Bakul Das

Bio: Bakul Das is an academic researcher from Cooch Behar Panchanan Barma University. The author has contributed to research in topics: Very low frequency & Ionosphere. The author has an hindex of 1, co-authored 8 publications receiving 3 citations.

D-region ionospheric disturbances associated with the Extremely Severe Cyclone Fani over North Indian Ocean as observed from two tropical VLF stations

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.

Impact of Three Solar Eclipses of 2019–2020 on the D‐Region Ionosphere Observed From a Subtropical Low‐Latitude VLF Radio Station

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.

VLF radio signal perturbations during two recent solar eclipses observed from a VLF receiving station, Cooch Behar, India

Abstract: Effects of two recent solar eclipses of December 2019 and June 2020 on the Very Low Frequency (VLF) radio signal amplitudes, observed from a single receiving station at Cooch Behar (CHB), India, have been presented in this paper. Signal amplitudes of VLF transmitters such as VTX (18.2 kHz), NWC (19.8 kHz), JJI (22.2 kHz), and DHO (23.4 kHz) experienced both enhancement and reduction during the eclipse period. Depending on the eclipse obscuration over the propagation path, propagation distance, and frequency of the signal, increase and decrease of signal amplitudes were observed. The amplitude perturbation on the VLF signals was also opposite for the two eclipses on each propagation path. The possible reason behind the amplitude perturbations is the changes induced in the earth ionosphere waveguide during the solar eclipses and corresponding constructive or destructive interference that occurred between the waveguide modes of the received signal at each frequency.

Response of the mesosphere and lower ionosphere to the Extremely Severe Cyclone ‘Fani’ of 2019 over the North Indian Ocean

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.

Effects of tropical cyclones on the VLF atmospherics observed from low latitude receiving stations

Abstract: The response of electric field intensity of VLF radio atmospherics during two tropical cyclones Fani (May 2019) and Amphan (May 2020) has been summarized in this paper. The amplitudes of VLF radio atmospherics (or VLF sferics) received at Cooch Behar (CHB) and Kolkata (CUB) at three discrete frequencies (4 kHz, 7 kHz, and 9 kHz) showed distinct variations with respect to the reference level during the two cyclonic storms, which is explained using the electrical structure and distribution of cloud-to-ground lightning associated with the cyclones.

Impact of Three Solar Eclipses of 2019–2020 on the D‐Region Ionosphere Observed From a Subtropical Low‐Latitude VLF Radio Station

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.

Response of the mesosphere and lower ionosphere to the Extremely Severe Cyclone ‘Fani’ of 2019 over the North Indian Ocean

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.

Effects of the super-powerful tropospheric western Pacific phenomenon of September–October 2018 on the ionosphere over China: results from oblique sounding

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.

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

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

Response of the Sub-Ionospheric VLF Signals to the Super Cyclonic Storm Amphan: First Observation from Indian Subcontinent

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.