D-region ionospheric disturbances associated with the Extremely Severe Cyclone Fani over North Indian Ocean as observed from two tropical VLF stations
01 Jan 2021-Advances in Space Research (Pergamon)-Vol. 67, Iss: 1, pp 75-86
TL;DR: In this article, the D-region ionospheric disturbances due to the tropical cyclone Fani over the Indian Ocean have been analyzed using Very Low Frequency (VLF) radio communication signals from three transmitters (VTX, NWC and JJI) received at two low latitude stations (Kolkata-CUB and Cooch Behar-CHB).
Abstract: The D-region ionospheric disturbances due to the tropical cyclone Fani over the Indian Ocean have been analysed using Very Low Frequency (VLF) radio communication signals from three transmitters (VTX, NWC and JJI) received at two low latitude stations (Kolkata-CUB and Cooch Behar-CHB). The cyclone Fani formed from a depression on 26th April, 2019 over the Bay of Bengal (Northeastern part of the Indian Ocean) and turned into an extremely severe cyclone with maximum 1-minute sustained winds of 250 km/h on 2 May, 2019 which made landfall on 3 May, 2019. Out of six propagation paths, five propagation paths, except the JJI-CHB which was far away from the cyclone track, showed strong perturbations beyond 3 σ level compared to unperturbed signals. Consistent good correlations of VLF signal perturbations with the wind speed and cyclone pressure have been seen for both the receiving stations. Computations of radio signal perturbations at CUB and CHB using the Long Wave Propagation Capability (LWPC) code revealed a Gaussian perturbation in the D-region ionosphere. Analysis of atmospheric temperature at different layers from the NASA’s TIMED satellite revealed a cooling effect near the tropopause and warming effects near the stratopause and upper mesosphere regions on 3 May, 2019. This study shows that the cyclone Fani perturbed the whole atmosphere, from troposphere to ionosphere and the VLF waves responded to the disturbances in the conductivity profiles of the lower ionosphere.
01 Aug 2022
TL;DR: In this paper , the effects of three solar eclipses (January 2019, December 2019, and June 2020) observed by very low frequency (VLF) radio signals from a single receiver at a subtropical low-latitude station in India is presented.
Abstract: Ionospheric impact of three solar eclipses (January 2019, December 2019, and June 2020) observed by Very Low Frequency (VLF) radio signals from a single receiver at a subtropical low‐latitude station in India is presented here. Perturbations in signal amplitudes of VLF transmitters such as the VTX (18.2 kHz), NWC (19.8 kHz), JJI (22.2 kHz), and DHO (23.4 kHz) are analyzed. Both positive, negative, and also mixed‐type of amplitude deviations are observed which are explained based on the solar eclipse obscuration profiles over the entire propagation paths. The difference in observed amplitude deviations for a VLF signal during the different solar eclipses are reproduced using the Long Wave Propagation Capability code and solar obscuration profile. Particularly, the effects of two solar eclipses on the VLF signal propagation characteristics have been analyzed. The modal attenuation coefficients of the VTX signal in the earth‐ionosphere waveguide are decreased with the increase of solar obscuration. A negative linear relationship of VLF signal attenuation with solar obscuration has been found for the first time during the solar eclipse. The relations are almost similar for the two solar eclipses of December 2019 and June 2020. Further, the percentage reduction of the D‐region electron density profiles as a function of altitude and solar obscuration during the two solar eclipses is presented.
01 Aug 2020
TL;DR: In this paper, very low frequency radio signals received from two places Kolkata and Cooch Behar, India revealed disturbances in the lower ionosphere namely in the D-region ionosphere.
Abstract: We present new observations of lower ionospheric disturbances due to the extremely severe cyclonic storm ‘Fani’ over northeastern part of the Indian Ocean. Very Low Frequency radio signals received from two places Kolkata and Cooch Behar, India revealed disturbances in the lower ionosphere namely in the D-region ionosphere. Mesospheric temperature and Ozone concentration data from the NASA’s TIMED satellite were also used to diagnose the disturbances in the lower ionosphere. Significant wavelike oscillations and strong amplitude anomalies in daytime and nighttime VLF signal were observed during the intense phase of the cyclone. Both the mesospheric Ozone concentration and temperature showed maximum anomalies beyond $3 \sigma$ during the cyclone period. Mesospheric temperature enhancement around VLF reflection heights indicates changes in the chemical composition and electron-neutral balance in the D-region ionosphere. Wavelet analysis of the VLF amplitudes indicates a strong anti-correlation of the total wavelet power in the wave-band of periods 10-30 min with the cyclone pressure which suggests a possibility of monitoring cyclone intensity from mesospheric gravity waves using VLF radio measurements.
TL;DR: In this article , Doppler measurements at oblique propagation paths from the city of Harbin, the People's Republic of China (PRC), to 10 high-frequency (HF) radio broadcast stations in the PRC, Japan, Mongolia, and the Republic of Korea captured the response in the ionosphere to the activity of the super typhoon, Typhoon Kong-rey, from 30 September to 6 October 2018.
Abstract: Abstract. Doppler measurements at oblique propagation paths from the city of Harbin, the People's Republic of China (PRC), to 10 high-frequency (HF) radio broadcast stations in the PRC, Japan, Mongolia, and the Republic of Korea captured the response in the ionosphere to the activity of the super typhoon, Typhoon Kong-rey, from 30 September to 6 October 2018. The Harbin Engineering University coherent software-defined radio system generates the database containing the complex amplitudes of the radio signals that have been acquired along 14 propagation paths since 2018. The complex amplitudes are used for calculating the temporal dependences of the Doppler spectra and signal amplitudes, and the Doppler spectra are used to plot the Doppler shift as a function of time, fD(t), for all rays. The scientific objectives of this study are to reveal the possible perturbations caused by the activity of Typhoon Kong-rey and to estimate the magnitudes of wave parameters of the ionospheric plasma and radio signals. The amplitudes, fDa, of the Doppler shift variations were observed to noticeably increase (factor of ∼2–3) on 1–2 and 5–6 October 2018, while the 20–120 min periods, T, of the Doppler shift variations suggest that the wavelike disturbances in the ionosphere are caused by atmospheric gravity waves. The periods and amplitudes of quasi-sinusoidal variations in the Doppler shift, which have been determined for all propagation paths, may be used to estimate the amplitudes, δNa, of quasi-sinusoidal variations in the electron density. Thus, T≈20 min and fDa≈0.1 Hz yield δNa≈0.4 %, whereas T≈30 min and fDa≈0.2 Hz give δNa≈1.2 %. If T≈60 min and fDa≈0.5 Hz, then δNa≈6 %. The periods T are found to change within the 15–120 min limits, and the Doppler shift amplitudes, fDa, show variability within the 0.05–0.4 Hz limits.
TL;DR: In this paper , the D-region ionospheric response during the lifespan 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.
Abstract: 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 ± 3 σ 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
TL;DR: In this paper, the lower ionospheric disturbances associated with the Super Cyclonic Storm Amphan over the Bay of Bengal have been observed by sub-ionospheric Very Low Frequency (VLF) radio signals from two navigational transmitters VTX and NWC, received at Cooch Behar, India.
Abstract: The lower ionospheric disturbances associated with the Super Cyclonic Storm Amphan over the Bay of Bengal have been presented in this paper. The disturbances were observed by sub-ionospheric Very Low Frequency (VLF) radio signals from two navigational transmitters VTX and NWC, received at Cooch Behar, India. Strong variations of the nighttime signal amplitudes exceeding three times their standard deviations have been observed during the peak intensity phase of the cyclone. Morning terminator time in the VLF signal was influenced just after the cyclone peak and evening terminator time was influenced significantly after landfall. The radius of the strongly perturbed ionospheric region associated with the cyclone during its peak intensity has been estimated approximately between 500 and 1000 km from VLF observations.
06 Oct 2018
TL;DR: In this paper, the average 1-σ variability of Nmax about the mean ranges from approx. ±35% (equator) to approximately ±45% (anomaly peak) for high frequencies, and from approximately ±25% to approximately.
Abstract: Hourly foF2 data from over 100 ionosonde stations during 1967–89 are examined to quantify F-region ionospheric variability, and to assess to what degree the observed variability may be attributed to various sources, i.e., solar ionizing flux, meteorological influences, and changing solar wind conditions. Our findings are as follows. Under quiet geomagnetic conditions (Kp 4), the average 1-σ variability of Nmax about the mean ranges from approx. ±35% (equator) to approx. ±45% (anomaly peak) to approx. ±55% (high-latitudes) for high frequencies, and from approx. ±25% (equator) to approx. ±45% (high-latitudes) at low frequencies. Some estimates are also provided on Nmax variability connected with annual, semiannual and 11-year solar cycle variations.
TL;DR: In this article, a brief overview of effects on the ionosphere of upward propagating waves from lower-lying regions is given, separately for the lower ionosphere, for the E-region ionosphere.
Abstract: Meteorological processes in the lower-lying layers, particularly in the troposphere, affect the ionosphere predominantly through the upward propagating waves and their modifications and modulations. Those waves are planetary waves, tidal waves, gravity waves, and almost forgotten infrasonic waves. A part of wave activity can be created in situ at ionospheric heights as primary (e.g., diurnal tide, gravity waves) or secondary waves (e.g., some gravity or planetary waves), but this paper is focused on the upward propagating waves from below the ionosphere. They propagate into the ionosphere mostly directly but the planetary waves can propagate upwards to the F region heights only indirectly, via various potential ways like modulation of the upward propagating tides. The waves may be altered during upward propagation via non-linear interactions, particularly in the MLT region. A brief overview of effects on the ionosphere of upward propagating waves from lower-lying regions is given, separately for the lower ionosphere, for the E-region ionosphere, and for the F-region ionosphere. The upward propagating waves of the neutral atmosphere origin are important both from the point of view of vertical coupling in the atmosphere–ionosphere system, and for applications in radio propagation/telecommunications, as they are responsible for a significant part of uncertainty of the radio wave propagation condition predictions.
TL;DR: A review of the development of ELF and VLF measurements, both from a historical point of view and from the view of their relationship to optical and other observations of ionospheric effects of lightning discharges is provided in this paper.
Abstract:  Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near‐Earth space. ELF (0.3–3 kHz) and VLF (3–30 kHz) wave energy released by lightning discharges is often the agent of modification of the lower ionospheric medium that results in the conductivity changes and the excitation of optical emissions that constitute transient luminous events (TLEs). In addition, the resultant ionospheric changes are best (and often uniquely) observable as perturbations of subionospherically propagating VLF signals. In fact, some of the earliest evidence for direct disturbances of the lower ionosphere in association with lightning discharges was obtained in the course of the study of such VLF perturbations. Measurements of the detailed ELF and VLF waveforms of parent lightning discharges that produce TLEs and terrestrial gamma ray flashes (TGFs) have also been very fruitful, often revealing properties of such discharges that maximize ionospheric effects, such as generation of intense electromagnetic pulses (EMPs) or removal of large quantities of charge. In this paper, we provide a review of the development of ELF and VLF measurements, both from a historical point of view and from the point of view of their relationship to optical and other observations of ionospheric effects of lightning discharges.
TL;DR: In this paper, the authors proposed a possible use of VLF/LF (very low frequency (3-30 kHz) /low frequency (30-300 kHz)) radio sounding of the seismo-ionospheric perturbations.
Abstract: It is recently recognized that the ionosphere is very sensitive to seismic effects, and the detection of ionospheric perturbations associated with earthquakes, seems to be very promising for short-term earthquake prediction. We have proposed a possible use of VLF/LF (very low frequency (3-30 kHz) /low frequency (30-300 kHz)) radio sounding of the seismo-ionospheric perturbations. A brief history of the use of subionospheric VLF/LF propagation for the short-term earthquake prediction is given, followed by a significant finding of ionospheric perturbation for the Kobe earthquake in 1995. After showing previous VLF/LF results, we present the latest VLF/LF findings; One is the statistical correlation of the ionospheric perturbation with earthquakes and the second is a case study for the Sumatra earthquake in December, 2004, indicating the spatical scale and dynamics of ionospheric perturbation for this earthquake.