Tracking of tropical cyclones and their intensity changes in the South Pacific region using world - wide lightning location network

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

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

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

Earthquakes associated subionospheric VLF anomalies recorded at two low latitude stations in the South Pacific region

The JJI VLF (22.2 kHz) transmitter signal received at two low-latitude stations, one in Port Vila (geog. coord., 17.73◦S, 168.33◦E), Vanuatu and other in Suva (18.14◦S, 178.44◦E), 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 -2σ (standard deviation) mark was found on the EQ day and an increase in the non-normalized and normalized NF and dispersion of above +2σ 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

Response of the low-latitude D region ionosphere to extreme space weather event of 14–16 December 2006

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) (19.8 kHz) and Lualualei, Hawaii (callsign NPM) (21.4 kHz) transmitters monitored at Suva (Geographic Coordinates, 18.10°S, 178.40°E), Fiji. Modeling of flare associated amplitude and phase enhancements of NWC (3.6 dB, 223°) and NPM (5 dB, 153°) using Long-Wave Propagation Capability code shows reduction in the D region reflection height (H′) by 11.1 km and 9.4 km, and enhancement in ionization gradients described by increases in the exponential sharpness factor (β) by 0.122 and 0.126 km�1, for the NWC and NPM paths, respectively. During the storm the daytime signal strengths of the NWC and NPM signals were reduced by 3.2 dB on 15 and 16 December (for about 46 h) and recovered by 17 December. Modeling for the NWC path shows that storm time values of H′ and β were reduced by 1.2 km and 0.06 km�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

VLF and ionospheric D - region perturbations associated with WWLLN - detected lightning in the South Pacific region

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°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

Lightning evolution and VLF perturbations associated with category 5 TC Yasa in the South Pacific region

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 hours 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 ~2.2-5.5 hours 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), , 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

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

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

Lightning - associated VLF perturbations observed at low latitude: occurrence and scattering characteristics

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

Initial results on solar flare effect on 24.8 kHz subionospheric propagation over long path to Suva

Solar flares are explosions on the surface of the sun that release a large amount of electromagnetic energy in the form of radio waves at the long wavelength end, through optical emission to X-rays at the short wavelength end. It has long been known that the solar flares, particularly associated with X-rays having wavelengths typically of tenths of nm, penetrate the lower region of the ionosphere (D-region) and increase the electron density via extra ionization (Mitra 1974). The normal unperturbed daytime D-region from which Very Low Frequency (VLF) signals are reflected is maintained mainly by Lyman-α radiation (121.6 nm) from the sun that partially ionizes the minor neutral constituent nitric oxide (at height around 70 km). Under normal conditions, the solar X-ray flux is too small to be a significant source for ionizing the D-region. However, when solar flare occurs, the X-ray flux increases significantly which with wavelengths below 1 nm penetrates down to the D-region and markedly increases the ionization of the neutral constituents particularly nitrogen and oxygen hence increases the electron density. The lower ionosphere can be characterized as the “Wait ionosphere” defined by a reference height H′in km and the exponential sharpness factor in km-1 (Wait and Spies 1964). Researchers have reported changes in the ionospheric parameters, H' and β as a function of solar X-ray flux (Thomson et al. 2004, 2005; Grubor et al. 2005; Zigman et al. 2007). The increase in the D-region electron density can produce significant perturbations in the phase and the amplitude of VLF signals propagating in the Earthionosphere waveguide