Browsing by Author "D.V. Phanikumar"

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Effect of total lunar eclipse of 27th July 2018 on the D-region ionosphere by using VLF observations
(Elsevier Ltd, 2022) Ajeet Kumar Maurya; D.V. Phanikumar; Rajesh Singh; K. Venkatesham; Abhay K. Singh
In this report, an effort for the first time has been made to investigate the effect of the total Lunar Eclipse that occurred on 27th July 2018 (the longest lunar eclipse in the 21st century) on the D-region ionosphere. Very Low Frequency (VLF) navigational transmitter signals recorded at Prayagraj (formally Allahabad) station in India is the primary dataset used. The signals from two VLF transmitters: NWC (19.8 kHz) from Australia, and JJI (22.2 kHz) from Japan, have been utilized. When compared to unperturbed night, NWC signal amplitude showed an anomalous decrease of ∼0.86 dB, while JJI amplitude showed an anomalous increase ∼0.26 dB during the lunar eclipse. The Wait's lower ionospheric parameters: reflection height (H’) and sharpness factor (β) are estimated by using a rather coarse method of modeling of VLF signals anomalies on eclipse day, which comes out to be H’ = 85.25 km & β = 0.689 km−1 and H’ = 84.59 km & β = 0.683 km−1 respectively for NWC and JJI path. Using wait parameters, the electron density (Ne) profile is estimated and this showed a decrease in Ne during eclipse time with reference to the nighttime ambient values. The Ne values for the NWC path, showed relatively higher decrease compared to the JJI path. The plausible reasons for observed Ne decrease on lunar eclipse night could be due to the reflected lunar Lyman‐α and X-ray's radiation which may contribute to the nighttime ionization is blocked during the lunar eclipse, thereby decreasing the nighttime lower ionospheric Ne. © 2021 COSPAR
Low-mid latitude D region ionospheric perturbations associated with 22 July 2009 total solar eclipse: Wave-like signatures inferred from VLF observations
(Blackwell Publishing Ltd, 2014) Ajeet K. Maurya; D.V. Phanikumar; Rajesh Singh; Sushil Kumar; B. Veenadhari; Y.-S. Kwak; Abhikesh Kumar; Abhay K. Singh; K. Niranjan Kumar
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. © 2014. American Geophysical Union. All Rights Reserved.
Solar flares induced D-region ionospheric and geomagnetic perturbations
(Elsevier Ltd, 2015) R. Selvakumaran; Ajeet K. Maurya; Sneha A. Gokani; B. Veenadhari; Sushil Kumar; K. Venkatesham; D.V. Phanikumar; Abhay K. Singh; Devendraa Siingh; Rajesh Singh
The D-region ionospheric perturbations caused by solar flares which occurred during January 2010-February 2011, a low solar activity period of current solar cycle 24, have been examined on NWC transmitter signal (19.8 kHz) recorded at an Indian low latitude station, Allahabad (Geographic lat. 25.75°N, long. 81.85°E). A total of 41 solar flares, including 21 C-class, 19 M-class and 01 X-class, occurred during the daylight part of the NWC-Allahabad transmitter receiver great circle path. The local time dependence of solar flare effects on the change in the VLF amplitude, time delay between VLF peak amplitude and X-ray flux peak have been studied during morning, noon and evening periods of local daytime. Using the Long Wave Propagation Capability code V 2.1 the D-region reference height (H/) and sharpness factor (β) for each class of solar flare (C, M and X) have been estimated. It is found that Dregion ionospheric parameters (H/, β) strongly depend on the local time of flare's occurrence and their classes. The flare time electron density estimated by using H/ and β shows maximum increase in the electron density of the order of ~80 times as compared to the normal day values. The electron density was found to increase exponentially with increase in the solar flux intensity. The solar flare effect on horizontal component (H) of the Earth's magnetic field over an equatorial station, Tirunelveli (Geographic lat., 8.7°N, long., 77.8°E, dip lat., 0.4°N), shows a maximum increase in H of ~8.5% for M class solar flares. The increase in H is due to the additional magnetic field produced by the ionospheric electrojet over the equatorial station. © 2015 Elsevier Ltd.