obtain the moment tensor, and the fault-normal and slip vectors are generally consistent with the geometry of the plate interface and the direction of plate motion. Therefore, these signals are probably radiated by shear slip on the plate interface. The ratio between the seismic energy rate estimated from the tremors and seismic moment rate in the VLF band is almost proportional, with a proportionality constant (i.e., scaled energy) of around 5 × 10−10. The spatial distribution of scaled energy may reflect spatial variations in the frequency-dependent characteristics of slow deformation.

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

Abstract: 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.

Low‐mid latitude D region ionospheric perturbations associated with 22 July 2009 total solar eclipse: Wave‐like signatures inferred from VLF observations

Abstract: We present first report on the periodic wave-like signatures (WLS) in the D region ionosphere during 22 July 2009 total solar eclipse using JJI, Japan, very low frequency (VLF) navigational transmitter signal (22.2 kHz) observations at stations, Allahabad, Varanasi and Nainital in Indian Sector, Busan in Korea, and Suva in Fiji. The signal amplitude increased on 22 July by about 6 and 7 dB at Allahabad and Varanasi and decreased by about 2.7, 3.5, and 0.5 dB at Nainital, Busan, and Suva, respectively, as compared to 24 July 2009 (normal day). The increase/decrease in the amplitude can be understood in terms of modal interference at the sites of modes converted at the discontinuity created by the eclipse intercepting the different transmitter-receiver great circle paths. The wavelet analysis shows the presence of WLS of period ~16–40 min at stations under total eclipse and of period ~30–80 min at stations under partial eclipse (~85–54% totality) with delay times between ~50 and 100 min at different stations. The intensity of WLS was maximum for paths in the partially eclipsed region and minimum in the fully eclipsed region. The features of WLS on eclipse day seem almost similar to WLS observed in the nighttime of normal days (e.g., 24 July 2009). The WLS could be generated by sudden cutoff of the photo-ionization creating nighttime like conditions in the D region ionosphere and solar eclipse induced gravity waves coming to ionosphere from below and above. The present observations shed additional light on the current understanding of gravity waves induced D region ionospheric perturbations.

Solar flare induced D-region ionospheric perturbations evaluated from VLF measurements

Abstract: The results of very low frequency (VLF) wave amplitude measurements carried out at the low latitude station Varanasi (geom. lat. 14∘55′N, long. 154∘E), India during solar flares are presented for the first time. The VLF waves (19.8 kHz) transmitted from the NWC-transmitter, Australia propagated in the Earth-ionosphere waveguide to long distances and were recorded at Varanasi. Data are analyzed and the reflection height H′ and the sharpness factor β are evaluated. It is found that the reflection height decreases whereas sharpness factor increases with the increase of solar flare power. The H′ is found to be higher and β smaller at low latitudes than the corresponding values at mid and high latitudes. The sunspot numbers were low during the considered period 2011–2012, being the rising phase of solar cycle 24 and as a result cosmic rays may impact the D-region ionosphere. The increased ionization from the flare lowers the effective reflecting height, H′, of the D-region roughly in proportion to the logarithm of the X-ray flare intensity from a typical mid-day unperturbed value of about 71–72 km down to about 65 km for an X class flare. The sharpness (β) of the lower edge of the D-region is also significantly increased by the flare but reaches a clear saturation value of about 0.48 km−1 for flares of magnitude greater than about X1 class.

Inversion of airborne tensor VLF data using integral equations

Abstract: The Geological Survey of Sweden has been collecting airborne tensor very low frequency data (VLF) over several decades, covering large parts of the country. The data has been an invaluable source o ...

Very low frequency subionospheric remote sensing of thunderstorm‐driven acoustic waves in the lower ionosphere

Abstract: We present observations of narrowband subionospheric VLF transmitter signals on 20 March 2001, exhibiting coherent fluctuations of over 1 dB peak to peak. Spectral analysis shows that the fluctuations have periods of 1-4 min and are largely coherent. The subionospheric propagation path of the signal from Puerto Rico to Colorado passes over two regions of convective and lightning activity, as observed by GOES satellite imagery and National Lightning Detection Network lightning data. We suggest that these fluctuations are evidence of acoustic waves launched by the convective activity below, observed in the 80-90 km altitude range to which nighttime VLF subionospheric remote sensing is sensitive. These observations show that VLF subionospheric remote sensing may provide a unique, 24 h remote sensing technique for acoustic and gravity wave activity. We reproduce this event in simulations using a fluid model of gravity and acoustic wave propagation to calculate the ionospheric disturbance, followed by an electromagnetic propagation model to calculate the perturbation amplitude at the location of the VLF receiver. Simulation results show that a very large and coherent convective source is required to produce these amplitude perturbations.

Unusual behavior of Very Low Frequency signal during the earthquake at Honshu/Japan on 11 March, 2011

Abstract: We present evidence of unusual Very Low Frequency (VLF) signal amplitude variation during the devastating earthquake of magnitude 9.0 which occurred at Honshu, in Japan on 11 March, 2011. We use the SoftPAL very low frequency receiver placed at Ionospheric and Earthquake Research Centre of Indian Centre for Space Physics, located at Sitapur (Lat. 22°30′N, Long. 87°47′E). We observe significant changes in signal amplitude from JJI (Lat. 32°05′N, Long. 131°51′E) station transmitting at a frequency of 22.2 kHz prior to the earthquake. We analyze signal amplitude for almost 2 weeks to establish a possible seismo–ionospheric correlation. We observe significant shift of the sunrise terminator time up to 2 days before the earthquake and the shift is found to be maximum on the day of the earthquake. In addition, we observe unusual increase of the D-layer disappearance time during the earthquake and the value becomes maximum on the day of the earthquake. These findings generally agree with our previous findings reported elsewhere.

A comparative study of measured amplitude and phase perturbations of VLF and LF radio signals induced by solar flares

Abstract: Very Low Frequency (VLF) and Low Frequency (LF) signal perturbations were examined to study ionospheric disturbances induced by solar X-ray flares in order to understand processes involved in propagation of VLF/LF radio signals over short paths and to estimate specific characteristics of each short path. The receiver at the Belgrade station is constantly monitoring the amplitude and phase of a coherent and subionospherically propagating LF signal operated in Sicily NSC at 45.90 kHz, and a VLF signal operated in Isola di Tavolara ICV at 20.27 kHz, with the great circle distances of 953 km and 976 km, respectively. A significant number of similarities between these short paths is a direct result of both transmitters and the receiver’s geographic location. The main difference is in transmitter frequencies. From July 2008 to February 2014 there were about 200 events that were chosen for further examination. All selected examples showed that the amplitude and phase of VLF and LF signals were perturbed by solar X-ray flares occurrence. This six-year period covers both minimum and maximum of solar activity. Simultaneous measurement of amplitude and phase of the VLF/LF signals during a solar flare occurrence was applied to evaluate the electron density profile versus altitude, to carry out the function of time over the middle Europe. [Projekat Ministarstva nauke Republike Srbije, br. 176002 i br. III4402]

Method and Device for Cancelling Interference

Abstract: The invention discloses a method and device for cancelling, from a radio signal received by the radio device in a first frequency band, the interference generated through a nonlinearity of the radio device when the radio device transmits a radio signal on at least a second frequency band simultaneously with the received radio signal, the at least one second frequency band being different from the first frequency band, the transmitted radio signal being obtained from digital signal.

Ground‐based ELF/VLF chorus observations at subauroral latitudes—VLF‐CHAIN Campaign

Abstract: We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17–25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

A comparative study of measured amplitude and phase perturbations of VLF and LF radio signals induced by solar flares

Abstract: Very Low Frequency, VLF and Low Frequency, LF signal perturbations were examined to study ionospheric disturbances induced by solar X-ray flares. The aim was to understand processes in propagation VLF/LF radio signals over short paths, and to estimate specific characteristics of each short path. The receiver at Belgrade station continuously monitor the amplitude and phase of coherent and subionospherically propagating LF signal operated in Sicily, NSC at 45.90, kHz and VLF signal operated in Isola di Tavolara ICV at 20.27 kHz, with great circle distances of 953 km and 976 km, respectively. Geographical locations of transmitters and receiver site result that these short paths have many similarity. The main difference is in transmitter frequencies. In period from 2008 to February 2014 were selected around 200 events for further examination. In all selected examples amplitude and phase on VLF and LF signals were perturbed by occurrence of solar X-ray flares. This six years period covers minimum and maximum of solar activity. Also, simultaneous measurement of amplitude and phase on the VLF/LF signals during occurrence of solar flare were employed for constructing electron density profile versus altitude and also in a function of time over middle Europe.

Very Low-Frequency Electromagnetic Method: A Shallow Subsurface Investigation Technique for Geophysical Applications

Abstract: The very low-frequency (VLF) electromagnetic (EM) method is the simplest EM method to delineate shallow subsurface conducting structures. Since the approach utilizes signals transmitted from worldwide transmitters located in coastal areas in the 5–30 kHz frequency band, it is suitable to depict conducting structures up to 200 m depth in highly resistive terrain. Freely and readily available primary field signals anywhere around the Earth make the VLF method very convenient and efficient for field data collection. Further, VLF data processing using digital linear filtering is quite accurate and very efficient in depicting the qualitative information about subsurface conductors, even though quantitative interpretation of VLF data is as complex as other EM data interpretation. In the present study, various aspects of the VLF method such as basic theory, worldwide VLF transmitters, quantities measured, and interpretation procedures are discussed in detail. Finally, the efficacy of the VLF method for groundwater investigation, mineral investigation, and landslide and subsurface pollution monitoring studies has been demonstrated. Even though the VLF method is a rapid technique for subsurface investigation, use of complementary geophysical methods such as gravity, direct current (DC) resistivity, self-potential, radiometric, etc., reduces the ambiguity in the interpretation and yields reliable subsurface information.

A comparative study of partial discharges under power and very low frequency voltage excitation

Abstract: Very low frequency (VLF) testing refers to AC testing in the frequency range of 0.01 Hz up to 1 Hz. VLF testing is increasingly becoming popular as an alternative approach for test objects which require significant reactive power such as power cables. However, reliable diagnosis of partial discharge characteristics under VLF voltage excitation is still lacking. The paper presents a comparative study between testing at normal power frequency and at VLF. Corona and surface discharges were produced using a needle/bowl and a short section of 11kV XLPE cable. Experiments were conducted over a range of frequencies from 50 Hz down to 0.08 Hz and the results were analyzed. They showed some similarity of PD characteristics in terms of their phase-resolved patterns. The discrepancies of PD results are described in details.

Whistler interaction with field‐aligned density irregularities in the ionosphere: Refraction, diffraction, and interference

Abstract: Field-aligned density irregularities (FAI) with kilometer-scale sizes transverse to the background magnetic field are a common feature in the ionosphere at all latitudes and local times. In this paper, we investigate the effect of these irregularities on the transionospheric propagation of very low frequency whistler waves and develop a quantitative description of FAI-related effects on whistler propagation through the lower ionosphere. Using an electron magnetohydrodynamics simulation, we provide two applications of our model. First, we show that the presence of kilometer-scale FAI in the ionosphere can reduce the power observed in the equatorial magnetosphere by more than 10 dB in some cases. Second, we demonstrate that multiple FAIs can act as a discrete lens for whistlers, providing a possible means for increasing wave power in artificial whistler ducting experiments.

Observation of local and conjugate ionospheric perturbations from individual oceanic lightning flashes

Abstract: Very low frequency (VLF) remote sensing observations of multifaceted local and conjugate ionospheric perturbations from geographically identified and well-characterized oceanic lightning discharges are presented for the first time. Lightning-induced electron precipitation (LEP) events are shown to produce disturbances first in the conjugate hemisphere and subsequently in the hemisphere of the causative lightning discharge in agreement with theoretical predictions. A rough threshold peak current of ∼100 kA is identified for lightning discharges to generate LEP events for the geomagnetic conditions present during observations. The occurrence of early VLF events and the subsequent duration of their recovery do not seem to fit any simple metric of lightning discharge peak current or proximity to great circle path. Knowledge of the full spectral density of the lightning electromagnetic pulse, not just its peak current, and the subionospheric mode structure are likely necessary to determine if a specific lightning discharge will generate an early VLF perturbation.

Ортостатическая тахикардия: диагностическое и прогностическое значение very low Frequency вариабельности ритма сердца

Abstract: Researched physiological mechanisms of development of orthostatic tachycardia syndrome (POTS) on the basis of a complex spectral structure analysis of heart rate variability (HRV), its nonlinear behavior in relationship to energy oscillations, baroreflex and parasympathetic activity. There were several stages of research. The first stage: created the method of spectral analysis of individual components of Very Low Frequency (VLF). On the basis of comparative Fast Furrier Transform data with Welch filters, autoregression, continuous wavelet analysis and Hilbert–Huang transform, for the first time it was shown that VLF has separate 200, 100 and 50 s oscillations (VLF200, VLF100, VLF50). The second stage: for evaluation of physiological properties of separate oscillations, was analyzed VLF structure in three groups of patients (100 subjects): with a predominance of parasympathetic activity (HF) in the HRV spectrum; with a dominance of 10 s oscillations (LF); with a severe depression of energy in all components of the spectrum. It was established that the individual components of VLF (VLF100 and VLF50) have a certain stability and partial independence from the changes of peripheral autonomic indices (LF/HF) at loads of low intensity. The third stage: at an active orthostatic test, 20 subjects with orthostatic tachycardia were researched in a comparison to a control group of 20 subjects without tachycardia. Analyzed the specifics of the VLF structure (VLF100 and VLF50) alone and in a conjunction with the LF and HF, as well as heart rate and blood pressure in subjects with orthostatic tachycardia with a predominance of parasympathetic activity at functional tests of low intensity (seven-test, deep breathing) and at an active orthostatic test. Based on these studies it was concluded that the individual components in the VLF structure (VLF100, VLF50) can demonstrate reciprocal relationships at a load among themselves and in the LF/HF ratio, and play an adaptive role in the mechanisms of autonomic provision. Revealed the importance of VLF and its components 100 and 50 s oscillations at an orthostatic test in the prognosis of orthostatic tachycardia development. In patients with dysautonomia at high level of VLF100 power, or high VLF50 indices, tachycardia did not develop. In this case, VLF and its component parts did an important adaptive function and conversely, the low VLF50 indices contribute to orthostatic tachycardia. In the proposed model of the descending neurosomatic control of autonomic regulation of HRV a very important role plays both neurogenic ways of regulation, and neuro-hormonal-metabolic ways, manifested in the complex VLF structure. Regarded the choice of ways and optimal neurosomatic models of regulation at POTS.

Design of a space-borne antenna for controlled removal of energetic Van Allen belt protons

Abstract: The energetic protons trapped in the inner Van Allen belt pose a risk to humans and spacecraft operating in Low Earth Orbit (LEO). These particles come from cosmic rays, solar storms and other processes, and they are a hindrance to development of space technologies. The Radiation Belt Remediation (RBR) idea has been proposed as a way to solve this problem through Ultra/Very Low Frequency (VLF/ULF) transmissions in the magnetosphere capable of inducing pitch angle scattering of the hazardous particles and precipitating them into the atmosphere. Whistler-type emissions (VLF band, tens of kHz) have been extensively studied for precipitation of energetic trapped electrons, but much less work has been devoted to the controlled removal of inner belt protons. The latter would require the man-made radiation of Electromagnetic Ion Cyclotron (EMIC) waves into the magnetosphere (ULF band, less than 10 Hz), the frequency of which is close to the cyclotron frequency of the trapped protons. In this paper we first identify the space-borne transmitter capable of radiating EMIC waves, and we estimate its radiation impedance and radiation pattern. The selected antenna configuration consists of a DC rotating coil, which is equivalent to two AC phased-orthogonal coils but with negligible self-inductance. However, the radiation resistance of magnetic dipoles is very small. For this reason, we propose a design based on superconductors and multiple turn arrangements. One of the most challenging aspects of using superconductors in space is their cooling system. This paper presents a preliminary thermal and mechanical design of a superconducting coil antenna capable of radiating EMIC waves into the magnetosphere. The coil is composed of high temperature superconducting tapes (HTS), which have to be kept below 77 K. Active thermal control and the use of cryogenics are therefore required to reject the heat coming from environmental sources. This preliminary design is used to calculate the power radiated from the antenna, its radiation pattern and its effect on the energetic proton population of the inner Van Allen belt. The feasibility of the remediation concept, as well as a scientific mission scaled down to detectability of the proton precipitating fluxes are finally addressed at the end of the paper.

Radio signal transmission method, radio signal transmitting device, radio signal receiving device, radio base station device and radio terminal device

Abstract: When a plurality of radio signals having different sub frame lengths are transmitted on the same radio carrier, a radio signal of a short sub frame length is arranged inside a carrier band, and a radio signal of a sub frame length longer than the sub frame length of the short sub frame length signal is arranged outside the carrier band.

VLF radio signal anomalies associated with strong earthquakes

Abstract: The VLF (Very Low Frequency) radio signal receiving network of Russian Alpha radio navigation system has been established in China since 2010. We applied a quartile-based method to study the variations of VLF signal propagation before the M≥6.0 shallow earthquakes in the network. Results found that possible anomalies occurred around the two strong earthquakes, 2010 M7.1 Yushu and 2013 M7.0 Lushan earthquake, respectively. For earthquakes with magnitude 5.0≥M≥7.0, such anomalies are not detected. Preliminary statistical analysis with Dst and Kp indices show that there exists a good correlation between geomagnetic storms and VLF propagation anomaly.

Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected by subionospheric VLF signals

Abstract: The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8–50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami.

Effects of ambient conditions on partial discharges at very low frequency (VLF) sinusoidal voltage excitation

Abstract: Effects of temperatures on corona discharges developed by needle/bowl electrode configuration at different frequencies are investigated. PD measurements are carried out under enclosed controlled air temperature range between 20°C and 40°C. The test chamber is airtight so the humidity is constant. Sinusoidal voltage waveforms in the frequency range of 0.05Hz to 50Hz are used to stress samples. There are evidences that PD behaviors at VLF and power frequency are dependent on ambient temperature in a similar manner. Nevertheless, there are still noticeable differences in PD patterns at VLF at various temperatures, especially in the phase position of PD events. These differences are discussed in details.

Study of long path VLF signal propagation characteristics as observed from Indian Antarctic station, Maitri

Abstract: To examine the quality and propagation characteristics of the radio waves in a very long propagation path, Indian Centre for Space Physics, Kolkata, participated in the 27th Indian scientific expedition to Antarctica during 2007–2008. One Stanford University made AWESOME Very Low Frequency (VLF) receiving system was installed at the Indian Antarctic station Maitri and about five weeks of data was recorded successfully from the Indian transmitter VTX and several other transmitting stations worldwide. The signal quality of VTX was found to be very good and the signal was highly stable. The signal shows the evidences of the presence of the 24 hours solar radiation in the Antarctic region during local summer. We compute the elevation angle of the Sun theoretically during this period and the spatial distribution of the signal by using the LWPC model. We compute the attenuation coefficient of the different propagation modes and observe that different modes are dominating in different propagation conditions. We also observe effects of the Antarctic polar ice in the propagation modes.

A synchronization system of very low-frequency interferometers

Abstract: A solution to the problem of synchronization of spaced autonomous data-acquisition systems and obtaining stable reference-frequency signals for measuring phases of very low-frequency (VLF) radio signals is described. The expediency of using GPS receivers and tunable crystal oscillators in the synchronization system of VLF interferometers is grounded. Measurement results of the phase stability of the reference signal, received from the designed device, are presented. The short-term frequency stability of a 200-kHz pulse signal at the output of the designed unit is no more than 1.5 × 10−7, allowing one to measure the phase of VLF signals with an accuracy of no worse than ±0.81°.

An effective FDTD scheme for the propagation of VLF-LF radiowaves in the Earth-Ionosphere waveguide

Abstract: Very low frequency (VLF) and low frequency (LF) waves remain used to transmitting toward submarines because they can be received a few meters under the sea surface, a possibility not allowed by other frequency bands. To predict the propagation of such waves, the waveguide and wavehop methods have been developed during years 1960–1980. Both are effective in terms of computational cost, but have some drawbacks. During years 1990–2000, a third method was introduced to compute VLF-LF propagation, the finite-difference time-domain (FDTD) method (Thevenot et al, Annales des Telecommunications, 1999). The price of the full wave method is a far larger computational time, but this is a drawback increasingly small as the computers become increasingly powerful.

First results of simultaneous recording of VLF emissions at two closely located points at auroral latitudes

Abstract: For the first time, simultaneous observations of very low frequency (VLF) emissions at auroral latitudes (L = 5.3) are carried out at two points located at close geomagnetic latitudes and spaced in longitude by 400 km: the Finnish Kannuslehto station (Φ = 64.2°) and the Russian Lovozero observatory (Φ = 64.1°). A recording equipment with similar frequency responses was used. The first results of a comparison of simultaneous observations showed that, in the overwhelming majority of cases, bursts of VLF emission appeared at both points synchronously with an identical (more often right-handed) polarization of the magnetic field of VLF waves, which can be evidence in favor of large dimensions of the ionosphere exit region of VLF waves. A simultaneous burst of quasi-periodic VLF emissions that occurred on February 02, 2013, during a substorm at 23–24 UT is discussed in detail. Additionally, VLF bursts were recorded which were observed only at one point, e.g., the appearance of left-hand polarized periodic emissions (PEs) in band 2.5–4.0 kHz with a repetition period of 3–4 s.

Characteristics of lightning associated transient perturbations in low latitude VLF paths

Abstract: Here we have reported some characteristics of lightning associated transient perturbations of Very Low Frequency signals detected over two tropical VLF paths; VTX-Kolkata and NWC-Kolkata. All types of events with positive and negative perturbations, very short to long recovery time have been found in nighttime. We explain positive and negative VLF amplitude deviations due to lightning events using the most well-known Long Wavelength Propagation Capability (LWPC) code. We find the location and size of ionospheric perturbations on the propagation paths by comparing the model results with observations.