المؤلفون: Ghosh, Soujan, Chowdhury, Swati, Kundu, Subrata, Biswas, Sagardweep, Dawn, Arjun, Ray, Suman, Choudhury, Asit K., Bari, Md. Wasimul, Bhowmick, Debashis, Manna, Souvik, Mondal, Sushanta K., Chakrabarti, Sonali, Maiti, Rajkumar, Das, Ram C., Basak, Tamal, Chakrabarti, Sandip K.

المصدر: Astrophysics & Space Science; Mar2023, Vol. 368 Issue 3, p1-18, 18p

مصطلحات موضوعية: IONOSPHERIC electron density, ECLIPSES, SOLAR eclipses, RADIO wave propagation, SOLAR oscillations, SIGNALS & signaling, TERRESTRIAL radiation

مصطلحات جغرافية: KOLKATA (India), INDIA

مستخلص: Solar eclipse is a unique phenomenon that produces an orderly disturbance in the ionosphere within a specific time frame. It provides us an opportunity to understand the ionospheric response due to its systematic variation during an eclipse. The amplitude and phase of a Very Low Frequency (VLF) signal carry the response of lower ionospheric perturbation due to the varying solar radiation impinging on Earth. During the Annular Solar Eclipse on December 26, 2019, (ASE2019), Indian Centre for Space Physics (ICSP), Kolkata, India conducted a nationwide VLF radio signal monitoring campaign to obtain the amplitude and phase variations of propagating VLF signal from fourteen different locations across India. The signal amplitude and phase profile exhibit unique profiles at these locations. These profiles in the VLF signal during ASE2019 are explained using the Long Wavelength Propagation Capability (LWPC) code and the modeled variation of solar disk obscuration by the moon. Furthermore, the lower ionospheric electron density ( N e ) computed from the model is in agreement with the observed lower ionospheric conditions. [ABSTRACT FROM AUTHOR]

: Copyright of Astrophysics & Space Science is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

المؤلفون: Mandal, Subir, Pallamraju, Duggirala, Pant, Tarun Kumar

المصدر: Journal of Geophysical Research. Space Physics; Aug2022, Vol. 127 Issue 8, p1-12, 12p

مصطلحات موضوعية: GRAVITY waves, ELECTRON distribution, RADIO wave propagation, ECHO, IONOSPHERIC disturbances, UPPER atmosphere

مصطلحات جغرافية: TRIVANDRUM (India), INDIA

مستخلص: The equatorial spread‐F (ESF) refers to the spread observed in return echoes of ionograms. Such spread in reflected echoes is found both in range and frequency, which can last for a few hours. These are due to large‐scale plasma irregularities occurring in the ionospheric heights on some nights. Under seemingly identical background conditions, ESF might occur on one night but remain absent on the other. These plasma irregularities adversely affect the trans‐ionospheric radio wave propagation but their occurrence on a given night continues to be one of the missing elements in our understanding of the equatorial ionospheric phenomena. In this work, the vertical propagation of gravity waves in daytime thermosphere has been shown to be a crucial parameter for the generation of ESF during post‐sunset hours. Using electron density profiles obtained from digisonde at Trivandrum, a dip‐equatorial location in India, we found that vertical propagation activity of gravity waves exists on 85% of the ESF days, whereas it is only 50% for the days without the occurrence of ESF during post‐sunset hours. Further, vertical propagation speeds of these gravity waves are higher on the ESF days than on the non‐ESF days. Also, ESF has been found to occur on 100% of the occasions, whenever the vertical speeds of these gravity waves are greater than 80 ms−1. This threshold value of vertical propagation speeds of gravity waves can be used to predict the ESF occurrence around 12–14 LT on that day, which is much in advance of the occurrence of ESF. Plain Language Summary: The plasma in the Earth's upper atmosphere enables trans‐ionospheric radiowave communications. Therefore, unforeseen or sudden changes in this region can severely affect GPS and satellite communications. One such phenomenon that creates these ionospheric disturbances is the occurrence of plasma irregularities in the nighttime equatorial ionosphere. These are generated over the magnetic equator and expand to latitudes farther away, so the radio communications in all these regions get adversely affected. However, the prediction of their occurrence has been evading the scientific community globally. In this work, we have discussed the role of upward propagating atmospheric gravity waves in explaining the day‐to‐day variability in the occurrence of these plasma irregularities. We found that if the vertical speeds of these waves in the daytime are higher, they perturb the ionosphere more efficiently and thereby contribute to the occurrence of plasma irregularities on that night. On the days when the propagation speeds are not as high, plasma irregularity occurrence was not favored. Our work shows that the upward propagation speed of the gravity waves in the daytime thermosphere is one of the crucial factors that can be used to predict, well in advance, the occurrence of these plasma irregularities. Key Points: Vertical propagation speeds of daytime gravity waves are higher on days with equatorial spread‐F (ESF) occurrence compared to those on days without ESF occurrenceThe time periods of the daytime gravity waves match with those found in the variations in ionospheric base heightThe occurrence of ESF can be predicted as early as 14 LT based on the gravity wave dynamics in the daytime thermosphere [ABSTRACT FROM AUTHOR]

: Copyright of Journal of Geophysical Research. Space Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

المؤلفون: Chauhan, Vishal¹, Agrawal, Shikha², Singh, O. P.¹, Singh, Birbal²

المصدر: AIP Conference Proceedings. 10/20/2010, Vol. 1286 Issue 1, p188-199. 12p.

مصطلحات موضوعية: *SOLAR eclipses, *VLF radio wave propagation, *IONOSPHERIC radio wave propagation, *IONOSPHERIC electron density, *GLOBAL Positioning System, *GEOMAGNETISM, *DATA analysis, 2009

مصطلحات جغرافية: AGRA (India), INDIA

مستخلص: Since The variations of Total Electron Content (TEC) and amplitude of the fixed frequency VLF transmitter signal (f = 19.8 kHz, NWC, Australia) are studied at Agra (Geographic Lat. 27.2°N, Long. 78°E), India during the total solar eclipse of 22 July, 2009 which was longest seen in India ever since 18 August, 1968. The equipment used for the study are a dual frequency GPS receiver (GSV 4004V) and a Soft PAL (Software based phase and amplitude logger) receiver. The data for a period of fifteen days (±7 days from the date of the event) are analysed and it is found that the TEC decreased by about 30% from normal days during the total solar eclipse, and the amplitude of the VLF signal also decreased likewise. The period of the data analysis is characterised by a low level of geomagnetic activity, hence the decrease in TEC and amplitude of the VLF signal is unlikely to be influenced by geomagnetic disturbances. The results are interpreted in terms of depression in electron densities at all ionospheric heights and are consistent with those obtained by earlier workers during similar eclipse events. [ABSTRACT FROM AUTHOR]

المؤلفون: Chakrabarti, Sandip K.^1,2, Sasmal, S.², Pal, S.¹, Mondal, S. K.²

المصدر: AIP Conference Proceedings. 10/20/2010, Vol. 1286 Issue 1, p61-76. 16p.

مصطلحات موضوعية: *VLF radio wave propagation, *IONOSPHERE, *WAVEGUIDES, *SOLAR eclipses, *SUMMER, *WINTER

مصطلحات جغرافية: INDIA, ANTARCTICA

مستخلص: VLF propagation effects are generally understood in terms of the earth-ionosphere waveguide. However, details of the theory are still incomplete. Particularly important are the newly emerging fields of VLF Astronomy where the ionosphere is treated as a giant detector for extraterrestrial energetic phenomena and the subject of lithosphere-ionosphere coupling where the the disturbances of this giant detector is influenced by terrestrial events, especially earthquakes and other seismic activities. We review the activities of our group in these fields. In particular, we concentrate on the results of the VLF campaigns we conducted using over a dozen receiving stations in Summer, in Winter and during the Total Solar eclipse in July, 2009. We also discuss briefly the results we obtained in Antarctica and their implications. [ABSTRACT FROM AUTHOR]

المؤلفون: Maji, Surya K.¹, Chakrabarti, Sandip K.^1,2, Mondal, Sushanta K.¹

المصدر: AIP Conference Proceedings. 10/20/2010, Vol. 1286 Issue 1, p214-219. 6p.

مصطلحات موضوعية: *VLF radio wave propagation, *IONOSPHERIC radio wave absorption, *SOLAR flares, *SOLAR eclipses, *WAVEGUIDES, *RADIO transmitter-receivers, *OCCULTATIONS (Astronomy), *EARTH (Planet), 2010

مصطلحات جغرافية: INDIA

مستخلص: The VLF radio waves propagate through the Earth-ionosphere waveguide. Irregularities caused by excesses or deficient soft X-rays which sustain the ionosphere changes the waveguide properties and hence the signals are modified. We report the results of our monitoring of the NWC transmitter from Khukurdaha (∼80 km away from Kolkata) during the partial solar eclipse (75%) of 15th January, 2010. The receiving station and the transmitter were on two opposite sides of the annular eclipse belt. We got clear depression in the data during the period of partial eclipse. There was also a solar flare (spot no. 1040) on that day during the time the eclipse was near maximum. The flare started from B, reaching maximum to C1.3 (as observed by GOES 14 satellite). We saw the partial effect of this flare since a part of the active region was blocked by the moon. To our knowledge this is the first such incident where the solar flare was observed through lunar occultation. [ABSTRACT FROM AUTHOR]

المؤلفون: Chakravarty, S. C.¹

المصدر: AIP Conference Proceedings. 10/20/2010, Vol. 1286 Issue 1, p83-102. 20p.

مصطلحات موضوعية: *VLF radio wave propagation, *ELECTROMAGNETIC waves, *EARTH stations, *IONOSPHERE, *WAVEGUIDES, *GEOMAGNETISM

مصطلحات جغرافية: INDIA

مستخلص: Since IGY period (1957-58), natural and artificially produced Very Low Frequency (VLF) electromagnetic radiations are being recorded at large number of ground stations and on board satellites to study various wave-plasma interactive phenomena. The terrestrial propagation of these VLF radio waves are primarily enabled through the earth ionosphere wave guide (EIWG) system to long horizontal distances around the globe and ducted along the geomagnetic field lines into the conjugate hemisphere through the ionosphere-plasmasphere-magnetosphere routes. The time frequency spectra indicate presence of dispersion and various cut-off frequencies providing several types of received signals like whistlers, chorus, tweeks, hiss, hisslers etc., which can be heard on an earphone with distinguishing audio structures. While the VLF technique has been a very effective tool for studying middle and high latitude phenomena, the importance of various anomalous characteristics over the Indian low latitude stations provide potentially new challenges for their scientific interpretation and modelling. The ducted and non-ducted propagation, low latitude TRIMPI/TLE effects, D-region ionisation perturbations due to solar and stellar x- and γ ray emissions and detecting precursors of seismic activities are a few problems which will gain from low latitude studies. Since the conjugate points of Indian stations lie over the Indian oceanic region, the VLF propagation effects would be relatively noise free to observe rare and new phenomena requiring better SNR to detect such changes. The VLF signals emanating from the active seismic zones would require high sensitivity of the system and suitable network of transmitting and receiving stations. Results obtained on whistlers and related studies from a number of Indian stations covering geomagnetic latitude range between 13-24 °N are mentioned and reviewed in the background of theoretical understanding of the lightning return stroke signal elements, VLF propagation through cold plasma, ionospheric wave guide mode, electron precipitation due to cyclotron resonance and production of ionisation in the D-region due to solar/stellar UV/X/γ-rays. Further use of the VLF technique in terms of improving both observational data for real time monitoring/modelling of geophysical phenomena and exploring space weather conditions are considered as part of a future Indian programme. [ABSTRACT FROM AUTHOR]

المؤلفون: Patel, R. P.¹ rppatel123@rediffmail.com, Singh, A. K.²

المصدر: AIP Conference Proceedings. 10/20/2010, Vol. 1286 Issue 1, p77-82. 6p.

مصطلحات موضوعية: *VLF emissions, *VLF radio wave propagation, *AUDIO frequency, *MAGNETOSPHERE, *GEOMAGNETISM, *ENERGY bands, *EARTH stations

مصطلحات جغرافية: VARANASI (Uttar Pradesh, India : District), UTTAR Pradesh (India), INDIA

مستخلص: Very Low Frequency emissions are the audio-frequency signals generated by the wave particle interaction in the magnetosphere. These waves propagate in the whistler mode from the source region to the observation point. Accurate analysis and interpretation of the dynamic spectra yield information about composition and structure of the intervening media alongwith the generation mechanism. We have observed hiss, periodic hiss, chorus emission and triggered emissions during routine recording of whistler waves but spectrogram of the discrete VLF emissions are not presented in this paper. Details of theoretical computation are presented. In our computation we have shown that the non-linear theory of wave-particle interaction could explain partly the observed spectra. We have computed resonant electron energy for different pitch angle for L = 2 and 4.5 and Bunching time, trapping time, etc are also computed. The computed results are in good agreement with those reported by other workers. [ABSTRACT FROM AUTHOR]

المؤلفون: Kumar, Ashok¹ akumar_belpk@yahoo.com, Sharma, Sarita²

المصدر: AIP Conference Proceedings. 2007, Vol. 923 Issue 1, p169-181. 13p. 1 Diagram, 4 Graphs.

مصطلحات موضوعية: *RAINFALL, *RADIO wave propagation, *MICROWAVE communication systems, *SYSTEMS design, *ZENITH distance

مصطلحات جغرافية: AMRITSAR (India), INDIA

مستخلص: The efficient design of reliable communication link requires the knowledge of rain fade margin to be provided in the system design to overcome the losses in the signal strength due to rain over the path. Hence there is need for the development of reliable and accurate propagation model for attenuation prediction at the operating frequency of the link at particular location. Therefore study of rain-induced effects on microwave propagation was undertaken at Guru Nanak Dev University, Amritsar. This paper describes the measurement of zenith path attenuation due to rain at Amritsar (31° 38′ N 74° 52′ E) India, using zenith looking radiometer operating at 20 GHz. The effective rain height has been obtained from the analysis of the radiometric data and point rainfall intensity. Specific attenuation for various rain rates in the present study has been evaluated from measured drop size data using lognormal drop size distribution. The average effective rain height at Amritsar has been estimated and compared to that of predicted by ITU-R model. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

المؤلفون: Banerjee, K.S.¹, Sharma, S.P.¹ spsharma@gg.iitkgp.ernet.in, Sarangi, A.K.², Sengupta, D.¹

المصدر: Physics & Chemistry of the Earth - Parts A/B/C. Dec2011, Vol. 36 Issue 16, p1345-1352. 8p.

مصطلحات موضوعية: *HYDROGEOLOGY, *VLF radio wave propagation, *ELECTRICAL resistivity, *TAILING ponds, *RADIATION measurements, *URANIUM mill tailings

مصطلحات جغرافية: JHARKHAND (India), INDIA

مستخلص: Abstract: The hydrogeological characteristics of the uranium mill tailings pond in the vicinity of Jaduguda (Jharkhand, India) were investigated to examine possible contamination and suggest suitable remedial measures, if required. As the hydrogeological characteristics of subsurface geology are closely related to the electrical properties of the subsurface, geophysical measurements using electrical resistivity coupled with Very Low Frequency electromagnetic method and radiation study were used to investigate the geophysical and geological condition of mill tailings in order to characterize the subsurface structures of the tailings pond. The resistivity interpretation depicted the thickness of the soil cover and thickness of tailings in the pond, as well as the depth to the basement. It also suggested the possible flow direction of leachate. It was observed that the resistivity of the top layer decreases in the direction opposite to the dam axis, which in turn, indicated that the groundwater movement occurs in the opposite direction of the dam axis (in the northwest direction). The VLF method depicted the fractures through which groundwater moves, and also showed the current density alignment in the northwest direction at 10m depth. The radiation measurement showed relatively higher counts in the northwest direction. This correlated well with the resistivity measurement. The current density at a depth of 20m showed a closed contour suggesting no groundwater movement in the area at this depth, and that high conductivity material was confined to the tailings area only. It was concluded that groundwater moves in opposite direction of the dam axis at shallower depth only. It was found that continuation of fractures do not extend to deeper depths, which suggested that the tailings storage facility at Jaduguda was reasonably safe from any downward contamination. [Copyright &y& Elsevier]

المؤلفون: Kumar De, Barin¹, Sundar De, Syam² de_syam_sundar@yahoo.co.in, Bandyopadhyay, Bijoy², Pal, Pinaki³, Ali, Rousan^1,4, Paul, Suman², Goswami, Prabir Kanta²

المصدر: Terrestrial, Atmospheric & Oceanic Sciences. Aug2011, Vol. 22 Issue 4, p435-442. 8p.

مصطلحات موضوعية: *VLF radio wave propagation, *SOLAR eclipses, *IONOSPHERE, *ATMOSPHERICS, *ABSORPTION, *ATMOSPHERIC ionization, *EARTH (Planet)

مصطلحات جغرافية: TRIPURA (India), INDIA, JAPAN

مستخلص: The results from the measurements of some of the fundamental parameters (amplitude of sferics and transmitted signal, conductivity of lower ionosphere) of the ionospheric responses to the 22 July 2009 solar eclipse (partial: 91.7%) are shown. This study summarizes our results from sferics signals at 81 kHz and subionospheric transmitted signals at 19.8 and 40 kHz recorded at Agartala, Tripura (latitude: 23°N, longitude: 91.4°E). We observed significant absorption in amplitude of these signals during the eclipse period compared to their ambient values for the same period during the adjacent 7 days. The signal strength along their propagation paths was controlled by the eclipse associated decrease in ionization in the D-region of the ionosphere. Waveguide mode theory calculations show that the elevation of the height of lower ionosphere boundary of the Earth-ionosphere waveguide to a value where the conductivity parameter was 1010 unit. The absorption in 81 kHz sferics amplitude is high compared to the absorption in the amplitude of 40 kHz signal transmitted from Japan. The simultaneous changes in the amplitudes of sferics and in the amplitude of transmitted signals assert some sort of coupling between the upper atmosphere and the Earth's near-surface atmosphere prevailing clouds during solar eclipse. [ABSTRACT FROM AUTHOR]