Verification of atmospheric reanalysis products is crucial for their application in extreme weather and climate-related research. This study evaluates tropical cyclone (TC) characteristics and related variables from 1990 to 2018 over Australia (95-160 degrees E and 0-30 degrees S) in three reanalysis products - the recently developed Australian Bureau of Meteorology's Atmospheric high-resolution Regional Reanalysis for Australia Version 2 (BARRA-R2), its predecessor BARRA-R (both at 12-km spatial resolution) and the widely used European Centre for Medium-Range Weather Forecasts (ECMWF)'s Global Reanalysis Version 5 (ERA5) at 31-km spatial resolution. TCs detected in these reanalyses, using the Okubo-Weiss-Zeta Parameter detection and tracking scheme, are compared with observations from the Bureau's TC database. All three products simulated more than 50% of the observed TC frequency, with ERA5 achieving a higher hit rate of 77% compared with BARRA-R2 (68%) and BARRA-R (53%). Most missed cases involved non-severe TCs. ERA5 showed a clear decline in annual TC frequency consistent with observations, whereas BARRA-R displayed a weak upward trend and BARRA-R2 a statistically insignificant decline. Large differences emerged in surface wind speed and gusts: BARRA-R represented TC surface winds better than ERA5, and BARRA-R2 produced slightly improved gusts compared with ERA-5. Case studies show that temporal evolution is generally well represented in all products, though ERA5 tends to maintain peak intensity for longer, whereas BARRA products sometimes show shifted timing of peak intensity. General discrepancies are attributed to resolution limitations, inherent differences between best-track and gridded data, underestimation of TC peak intensity and wider model forecast constraints.

We present unusually strong daytime signal amplitude anomalies of 11.1 dB on JJI and 12.6 dB on VTX very low-frequency (VLF) transmitter signals recorded at Suva, Fiji, due to the powerful Hunga Tonga-Hunga Ha'apai (HTHH) volcanic eruption (VE) on 15 January 2022 around 04:00 UT. Statistical analysis of daytime trends, daytime and nighttime fluctuations, and dispersion of VLF anomalies showed that VLF anomalies were indeed associated with HTHH VE. The combined effect of long-duration strong ionization produced by exceptionally intense volcanic lightning detected and Atmospheric Gravity Waves (AGWs) and Lamb Waves (LWs) associated with HTHH VE are reported here as causes of unusually strong VLF amplitude anomalies and D-region electron density changes. The daytime VLF anomalies extended into nighttime with reduced level that can be attributed to AGWs due to sustained HTHH volcanic lightning activity, as reported in the literature. Using the Long Wave Propagation Capability (code V2.1), we modeled VLF anomalies and found significant changes in the D-region VLF reference height (H ') and electron density gradient (beta). The H ' for JJI path decreased significantly by 3.2 km, and beta increased by 0.319 km-1, giving an electron density increase from 1.83 x 102 cm-3 to 1.34 x 103 cm-3 at the normal daytime H ' of 75.1 km. The perturbations in the D-region ionosphere are attributed to strong AGWs, mostly of frequency 0.10-0.30 mHz and LWs obtained using Morlet wavelet analysis of VLF anomalies, along with tremendously high lightning strokes demonstrating coupling between the atmosphere and the D-region.

GNSS data collected from Tarawa Station

Tropical cyclone (TC) best track datasets have temporal inhomogeneity, mostly associated with changes in monitoring practices and technological improvements. Temporal inconsistencies are often mitigated by using TC data from more homogeneous periods. For example, TC records since 1980 are preferred for frequency and track analysis, while records for intensity analysis have become more consistent since similar to 2000. However, such measures reduce the sample size for trend analysis, potentially leading to conflicting conclusions due to natural climate-variability. Inter-agency best track data can also vary, due to differences in the way best track information-such as centre fix locations and associated intensity estimates-are defined and assessed. When comparing global datasets and regional datasets, additional inconsistencies can be introduced where TCs form or track just outside the official area of responsibility for each agency. We highlight discrepancies in Australian TC best track data from various sources by comparing it to a more rigorously scrutinized dataset compiled by the Australian Bureau of Meteorology. This dataset is found to have highly accurate TC records for the Australian region. We also highlight the implications of data differences on TC-related trend analysis, aiming to increase awareness of dataset inconsistencies while guiding credible climate-change detection and attribution messages.

During its earth-bound phase of the Aditya-L1 spacecraft of India, the Supra-Thermal and Energetic Particle Spectrometer (STEPS) of the Aditya Solar wind Particle EXperiment (ASPEX) was operated whenever the orbit was above 52000 km during 11 - 19 September 2023. This phase of operation provided measurements of energetic ions (with energies 0.1–2 MeV) in the magnetosphere, magnetosheath, and interplanetary medium. Three interplanetary coronal mass ejections (ICME) hit the magnetosphere during this period. This provided opportunity to examine the relative roles of ICME-generated solar energetic particles (SEPs) and substorm generated energetic ions on the magnetosphere. We approach this objective by detailed spectral analyses of energetic ion fluxes measured by two units of ASPEX-STEPS. We identify three distinctly different conditions of the north-south component of the interplanetary magnetic field (IMF B_z = 0, > 0, and < 0) and use the derived spectral indices to understand this relative role. By combining these with the simultaneous energetic ion flux variations from the Advanced Composition Explorer (ACE) around the Sun-Earth first Lagrangian (L1) point and the Geostationary Operational Environmental Satellite (GOES) in the Earth's magnetosphere, we show that the polarity of IMF B_z influences the energetic ion spectra in the magnetosphere by modulating the interplay of the ICME-generated SEP with the energetic particles generated inside the magnetosphere by substorms. Interestingly, ASPEX-STEPS observations also indicate towards directional anisotropy based on spectral indices. This suggests spatially inhomogeneous mixing of energetic ions coming from different source processes.

Ground simulation methods have gained significant research attention due to their effectiveness. We propose a ground simulation method for space laser communication with a transmission distance of 53,000 km in free space using the laser as a means of information transmission. The simulation is verified using artificial intelligence and pattern recognition techniques, with Fresnel diffraction adopted as a mathematical model to represent the system and its architecture, facilitating the creation of Matlab/Simulink blocks for simulation. OptiSystem was used to calculate the power from the transmission distance. The transmission distance determined from the system is 53,000 km, with a receiving power of 2.72 x 10(-10) W. The simulations demonstrated that the power transmitted from an Earth station at arbitrary distances up to 53,000 km in space could be used to determine the power received by the receiving station. Prediction models were integrated into mathematical models to validate the results. The model's performance was tested with up to 20% additive noise, including signal scattering, background radiation, and interference. A ladder network with entanglement and an isochronous neighbourhood function exhibited high performance in accurately predicting the output at low computation cost compared to other models. The optimized deep Bayesian network technique, on the other hand, showed a high prediction rate and accuracy, albeit at the expense of high computational cost.

D-region effect of intense storm of 23 March (Dst = -163 nT) and super storm of 23 April 2023 (Dst = -212 nT) have been investigated using VTX and NWC transmitter signals recorded at low latitude station Dehradun (DDN; 30.31 degrees N, 78.03 degrees E), India. During March storm on 23 March, amplitude anomalies of +2.92 (NWC) and +1.91 dB (VTX) were observed. April storm showed amplitude anomalies of -9.41 (NWC) and -2.68 dB (VTX) on 23 April. Long Wave Propagation Capability code (LWPC) has been used to model signal anomalies to obtain D-region Very low frequency (VLF) reference height (H ') and electron density gradient (beta). During March storm, NWC-Dehradun path showed a decrease in H ' by 12.04 km and an increase in beta by 0.301 km-1, while VTX-Dehradun path showed an increase in H ' by 0.07 km and beta by 0.032 km-1 on 23 March. During April storm, VTX-Dehradun path showed increase in H ' by 0.315 km and an increase in beta by 0.024 km-1, while H ' increased by 2.91 km and beta by 0.23 km-1 for NWC-Dehradun path on 23 April. Wavelet analysis of VLF anomalies showed signatures of atmospheric gravity waves of periods between 30 and 100 min during both storms associated with joule heating in the high/auroral latitudes. Results suggest that storm time prompt penetration (PP) of electric fields has contributed to VLF signal anomalies during April storm's main phase, which is also supported by PP Equatorial Electric Field Model run for the location of DDN station.

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).

A comprehensive analysis of subionospheric Very Low Frequency (VLF) signals from transmitters with call‐signs NWC, NPM, and NLK monitored at a low‐latitude station, Suva, Fiji, has been carried out to determine the D‐region effects of the intense geomagnetic storms for a period of 7 years (2011–2016 and 2018). The periods of 2011–2016 and 2018 fall under the 24th solar cycle with yearly mean sunspot numbers 80.8–39.8 and 7.0, respectively. Seven out of 12 geomagnetic storms revealed VLF anomalies during the storm main phase day and storm recovery phase days. A pronounced decrease in the VLF amplitude and phase for the storms having main phase onset in the nighttime was found. Out of seven geomagnetic storms, a storm of St Patrick's Day of 17–18 March 2013 is presented as a case study and the same analysis was done for all the storms. The anomalies in the VLF amplitude and phase have been modeled using Long‐Wave Propagation Capability code V2.1 to obtain the changes in Wait parameters (reference height, H ′ and sharpness factor, ꞵ ) and changes in the D‐region electron density. In most cases of storms, an increase in the H ′ and a decrease in the ꞵ during the storm main and recovery phases were obtained which for the 17–18 March 2013 storm increased by 7.4–7.6 km and decreased from 0.03 to 0.07 km −1 , respectively. Wavelet analysis of signal anomalies showed clear wave‐like spectra (0.05–0.18 mHz) of atmospheric gravity waves indicating traveling ionospheric disturbances propagating toward low latitudes due to storm‐associated Joule heating at high latitudes.

Global Navigation Satellite Systems (GNSSs) provide the services on positioning, time, and navigation aspects of ground, marine, and aviation to our society. The integration of GNSS, telecommunication, geographic information systems, and remote sensing technologies have demonstrated their utility in the remote sensing of earth’s atmosphere and ionosphere. In this chapter, the basics of the Global Positioning System (GPS), its components and different transmitted GPS signals are described. The different error sources introduced in GPS/GNSS measurements are described with the main emphasis on the tropospheric error sources. GNSS meteorology is now an established atmospheric remote technique that can provide the data on integrated water vapor (IWV) which is the most abundant and important greenhouse gas for meteorological and climatic processes. The IWV is one of the key parameters to understand atmospheric structure and processes, including severe terrestrial weather phenomena and climate change.

Large-scale travelling ionospheric disturbances (LSTIDs) are detected using the critical frequency of the $F_{2}$ layer $( f_{o}F_{2})$ of the ionosphere. The HF interferometry (HF-Int) technique is applied to the network of ionosondes in the Australian region to detect and estimate propagation parameters. Here we present traveling ionospheric disturbances (TIDs) characteristics (period, velocity, and intensity) associated with selected geomagnetic storms. The TID response during geomagnetic storms varies significantly owing to the differing storm evolution patterns. Results show the morphology of parameters detected continually changes with time corresponding to highly dynamic response effects on the ionosphere. TIDs detected had a wide range of velocity magnitudes (mean ± standard deviation) such as $687 \pm 16\mathrm{m} /\mathrm{s}$ during onset phase, $516 \pm 125\mathrm{m} /\mathrm{s}$ during main phase and $569 \pm 91\mathrm{m} /\mathrm{s}$ during the recovery phase of the storm presented here. Propagation directions are predominantly equatorward with east and west deviations owing to higher latitude source generations associated with storm induced intensifications. The HF-Int system uses spectral techniques to estimate the dominant period and outputs the spectral energy contribution (SEC) which is a measure of the contribution of the TID to the total variability of the time series. TID activity levels are based on SEC criteria mostly indicated moderate and weak events with a range of periods 60-140 min.

Large scale traveling ionospheric disturbances (LSTIDs) can be detected using the critical frequency of F 2 layer ( f o F 2 ) of the ionosphere. The HF interferometry (HF‐int) technique is applied to the ionosonde data from low and mid latitude stations over Australia. For the first time, we present a comparison of TIDs detected during the geomagnetic storms of 17 March and 23 June 2015. The TID response for these two comparable superstorms is significantly different owing to the differing storm evolution patterns. The storm associated TIDs are more intense and longer compared to the solar terminator. Results demonstrate that the morphology of parameters continually varies over time due to highly dynamic ionospheric response to geomagnetic storms. For the March storms, TIDs velocities were 400–700 m/s at the onset phase, ∼500 m/s during the main phase and ∼600 m/s during the recovery phase. Their propagation directions were equatorward with east/west deviations. Lower period (∼60 min) TIDs were determined during storm onset whereas higher periods (∼140 min) were detected during the recovery phase. The June storm produced a different response. The TID velocity at the onset phase was ∼500 m/s but during the main and recovery phases the velocities were greater than 1,000 m/s. There were large deviations in velocity vectors at the different stations suggesting multiple sources of TID generation and possible resonating effects at middle and low latitude stations. Our findings indicate that while TID detection and storm phases co‐occur, the propagation characteristics of TIDs change dramatically throughout the storm.

The ionospheric effects of six intense geomagnetic storms with Dst index ≤ −100 nT that occurred in 2012 were studied at a low-latitude station, Darwin (Geomagnetic coordinates, 21.96° S, 202.84° E), a low-mid-latitude station, Townsville (28.95° S, 220.72° E), and a mid-latitude station, Canberra (45.65° S, 226.30° E), in the Australian Region, by analyzing the storm–time variations in the critical frequency of the F2-region (foF2). Out of six storms, a storm of 23–24 April did not produce any ionospheric effect. The storms of 30 September–3 October (minimum Dst = −122 nT) and 7–10 October (minimum Dst = −109 nT) are presented as case studies and the same analysis was done for the other four storms. The storm of 30 September–3 October, during its main phase, produced a positive ionospheric storm at all three stations with a maximum percentage increase in foF2 (∆foF2%) of 45.3% at Canberra whereas during the recovery phase it produced a negative ionospheric storm at all three stations with a maximum ∆foF2% of −63.5% at Canberra associated with a decrease in virtual height of the F-layer (h’F). The storm of 7–10 October produced a strong long-duration negative ionospheric storm associated with an increase in h’F during its recovery phase at all three stations with a maximum ∆foF2% of −65.1% at Townsville. The negative ionospheric storms with comparatively longer duration were more pronounced in comparison to positive storms and occurred only during the recovery phase of storms. The storm main phase showed positive ionospheric storms for two storms (14–15 July and 30 September–3 October) and other three storms did not produce any ionospheric storm at the low-latitude station indicating prompt penetrating electric fields (PPEFs) associated with these storms did not propagate to the low latitude. The positive ionospheric storms during the main phase are accounted to PPEFs affecting ionospheric equatorial E × B drifts and traveling ionospheric disturbances due to joule heating at the high latitudes. The ionospheric effects during the recovery phase are accounted to the disturbance dynamo electric fields and overshielding electric field affecting E × B drifts and the storm-induced circulation from high latitudes toward low latitudes leading to changes in the natural gas composition [O/N2] ratio.

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

We report rare simultaneous observations of columniform sprites and associated gravity waves (GWs) using the Transient Luminous Events (TLEs) camera and All-sky imager at Prayagraj (25.5° N, 81.9° E, geomag. lat. ~ 16.5° N), India. On 30 May 2014, a Mesoscale Convective System generated a group of sprites over the north horizon that reached the upper mesosphere. Just before this event, GWs (period ~ 14 min) were seen in OH broadband airglow (emission peak ~ 87 km) imaging that propagated in the direction of the sprite occurrence and dissipated in the background atmosphere thereby generating turbulence. About 9–14 min after the sprite event, another set of GWs (period ~ 11 min) was observed in OH imaging that arrived from the direction of the TLEs. At this site, we also record Very Low Frequency navigational transmitter signal JJI (22.2 kHz) from Japan. The amplitude of the JJI signal showed the presence of GWs with ~ 12.2 min periodicities and ~ 18 min period. The GWs of similar features were observed in the ionospheric Total Electron Content variations recorded at a nearby GPS site. The results presented here are important to understand the physical coupling of the troposphere with the lower and upper ionosphere through GWs.

Lightning data used in 'Lightning activity and wind speed variations in Tropical Cyclones of the Southwest Pacific Region'.

The long-term variations in foF2 at Hobart (52.88 degrees S, 147.32 degrees E), Canberra (35.28 degrees S, 149.13 degrees E) and Christchurch (43.53 degrees S, 172.64 degrees E) stations, located in the mid-latitude zone in the Southern Hemisphere were analyzed using 1947-2006 years of the data. The saturation, solar and geomagnetic activity and seasonal effects were removed mainly by using 12-month running mean, linear and multiple regression (twofold regression) methods to find possible signatures of climate change in long-term trends in the foF2. The solar activity proxies, sunspot number, RZ, and F10.7 solar radio flux were used in regression to find the foF2 residuals at midday (12 LT) and midnight (00 LT) of the stations. The long-term trends obtained at 12 LT are more significant and consistent with the model results. The trends estimated with F10.7 solar flux are negative and the trends estimated with RZ are positive (small and not significant). The foF2 decreased by 0.1-0.4 MHz for the 5 solar cycles period which could be mainly due to enhanced CO2 in the troposphere that is cooling the upper atmosphere. Further research is needed to see if the foF2 trends are also affected by other factors such as thermospheric winds, neutral constituents, the secular variation of Earth's magnetic field, long-term changes in stratospheric ozone, solar and geomagnetic activities.