Transmitter-receiver Great Circle Path Research Articles

VLF and Ionospheric D‐Region Perturbations Associated With WWLLN‐Detected Lightning in the South Pacific Region

AbstractThe subionospheric early very low frequency (VLF) perturbations observed on NWC (19.8 kHz) navigational transmitter signal monitored at a low‐latitude station, Suva (18.1°S, 178.5°E), Fiji, during campaign periods of November 2011, 2012, and 2014 and December 2014, are presented. Early VLF events are associated with D‐region conductivity changes mainly produced by lightning‐generated transient luminous events (TLEs). Early VLF events occurred both during daytime and nighttime, with a considerably higher occurrence at nighttime. VLF perturbations caused by lightning strokes located up to 100 km off the transmitter‐receiver great circle path (TRGCP) are attributed to narrow‐angle scattering, while lightning strokes 100–500 km off the TRGCP are considered to cause early VLF events by wide‐angle scattering. Using the World Wide Lightning Location Network data, for the first time, we have studied the relationship between the energy of lightning strokes and the level of VLF perturbations. Greater is the energy of lightning, greater would be the strength of the VLF perturbation. However, the low‐energy lightning stroke can also produce a comparable level of perturbation to that of strong lightning. The modeling results of scattered amplitude (M) and echo phase (ϕE) of the unusually long recovery early/fast VLF event showed a better exponential fit (r ∼ 0.9) than the logarithmic fit. Long‐wavelength propagation capability (LWPC) code modeling of nighttime early VLF events considering causative TLE width of 50 km column indicated a decrease in the D‐region reference height (Hʹ) by up to 30 km and an increase in the sharpness factor (β) by 0.25 km−1.

Earth-Ionosphere Waveguide Model Parameters Using VLF Transmissions Received in the South Pacific Region

Very Low Frequency (VLF) signals form a novel tool to study the earth-ionosphere waveguide mode parameters. The waveguide model analysis of four VLF transmitter signals: 1) NWC, Australia (19.8 kHz), 2) NPM, Hawaii (21.4 kHz), 3) JJI, Elbino, Japan (22.2 kHz) and 4) NLK, Seattle, USA (24.8 kHz) propagating over long propagation paths to Suva, Fiji, has been carried out using the VLF amplitude and phase data recorded during 2014. The Transmitter Receiver Great Circle Path distances to the receiving station are 6.69 Mm for NWC, 5.07 Mm for NPM, 7.50 Mm for JJI and 9.43 Mm for NLK transmitter signal. Our results show good consistency between experimental and theoretical values of waveguide mode parameters for the west-east (W-E) (NWC/JJI-Suva) and east-west (E-W) (NLK/NPM-Suva) component of the VLF propagation paths. The waveguide mode parameters estimated in our work were found to be higher for the E-W component of the VLF propagation path compared to the W-E component path. We have also employed Long Wave Propagation Capability (V2.1) code to estimate the daytime and nighttime signal strength and daytime to nighttime signal strength ratio ( $E_{\mathrm {DN}}$ ) for all four VLF transmitter signals and found that the nighttime signal strength is generally higher compared to the daytime.

Solar flares induced D-region ionospheric and geomagnetic perturbations

The D-region ionospheric perturbations caused by solar flares which occurred during January 2010–February 2011, a low solar activity period of current solar cycle 24, have been examined on NWC transmitter signal (19.8kHz) recorded at an Indian low latitude station, Allahabad (Geographic lat. 25.75°N, long. 81.85°E). A total of 41 solar flares, including 21 C-class, 19 M-class and 01 X-class, occurred during the daylight part of the NWC-Allahabad transmitter receiver great circle path. The local time dependence of solar flare effects on the change in the VLF amplitude, time delay between VLF peak amplitude and X-ray flux peak have been studied during morning, noon and evening periods of local daytime. Using the Long Wave Propagation Capability code V 2.1 the D-region reference height (H/) and sharpness factor (β) for each class of solar flare (C, M and X) have been estimated. It is found that D-region ionospheric parameters (H/, β) strongly depend on the local time of flare's occurrence and their classes. The flare time electron density estimated by using H/ and β shows maximum increase in the electron density of the order of ~80 times as compared to the normal day values. The electron density was found to increase exponentially with increase in the solar flux intensity. The solar flare effect on horizontal component (H) of the Earth's magnetic field over an equatorial station, Tirunelveli (Geographic lat., 8.7°N, long., 77.8°E, dip lat., 0.4°N), shows a maximum increase in H of ~8.5% for M class solar flares. The increase in H is due to the additional magnetic field produced by the ionospheric electrojet over the equatorial station.

Response of the mid-latitude D-region ionosphere to the total solar eclipse of 22 July 2009 studied using VLF signals in South Korean peninsula

In this paper, we analyze VLF signals received at Busan to study the the D-region changes linked with the solar eclipse event of 22 July 2009 for very short (∼390km) transmitter–receiver great circle path (TRGCP) during local noon time 00:36–03:13UT (09:36–12:13KST). The eclipse crossed south of Busan with a maximum obscuration of ∼84%. Observations clearly show a reduction of ∼6.2dB in the VLF signal strength at the time of maximum solar obscuration (84% at 01:53UT) as compared to those observed on the control days. Estimated values of change in Wait ionospheric parameters: reflection height (h′) in km and inverse scale height parameter (β) in km−1 from Long Wave Propagation Capability (LWPC) model during the maximum eclipse phase as compared to unperturbed ionosphere are 7km and 0.055km−1, respectively. Moreover, the D-region electron density estimated from model computation shows 95% depletion in electron density at the height of ∼71km. The reflection height is found to increase by ∼7km in the D-region during the eclipse as compared to those on the control days, implying a depletion in the Lyman-α flux by a factor of ∼7. The present observations are discussed in the light of current understanding on the solar eclipse induced D-region dynamics.

Rare examples of early VLF events observed in association with ISUAL-detected gigantic jets

[1] We examine narrowband VLF observations and investigate the association of early VLF perturbations with gigantic jets recorded by the Imager of Sprites and Upper Atmospheric Lightnings (ISUAL) instrument aboard FORMOSAT-2. From its inception in 2004 to April 2013, the ISUAL instrument has recorded 90 gigantic jets using a triggered camera. Stanford VLF receivers located around the world are used to detect perturbations to VLF transmitter signals associated with lightning. While nine gigantic jet events occurred within 100 km of a VLF transmitter-receiver great circle path, only four early VLF events were detected in association with three ISUAL gigantic jets. One of these is a moderate event of 0.4 dB amplitude change, and the others are very small. The recovery time of these events are less than a couple of minutes and so do not constitute the “long recovery” early VLF events that have been postulated to be associated with gigantic jets. We speculate on possible explanations for the lack of other events on monitored paths, including a lack of significant ionization produced in the D region ionosphere by the gigantic jet event, weak transmitter signals recorded by the receivers, or mode effects on transmitter paths.

Subionospheric VLF perturbations observed at low latitude associated with earthquake from Indonesia region

Subionospheric propagation from a Very Low Frequency (VLF) transmitter (VTX, 18.2kHz) received at a low latitude station Suva, Fiji over a Transmitter–Receiver Great Circle Path (TRGCP) length of 11,400km has been utilized to identify any possible ionospheric perturbations associated with the earthquakes that occurred in the Indonesia region during the period December 2006–October 2010. Out of five earthquakes that occurred with their epicenter in the fifth Fresnel zone, only an earthquake on 18 December 2006, in the North Sumatra region, has shown convincing evidence of lower ionospheric perturbations on the VTX transmission. The magnitude of this earthquake was 5.8 measured on the Richter scale and occurred at a depth of 53km with its epicenter located 45km off the TRGCP. The VLF amplitude data for this earthquake was analyzed using (1) terminator time (TT), (2) average nighttime and daytime amplitude variation, and (3) nighttime fluctuation (NF) methods. The results show that the sunrise TTs deviated considerably in the period 14–22 December 2006 measuring up to ~20min on the day of the earthquake. The results also show that the average nighttime as well as the average daytime signal amplitudes decreased by about 5dB and 3dB, respectively, during the period of the earthquake. The NF method revealed a decline in the trend at least 2 days before the earthquake though not exceeding the 2σ criteria and enhancements in the NF exceeding 2σ mark, however, the normalized values of the trend, NF and dispersion did not reveal an increase above the 2σ marks as reported by previous researchers. This could be due to the very long path length and the prevalence of lightning activity along the TRGCP in the Asia-Oceania Region.

Waveguide Parameters of 19.8 kHz Signal Propagating over a Long Path

The amplitude and phase of 19.8 kHz signal from navigational transmitter located in North West Cape, Australia, recorded at Suva, Fiji, have been utilized to determine the waveguide mode parameters. The propagation path is mixed over land and sea having Transmitter-Receiver Great Circle Path distance 6.7 Mm. The experimental values of the parameters were found to be consistent with the theoretical values calculated using the mode theory of VLF wave propagation in the waveguide.

Early VLF perturbations observed in association with elves

Abstract. VLF remote sensing is used to detect lower-ionospheric electron density changes associated with a certain type of transient luminous events known as elves. Both ground- and satellite-based observations of elves are analysed in relation to VLF data acquired at various receiver sites in Europe, the United States and Antarctica. Ground-based observations were performed during the EuroSprite2003 campaign, when five elves were captured by low-light cameras located in the Pyrenees. Analysis of VLF recordings from Crete shows early VLF perturbations accompanying all of the elves. A large dataset consisting of elves captured by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) payload on Taiwan's FORMOSAT-2 satellite over Europe and North America has also been analysed. Early/fast VLF perturbations were found to accompany some of the elves observed over Europe. However, no VLF perturbations were detected in relation to the elves observed by ISUAL over North America. The present analysis – based on the largest database of optical elve observations used for VLF studies so far – constitutes evidence of processes initiated by the lightning EMP (electromagnetic pulse) causing electron density changes in the lower ionosphere in line with theoretical predictions. It also proves that sub-ionospheric electron density changes associated with elves can intrude to lower heights and thus perturb VLF transmissions. The possibility of VLF detection, however, depends on several factors, e.g., the distance of the elve from the receiver and the transmitter–receiver great circle path (GCP), the altitude of the ionised region and the characteristics of the VLF transmitter, as well as the EMP energy, which occasionally may be sufficient to cause optical emissions but not ionisation.

Numerical simulation of LEP Trimpis observed at Poitiers, France, on signals from VLF transmitters

Transient amplitude and phase perturbations on subionospheric VLF signals, known as the Trimpi effect, are caused by the scattering of VLF radiation from localized ionization enhancements in the nighttime D-region. The patches of ionization are due to precipitation from the radiation belts of keV electrons, that is induced by lightning-generated whistlers.This work is concerned with the numerical simulation of such VLF perturbations, termed LEP (lightning-induced electron precipitation) or classic Trimpis. Two different codes are used to compute the VLF propagation in the Earth-ionosphere waveguide in the presence of a D-region inhomogeneity. The first is based on mode theory, and the second on the FDTD (finite-difference time-domain) method. Both codes are two-dimensional and, therefore, relevant only to LEP events lying on the transmitter-receiver great circle path (TRGCP). A method of simulation is proposed to interpret quantitatively VLF amplitude and phase changes in terms of the approximate location and size of the associated ionospheric perturbation along the TRGCP. The method is applied to LEP Trimpis observed at Poitiers ( L = 2) on signals from the NAA and GQD transmitters. Results are discussed in the light of the information that may be deduced from the high-resolution analysis of VLF temporal signatures. A sequence of simultaneous LEP Trimpis observed at Poitiers on three widely separated transmission paths is studied; two alternative interpretations are proposed.