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Veenadhari B.,Indian Institute of Geomagnetism | Kumar S.,Indian Institute of Geomagnetism | Tulasi Ram S.,Indian Institute of Geomagnetism | Singh R.,KSK Geomagnetic Research Laboratory | Alex S.,Indian Institute of Geomagnetism
Indian Journal of Radio and Space Physics | Year: 2012

The corotating interaction region (CIR) induced magnetic storms which occurred during solar minimum are investigated to study their effects on equatorial and low-latitude ionosphere and geomagnetic field at Indian sector. This work was a part of Indian CAWSES campaign for the period of March - April 2006. The penetration electric fields play a crucial role in modifying equatorial electric fields during severe magnetic storms and these effects are not studied in detail for moderate magnetic storm originated by CIRs. The present paper investigates the effects of CIR related moderate storms and found that during the moderate magnetic storms, the short lived (duration of 2 - 3 h) penetration of high latitude electric fields to equatorial ionosphere exists during local morning and noon hours during main phase as inferred from changes in EEJ with the response to interplanetary electric field. The variations of EEJ are linearly correlated with the sudden changes in interplanetary electric field (IEFY) around morning to noon hours, which confirms existence of the penetration of electric fields.

Singh A.K.,Banaras Hindu University | Singh R.,KSK Geomagnetic Research Laboratory | Veenadhari B.,Indian Institute of Geomagnetism
Advances in Space Research | Year: 2012

Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (Geog lat.; 25.27°N; Geog long.; 82.98°E; Geomag lat = 14°55' N) was investigated using ELF/VLF radio signal Tweeks, a naturally occurring VLF signal and radio signals from various VLF navigational transmitters are first time used simultaneously to study the effect of total solar eclipse (TSE) Tweeks occurrence is a nighttime phenomena but the obscuration of solar disc during TSE in early morning leads to tweek occurrence The changes in D-region ionospheric VLF reflection heights (h) and electron density (n e: 22.6-24.6 cm -3) during eclipse have been estimated from tweek analysis The reflection height increased from ∼89 km from the first occurrence of tweek to about ∼93 km at the totality and then decreased to ∼88 km at the end of the eclipse, suggesting significant increase in tweek reflection height of about 5.5 km during the eclipse The reflection heights at the time of totality during TSE are found to be less by 2-3 km as compared to the usual nighttime tweek reflection heights This is due to partial nighttime condition created by TSE A significant increase of 3 dB in the strength of the amplitude of VLF signal of 22.2 kHz transmitted from JJI-Japan is observed around the time of the total solar eclipse (TSE) as compared to a normal day The modeled electron density height profile of the lower ionosphere depicts linear variation in the electron density with respect to solar radiation as observed by tweek analysis also These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days © 2012 COSPAR Published by Elsevier Ltd All rights reserved.

Singh A.K.,Banaras Hindu University | Singh R.,KSK Geomagnetic Research Laboratory | Singh R.P.,Banaras Hindu University | Adams K.,New Hill | Dowden R.L.,New Hill
Advances in Space Research | Year: 2015

Initial results of subionospheric very low frequency (VLF) perturbations observed on NWC (19.8 kHz) and NPM (21.4 kHz) transmitter signals at low latitude ground station Varanasi, India, during the period January-June 2010, are presented. Characteristics and occurrence rates of these events mainly observed during nighttime are studied. Most of the early VLF events had slow recovery with amplitude perturbations of 0.5-4.5 dB and phase changes of 3-12°. Temporal variation of the events is studied. World Wide Lightning Location Network data along with the broadband VLF data are analyzed to find the location of causative lightning discharge and/or the sferics associated with these early VLF events. Lightning induced changes in D-region ionospheric conductivity are attributed to the perturbations in the VLF signals. © 2014 COSPAR.

Maurya A.K.,KSK Geomagnetic Research Laboratory | Singh R.,KSK Geomagnetic Research Laboratory | Veenadhari B.,Indian Institute of Geomagnetism IIG | Kumar S.,University of The South Pacific | Singh A.K.,Banaras Hindu University
Natural Hazards and Earth System Sciences | Year: 2013

The present study reports the VLF (very low frequency) sub-ionospheric perturbations observed on transmitter JJI (22.1 kHz), Japan, received at the Indian low-latitude station, Allahabad ( geographic lat. 25.41 N, long 81.93 E), due to Wenchuan earthquake (EQ) that occurred on 12 May 2008 with the magnitude 7.9 and at the depth of 19 km in Sichuan province of Southwest China, located at 31.0 N, 103.4 E. The nighttime amplitude fluctuation analysis gives a significant increase in fluctuation and dispersion two days before EQ, when it crosses 2σ criterion. However, there was no significant change observed in the amplitude trend. The diurnal amplitude variation shows a significant increase in the amplitude of JJI signal on 11 and 12 May 2008. The gravity wave channel and changes in the electric field associated with this EQ seem to be the potential factors of the observed nighttime amplitude fluctuation, dispersion, and significant increase in the signal strength. © Author(s) 2013. CC Attribution 3.0 License.

Maurya A.K.,Indian Institute of Geomagnetism | Singh R.,KSK Geomagnetic Research Laboratory | Singh R.,Stanford University | Veenadhari B.,Indian Institute of Geomagnetism | And 8 more authors.
Journal of Geophysical Research: Space Physics | Year: 2012

A total of 19,308 tweeks recorded during April 2007 to March 2008, at Allahabad, a low-latitude station in the Indian sector, has been used to study seasonal occurrence which shows maximum tweek occurrence of about 63% during summer season and about 19% and 18% occurrences during equinox and winter seasons. Maximum occurrence of tweeks during summer season is consistent with the larger number of lightnings detected by World Wide Lightning Location Network in the Indian and Asia Oceania regions during summer as compared to that during equinox and winter. Seasonally, tweek (ionospheric) reflection height in the premidnight (18:00-00:00 LT) during winter is less as compared to that during equinox and summer. Annual (seasonal average) variation of the mean ionospheric reflection height shows a gradual increase in the reflection height from about 19:30 to 04:30 LT. The annual average of postmidnight (00:00-06:00 LT) reflection height is about 5 km higher as compared to that in the premidnight. Our initial results on the variability in the ionospheric reflection height under pure nighttime propagation (21:00-03:00 LT) on magnetically quiet days show a day-to-day variability of up to 8 km. Theoretically calculated attenuation of the Earth-ionosphere waveguide for first six tweek modes in the early and late night periods (21:00-00:00 LT; 00:00-03:00 LT) is less compared to that in the dusk (18:00-21:00 LT) period. The higher attenuation in the dusk period and most of the tweeks in the dawn period traveling partially under daytime conditions explain the lower occurrence of tweeks in these periods. Copyright 2012 by the American Geophysical Union.

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