The 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 degrees S, 178.5 degrees 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 (phi(E)) of the unusually long recovery early/fast VLF event showed a better exponential fit (r similar to 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 (H') by up to 30 km and an increase in the sharpness factor (beta) by 0.25 km(-1).

The JJI VLF (22.2 kHz) transmitter signal received at two low-latitude stations, one in Port Vila (geog. coord., 17.73S, 168.33E), Vanuatu and other in Suva (18.14S, 178.44E), Fiji, was analyzed for any VLF changes due to 16 Earthquakes (EQs) with magnitudes 5.5 to 7.7, during 2018 (JJI-Vanuatu path, 6.8 Mm) and 2007 to 2018 (JJI-Suva path, 7.5 Mm). The VLF signal amplitude analysis included terminator time (TT), average daytime and nighttime amplitude variation, nighttime fluctuation, and mother Morlet wavelet methods. Out of 16 EQs only eleven EQs have shown subionospheric VLF changes including the decrease in the amplitude for about 2-8 h on the EQ day, unusual shifts in the TT of up to 5-9 min, and the decrease in the average daytime and nighttime signal amplitude of about 1-1.5 dB and 1-5 dB, respectively, on the mainshock day of the EQs. The dA(t) < 0 condition was observed about 4-5 days before the EQ which stabilized after 3-4 days from the EQ day. A decrease in the non-normalized and normalized trend of below -2 sigma (standard deviation) mark was found on the EQ day and an increase in the non-normalized and normalized NF and dispersion of above +20 mark on the day of seismic activity was found. Mother wavelet analysis of EQ associated changes in the signal amplitude showed a strong and enhanced presence of short frequency (~0.05-0.10 mHz) wave-like signatures, a few days prior, on the day of EQ, and after the EQ day as compared to normal days.

Abstract In this paper, we present the D-region ionospheric response during the lifespan (10–19 December 2020) of a severe category 5 tropical cyclone (TC) Yasa in the South Pacific by using the very low frequency (VLF, 3–30 kHz) signals from NPM, NLK, and JJI transmitters recorded at Suva, Fiji. Results indicate enhanced lightning and convective activity in all three regions (eyewall, inner rainbands, and outer rainbands) during the TC Yasa that are also linked to the wave-sensitive zones of these transmitter–receiver great circle paths. Of the three regions, the outer rainbands showed the maximum lightning occurrence; hence convective activity. Prominent eyewall lightning was observed just before the TC started to weaken following its peak intensity. Analysis of VLF signals amplitude showed both negative and positive perturbations (amplitudes exceeding ± 3σ mark) lasting for more than 2 h with maximum change in the daytime and nighttime signal amplitudes of − 4.9 dB (NPM) and − 19.8 dB (NLK), respectively. The signal perturbations were wave-like, exhibiting periods of oscillations between ~ 2.2 and 5.5 h as revealed by the Morlet wavelet analysis. Additionally, the LWPC modeling of the signal perturbations indicated a 10 km increase in the daytime D-region reference height, H′, and a 12 km decrease in the nighttime D-region H′ during TC Yasa. The D-region density gradients (sharpness), β, showed small perturbations of 0.01–0.14 km−1 from its normal values. We suggest that the observed changes to the D-region parameters are due to the enhanced convection during TC Yasa which excites atmospheric gravity waves producing traveling ionospheric disturbances to the D-region. Graphical Abstract

The diurnal variations in the phase and amplitude of very low frequency (VLF) transmissions with the call signs NWC, NPM, and NLK, received at Suva, Fiji, have been modeled using the Long Wave Propagation Capability (V2.1) code to determine the ionospheric D region parameters, H. (reference height), and beta (rate of increase of electron density with height), for different daytime and nighttime conditions along the transmitter-receiver great circle paths (TRGCPs). Measured VLF signal amplitude and phase show explicit variation over the day and nighttime along a TRGCP, also revealing amplitude minima and phase steps during sunrise and sunset as the day/night terminator traverses a TRGCP. While the daytime signal strength is reasonably smooth, at nighttime, the signal exhibits a great deal of variability. For three signal paths, the mean daytime H' and beta were found to be 70.7 km and 0.40 km(-1), respectively, while nighttime mean values of these parameters were found to be 84.2 km and 0.68 km(-1), respectively. The temporal and day-to-day variability of the nighttime D region parameters shows that H' and beta ranges in between 83.0 and 85.0 km and 0.58 and 0.80 km(-1), respectively. One of the possible sources of nighttime signal variability is increase in the number of modes propagating and relative complex interference between them along the TRGCPs, whereby the weaker modes also become significant at night due to reduced attenuation. In addition, the variations in the nighttime D region may also be a cause of high signal variability.

The effects of solar flares on the propagation of subionospheric VLF signals from NWC and NLK transmitter stations monitored at a low-latitude station, Suva (18.2°S, 178.4°E), Fiji, between December 2006 and December 2010 (an unprecedented solar minimum of solar cycles 23 and 24) and between January 2012 and December 2013 (moderate solar activity at the peak of solar cycle 24) have been analyzed to find solar flare time D-region changes. The amplitude and phase enhancements associated with solar flares were observed in the signals from both stations which are due to an increase in the electron density of the D-region as a result of extra ionization caused by the solar flares. The solar flare-induced perturbations in both the amplitude and phase of VLF signals were used to determine D-region ionospheric parameters: H ′ (the ionospheric reflection height) and β (rate of increase in electron density with height) using Long Wave Propagation Capability (LWPC) version 2.1 modeling. A comparative analysis of the ionospheric D-region parameter changes carried out for this location shows a greater increase in β and decrease in H ′ during low-solar activity period than during moderate-solar activity period, for the same class of flares. Our results also show greater differences in the values of β and H ′ for strong flares in comparison with weak flares under both low- and moderate-solar activity conditions.

We estimate D region changes due to 22 July 2009 total solar eclipse (SE), 13–14 November 2012 total SE, and 9–10 May 2013 annular SE, using VLF navigational transmitters signal observations at Suva, Fiji. The North West Cape (NWC) signal (19.8 kHz) showed an amplitude and phase decrease of 0.70 dB and 23° during November SE and 2.0 dB and 90° during May SE. The modeling using Long Wave Propagation Capability code for NWC‐Suva path during November and May SEs showed an increase in average D region reflection height (H′) and sharpness factor (β) by 0.6 and 0.5 km and 0.012 and 0.015 km−1, respectively. The July total SE for JJI‐Suva path showed an increase in H′ of 1.5 km and a decrease in β of 0.055 km−1. The decrease in the electron density calculated using SE time H′ and β is maximum for July total SE and minimum for May annular SE. The effective recombination coefficient estimated from the decay and recovery of signal phase associated with May annular SE was higher (27%) than normal daytime value 5.0 × 10−7 cm−3 s−1 and varied between 1.47 × 10−6 and 1.15 × 10−7 cm−3 s−1 in the altitude 70 to 80 km. Morlet wavelet analysis of signals amplitude shows strong wave‐like signatures (WLS) associated with three SEs with period ranging 24–66 min, but the intensity and duration of WLS show no clear dependence on SE magnitude and type. Apart from the cooling spot, the eclipse shadow can also generate WLS associated with atmospheric gravity waves.

The response of the D region low-latitude ionosphere has been examined for extreme space weather event of 14-16 December 2006 associated with a X1.5 solar flare and an intense geomagnetic storm (Dst=-146 nT) using VLF signals from Northwest Cape, Australia (NWC) (19.8kHz) and Lualualei, Hawaii (callsign NPM) (21.4kHz) transmitters monitored at Suva (Geographic Coordinates, 18.10 degrees S, 178.40 degrees E), Fiji. Modeling of flare associated amplitude and phase enhancements of NWC (3.6dB, 223 degrees) and NPM (5dB, 153 degrees) using Long-Wave Propagation Capability code shows reduction in the D region reflection height (H) by 11.1km and 9.4km, and enhancement in ionization gradients described by increases in the exponential sharpness factor () by 0.122 and 0.126km(-1), for the NWC and NPM paths, respectively. During the storm the daytime signal strengths of the NWC and NPM signals were reduced by 3.2dB on 15 and 16 December (for about 46h) and recovered by 17 December. Modeling for the NWC path shows that storm time values of H and were reduced by 1.2km and 0.06km(-1), respectively. Morlet wavelet analysis of signal amplitudes shows no clearly strong signatures of gravity wave propagation to low latitudes during the main and recovery phases. The reduction in VLF signal strength is due to increased signal attenuation and absorption by the Earth-ionosphere waveguide due to storm-induced D region ionization changes and hence changes in D region parameters. The long duration of the storm effect results from the slow diffusion of changed composition/ionization at D region altitudes compared with higher altitudes in the ionosphere.

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/m 2 ) 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.

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 (22.2kHz) 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 7dB at Allahabad and Varanasi and decreased by about 2.7, 3.5, and 0.5dB 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 similar to 16-40min at stations under total eclipse and of period similar to 30-80min at stations under partial eclipse (similar to 85-54% totality) with delay times between similar to 50 and 100min 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.

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 World-Wide 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.

Subionospheric propagation from a Very Low Frequency (VLF) transmitter (VIX, 18.2 kHz) received at a low latitude station Suva, Fiji over a Transmitter Receiver Great Circle Path (TRGCP) length of 11,400 km 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 53 km with its epicenter located 45 km off the TRGCR 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 Us deviated considerably in the period 14-22 December 2006 measuring up to similar to 20 min 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 5 dB and 3 dB, 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 2 sigma criteria and enhancements in the NF exceeding 2 sigma mark, however, the normalized values of the trend, NF and dispersion did not reveal an increase above the 2 sigma 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. (C) 2013 Elsevier Ltd. All rights reserved.

The cyclonic storms are associated with strong winds, rainfall, and thunderstorms generating strong lightning discharges. Tracking of thunderstorms and rapid intensification of cyclones are important challenges in weather forecasting in order to warn the potential threats to the communities. Our analysis of World-Wide Lightning Location Network (WWLLN) detected lightning data suggests that lightning activity is greatly enhanced in the rainbands with secondary maximum in the eyewall of a mature tropical cyclone. The movement of a March 2010 tropical cyclone in the South Pacific region and associated lightning activity in the eyewall and the rainbands are presented to demonstrate the potential of WWLLN data in timely forecasting of thunderstorms associated with cyclonic storms thus reducing the overall threat to the Pacific societies as well as to ocean shipping and airborne carrier services flying in and/or over the Southwest Pacific Ocean.

In this paper, the echo amplitude (M) and echo phase (ø) of typical early/fast events on NWC (19.8 kHz) signal received at Suva, Fiji, are modelled to determine their form of recovery (decay). We applied logarithmic and exponential fitting formulas for M and ø obtained using a simple theoretical model of VLF wave scattering from lightning-induced electron density perturbations in the lower ionosphere and found that they are highly logarithmic.

The South Pacific Journal of Natural Science (SPJNS) is published online by CSIRO PUBLISHING on behalf of the Faculty of Science and Technology, The University of the South Pacific, Fiji. The normal focus of the journal is to disseminate scientific research work carried out in the Pacific.

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 World‐Wide 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.

The South Pacific Journal of Natural Science (SPJNS) is published online by CSIRO PUBLISHING on behalf of the Faculty of Science and Technology, The University of the South Pacific, Fiji. The normal focus of the journal is to disseminate scientific research work carried out in the Pacific.