A study of sub-ionospheric early VLF perturbations observed at Agra (L=1.15), India (original) (raw)
Related papers
Earth, Planets and Space, 2013
The occurrence of short-timescale (∼1-100 s) perturbations (early VLF events) on four Very Low Frequency (VLF) transmitter signals (call signs: NWC, NPM, VTX, NLK), recorded at Suva (18.1 • S, 178.5 • E, L = 1.16), shows the most frequent occurrence on the NWC signal and least on the VTX. Daytime early/fast events on the NWC transmission are (0.2-0.5 dB) with only negative amplitude perturbations with comparatively lower recovery times (10-30 s) as compared with most nighttime events with amplitude perturbations of 0.2-1.5 dB and recovery times of 20-80 s. The WorldWide Lightning Location Network detected causative lightnings for 74 of 453 early VLF events out of which 54 (73%) were produced due to narrow-angle scattering, and by 20 (27%) due to wide-angle scattering. The recovery (decay) of the scattered amplitude of early/fast events on the NWC signal shows both exponential and logarithmic forms, but the linear correlation coefficient is better with a logarithm fit. The first observations of early/slow events in daylight propagation are presented. Initial results on early/fast events with unusually long recoveries (≥5 min) and strong perturbations (≥1 dB) indicate that they are mainly observed on the transmissions from NPM and NLK in the nighttime only, with rare occurrence on other transmissions. Such unusually long recovery of early/fast events may be associated with large ionic conductivity perturbations associated with gigantic jets.
Lightning-induced perturbations on VLF subionospheric transmissions
Journal of Atmospheric and Terrestrial Physics, 1990
Phase and amplitude perturbations on VLF subionospheric transmissions from transmitter NWC to Dunedin have been studied on both MSK frequencies and at spaced receivers, 9 km apart. In any one event (a 'Trimpi') the phase and amplitude perturbation can be expressed in terms of a perturbation phasor. This is generally believed to be the result of lightning-induced electron precipitation (LEP) producing a localized increase in ionization near the normal reflection height for subionospheric (waveguide) VLF waves. Most of the Trimpis received on the NWC-Dunedin path can be best explained if the LEP ionization is sufficiently localized so that it acts as a scattering centre for the subionospheric VLF wave from the transmitter. It is then this scattered wave or echo at the receiver which makes the perturbation phasor. We call these 'echo Trimpis'. The phase of the echo relative to the direct signal will differ on spaced antennae if the angle of arrival of the two signals differ. Similarly, this relative phase will vary with frequency if the group delay of the signals differ. Thus measurement of these differences allows location of the scattering centres, and so too the LEP. Locations made show a significant grouping in a region where the lightning intensity is high. This and other features strongly suggest that these echo Trimpis originate from local (southern hemisphere) lightning. This and other reasons are suggested to explain the high proportion of echo Trimpis on this path.
Subionospheric VLF perturbations observed at a low latitude station Varanasi (L=1.07)
Advances in Space Research, 2015
Initial results of subionospheric very low frequency (VLF) perturbations observed on NWC (19.8 kHz) and NPM (21.4 kHz) transmitter signals at low latitude ground station Varanasi, India, during the period January-June 2010, are presented. Characteristics and occurrence rates of these events mainly observed during nighttime are studied. Most of the early VLF events had slow recovery with amplitude perturbations of 0.5-4.5 dB and phase changes of 3-12°. Temporal variation of the events is studied. World Wide Lightning Location Network data along with the broadband VLF data are analyzed to find the location of causative lightning discharge and/or the sferics associated with these early VLF events. Lightning induced changes in D-region ionospheric conductivity are attributed to the perturbations in the VLF signals.
Broadband VLF measurements of lightning-induced ionospheric perturbations
2008
Very low frequency (VLF) electromagnetic pulses radiated by lightning are an effective tool for probing the D region ionosphere. We detect and measure the D region ionospheric disturbances caused by the strong lightning flash by analyzing the broadband VLF spectrum from lightning that occurred just before and after a nearby intense lightning discharge. Comparing the measured electron density changes to those from previous measurements and the theoretical expectations, we find the detected perturbations are consistent with the theoretically predicted ionization changes produced directly by the lightning electromagnetic pulse.
Journal of Geophysical Research: Space Physics, 2009
TLEs are optically observed from the U.S. Langmuir Laboratory, while ELF/VLF waveform data are simultaneously recorded on board the Centre National d'Etudes Spatiales microsatellite DEMETER and on the ground at Langmuir. Analyses of ELF/VLF measurements associated with sprite events observed on 28 July 2005 and 3 August 2005 are presented. Conditions to trace back the wave emissions from the satellite to the source region of the parent lightning discharge are discussed. The main results concern: (1) the identification from a low Earth orbit satellite of the 0+ whistler signatures of the TLE causative lightning; (2) the identification of the propagation characteristics of proton whistlers triggered by the 0+ whistlers of the causative lightning, and the potential use of those characteristics; (3) recognition of the difficulty to observe sprite-produced ELF bursts in the presence of proton-whistlers; (4) the use of geographical displays of the average power received by the DEMETER electric field antennas over the U.S. Navy transmitter North West Cape (NWC) located in Western Australia to evaluate VLF transmission cones which explain the presence (28 July events) or the absence (3 August events) of propagation links between sferics observed at ground and 0+ whistlers observed on DEMETER; and (5) owing to electron-collisions, an optimum transfer of energy from the atmosphere to the ionosphere for waves with k vectors antiparallel, or quasi-antiparallel, to Earth's magnetic field direction.
2011
Two long duration solar eclipses having totality over Earth's equatorial region, within a period of six months is a rare event. The results from such two solar eclipses, one on 22nd July, 2009 and another on 15th January, 2010, on VLF propagation is presented in this paper. From the amplitude variation of a transmitted VLF navigational signal at 18.2 kHz (VTX3, 8.38° N, 77.75° E) over a Great Circle Path (GCP) of 2200 km, the virtual reflection height H' and inverse scale height parameter (β) of the eclipsed lower ionosphere are estimated. Modeling shows that during maximum eclipse over the path, an average 80% drop in electron density occurs at a height of 71 km at the equatorial lower ionosphere. At the same time, the intensity of sferics from lightning at different frequencies is found to increase during both the eclipses. The spectral variations of sferics increment in both the eclipses are almost identical having a correlation coefficient of 0.88 with a statistical significance of 98%. The results are explained on the basis of decrease in electron density at the lower ionosphere, modifying the reflection coefficient which affected the propagation of VLF sferics in Earth-ionosphere waveguide under eclipsed condition.
Subionospheric early VLF perturbations observed at Suva: VLF detection of red sprites in the day?
Journal of Geophysical Research: Space Physics, 2008
First observations of early Very Low Frequency (VLF) perturbations on signals from NWC (19.8 kHz) and NPM (21.4 kHz) monitored at Suva, in the month of November 2006, are presented. The early/fast, early/slow, early/short (RORD), and step-like early VLF perturbations are observed on signals from both the transmitters. The early/fast VLF events are found to occur more often in the nighttime than in the daytime whereas step-like early events predominantly occur in the daytime. Most of the early VLF events are associated with amplitude changes between 0.2-0.8 dB with only a few cases > 0.8 dB. In general, the recovery time of daytime early/fast VLF events is less when compared to the nighttime early/fast VLF events. The lightning location data provided by the WorldWide Lightning Location Network and broadband VLF data recorded at Suva have been analyzed to identify the location of causative lighting discharges along the great circle paths between transmitter and receiver, and the sferics associated with causative lightning of early VLF events. This research is the first to report both daytime early/fast VLF perturbations with faster recovery and also step-like early VLF perturbations initiated and ended by the lightnings which are most likely associated with red sprites and/or elves occurring in the daytime.
Journal of Geophysical Research: Space Physics, 2013
Subionospheric VLF recordings are investigated in relation with intense cloud-to-ground (CG) lightning data. Lightning impacts the lower ionosphere via heating and ionization changes which produce VLF signal perturbations known as early VLF events. Typically, early events recover in about 100 s, but a small subclass does not recover for many minutes, known as long-recovery early events (LORE). In this study, we identify LORE as a distinct category of early VLF events, whose signature may occur either on its own or alongside the short-lived typical early VLF event. Since LORE onsets coincide with powerful lightning strokes of either polarity (±), we infer that they are due to long-lasting ionization changes in the uppermost D region ionosphere caused by electromagnetic pulses emitted by strong ± CG lightning peak currents of typically > 250 kA, which are also known to generate elves. The LORE perturbations are detected when the discharge is located within~250 km from the great circle path of a VLF transmitter-receiver link. The probability of occurrence increases with stroke intensity and approaches unity for discharges with peak currents ≥~300 kA. LOREs are nighttime phenomena that occur preferentially, at least in the present regional data set, during winter when strong ± CG discharges are more frequent and intense. The evidence suggests LORE as a distinct signature representing the VLF fingerprint of elves, a fact which, although was predicted by theory, it escaped identification in the long-going VLF research of lightning effects in the lower ionosphere.
VLF signatures of lightning-induced heating and ionization of the nighttime D-region
Geophysical Research Letters, 1993
48.5 kHz signals from a transmitter in Silver Creek, Nebraska, propagating to Huntsville (HU), Alabama over a • 1200 km Great Circle Path (GCP) exhibit characteristic amplitude changes which appear within 20 ms of cloudto-ground (CG) flashes located within 50 km of the path. Data are consistent with the heating of ionospheric electrons by the electromagnetic (EM) pulse from lightning producing ionization changes in the D-region over the thunderstorm. Yip, W.-Y., U.S. Inan, and R. E. Orville, On the spatial relationship between lightning discharges and propagation paths of perturbed subionospheric VLF/LF signals, J. Geophys. Res., 96, 249, 1991.
Disturbance detection due to lightning at ionospheric D-region over Malaysia
International Journal of Advances in Applied Sciences (IJAAS), 2024
Previous research on the interference of very low frequency (VLF) signals in the equator region was inadequate and largely concentrated in the middle and high latitude regions. Therefore, this research aims to determine the disruption of VLF waves in the ionospheric D-region above Malaysia, which is in the equator area. This paper presents observations of early/fast, early/slow, and lightning-induced electron precipitation (LEP) events in January 2010. Broadband and narrowband data are monitored and investigated using Japan’s JJI Ebino transmitter (32°40' N, 130°81' E) to the receiver at the Universiti Kebangsaan Malaysia (2°55' N, 101°46' E). Broadband and narrowband data are analyzed with theoretical considerations and linked to events from interference in the ionospheric D-region. Many early/fast, early/slow, and LEP events are found to originate from the lightning release activity emitted and may alter the amplitude and VLF signal phase in the lower layer ionosphere over Malaysia.