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  • 1
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    25 April 2015 Nepal Earthquake: Investigation of precursor in VLF sub-ionospheric signal (2016)
    Wiley
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    Publication Date: 2016-09-30
    Description: We present a critical analysis of the observations and interpretation of VLF sub-ionospheric measurements related to the main Nepal Gorkha earthquake which occurred on April 25, 2015 (M w 7.8) and its major aftershock on May 12, 2015 (M w 7.3). The VLF narrowband signal used is from NWC (19.8 kHz) VLF transmitter located in Australia and recorded at Allahabad (lat. 25.41° N; long. 81.93° E). Allahabad is located very close (~360 km) to these earthquake (EQ) epicenters. Two widely used analysis, viz., (1) terminator time and (2) night time fluctuation techniques are applied to extract seismic related effects in the NWC narrowband VLF data. The terminator time analysis yields statistically significant shifts of ~45 and ~26 minutes respectively in evening terminator time in the NWC VLF amplitude signal, one day before both the earthquakes. The nighttime fluctuation method shows a consistent, statistically significant, increase in three parameters one day before the earthquake. The observed terminator time and nighttime fluctuation shifts were associated with these earthquakes only after scrutinizing possible contributions from other potential sources such as solar activity, other earthquakes on the signal path, meteorological disturbances such as lightning activity, wind speed, temperature etc. along the transmitter-receiver great circle path. The VLF sub-ionospheric signal analysis results unambiguously point towards the presence of seismically excited atmospheric gravity waves (AGWs) during these major earthquakes and their important role in providing the coupling between the seismic source region and overlying ionosphere.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Substrate Controlled Synthesis of Benzisoxazole and Benzisothiazole Derivatives via PhI(OAc)2-Mediated Oxidation Followed by Intramolecular Oxidative O–N/S–N Bond Formation (2015)
    American Chemical Society (ACS)
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    Publication Date: 2015-11-21
    Description: The Journal of Organic Chemistry DOI: 10.1021/acs.joc.5b02276
    Print ISSN: 0022-3263
    Electronic ISSN: 1520-6904
    Topics: Chemistry and Pharmacology
    Published by American Chemical Society (ACS)
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  • 3
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    Changes in the D-region associated with three recent solar eclipses in the South Pacific Region (2016)
    Wiley
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    Publication Date: 2016-06-11
    Description: We estimate D-region changes due to 22 July 2009 total solar eclipse (SE), 13-14 November 2012 total SE and 9-10 May 2013 annular SE, using VLF navigational transmitters signal observations at Suva, Fiji. The NWC signal (19.8 kHz) showed an amplitude and phase decrease of 0.70 dB and 23° during November SE and 2.0 dB and 90 o during May SE. The modeling using Long Wave Propagation Capability code for NWC-Suva path during November and May SEs showed an increase in average D-region reflection height ( H ') and sharpness factor ( β ) by 0.6 and 0.5 km and 0.012 and 0.0015 km -1 , respectively. The July total SE for JJI-Suva path showed an increase in H ' of 1.5 km and a decrease in β of 0.055 km -1 . The decrease in the electron density calculated using SE time H ' and β is maximum for July total SE and minimum for May annular SE. The effective recombination coefficient estimated from the decay and recovery of signal phase associated with May annular SE was higher ( 27%) than normal daytime value 5.0 × 10 -7 cm -3 s -1 and varied between 1.47 × 10 -6 to 1.15 × 10 -7 cm -3 s -1 in the altitude 70 to 80 km. Morlet wavelet analysis of signals amplitude shows strong wave-like signatures (WLS) associated with three SEs with period ranging 24-66 min but the intensity and duration of WLS show no clear dependence on SE magnitude and type. Apart from the cooling spot, the eclipse shadow can also generate WLS associated with AGWs.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
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    Superconductivity by Sr intercalation in the layered topological insulator Bi_{2} Se_{3} (2015)
    American Physical Society (APS)
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    Publication Date: 2015-07-17
    Description: Author(s): Shruti, V. K. Maurya, P. Neha, P. Srivastava, and S. Patnaik Strontium intercalation between van der Waals bonded layers of the topological insulator Bi 2 Se 3 is found to induce superconductivity with a maximum T c of 2.9 K. Transport measurement on a single crystal of the optimally doped sample Sr 0.1 Bi 2 Se 3 shows weak anisotropy ( Γ ∼ 1.5 ) and an upper critical fie… [Phys. Rev. B 92, 020506(R)] Published Wed Jul 15, 2015
    Keywords: Superfluidity and superconductivity
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
    Published by American Physical Society (APS)
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  • 5
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    Very low latitude (L=1.08) whistlers and correlation with lightning activity (2015)
    Wiley
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    Publication Date: 2015-07-15
    Description: We present analysis of more than 2000 lightning-generated whistlers recorded at a low latitude station, located at Allahabad (Geog. lat., 25.40° N; Geog. long. 81.93° E; L =1.081), India, during December 2010 to November 2011. The main focus of this work is on the correlation between observed low latitude whistlers and lightning activity detected by the World-Wide Lightning Location Network near the conjugate point (Geog. 9.87° S; 83.59° E) of Allahabad. Whistler occurrence is higher in the post-midnight period as compared to the pre-midnight period. Whistlers were observed in the daytime only on two days that too before 8:30 LT (morning). Seasonally, occurrence is maximum during winter months, which is due to more lightning activity in the conjugate region and favourable ionospheric conditions. About 63% of whistlers were correlated with lightning strokes in the vicinity of the conjugate point within spatial extent of 1000 km (conjugate area). Most (about 53%) whistlers were found to be associated with lightning strokes that were offset to the southeast of the conjugate point. The results indicate an energy range of 7.5 -17.5 kJ of lightning strokes generate most of whistlers at this station. The L -shell calculations show that propagation paths of the observed whistlers were embedded in the topside ionosphere. Based on these results we suggest a possibility of ducted mode of propagation even for such very low latitude whistlers.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 6
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    D-region ionosphere response to the total solar eclipse of 22 July 2009 deduced from ELF-VLF tweek observations in the Indian sector (2011)
    Wiley
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    Publication Date: 2011-10-01
    Description: Observations of tweeks with higher harmonics (n 〉 1) at low latitude stations Allahabad and Nainital, in the Indian sector, during the total solar eclipse on 22 July 2009, are presented. Allahabad and Nainital stations were in 100% and 85% of the totality paths. Observations suggest that about 30–40% obscuration of solar disc can lead to the tweeks occurrence which otherwise occur only in nighttime. A total of 148 tweeks at Allahabad and 20 tweeks at Nainital were recorded with some of them up to 3rd harmonics. The World Wide Lightning Location Network data indicated that tweeks observed were generated by lightning's located in the partial eclipse area of Asia-Oceania region. The changes in D-region ionospheric VLF reflection height and electron density (∼22–23 cm−3) during eclipse have been estimated from the first cut-off frequency of the tweeks. The reflection height increased from ∼89 km from the first occurrence of tweek to about 91–92 km at the totality and then decreased to ∼87 km at the end of the eclipse, suggesting a change of about 5 km in the reflection height during eclipse. The reflection heights are lower by 2–3 km as compared to normal nighttime tweek reflection heights. The above increase in the reflection height indicate that the partial nighttime condition is created during eclipse, as the main D-region ionizing radiation Lyman α is blocked but solar soft X-ray and EUV radiations originating from the limb solar corona are not totally blocked which produce some of ionization in the D-region.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 7
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    Very low latitude (L = 1.08) whistlers (2012)
    Wiley
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    Publication Date: 2012-12-08
    Description: For decades, whistlers observed on the ground at mid and high latitudes have been used for diagnostics of Earth's plasmasphere. Whistlers have also been observed at low latitudes however, the propagation characteristics of low latitude whistlers are poorly understood thus they have not been used effectively as a diagnostic for the low latitude ionosphere. One key limitation with past studies has been lack of knowledge of the whistler source lightning location. Here we present the first cases of low latitude ground whistlers most likely linked with their causative lightning discharges in the conjugate zone. The Global Lightning Dataset 360 (GLD360) detected lightning discharges were found to be located close to the conjugate location of the recording stations, providing direct evidence of inter-hemispheric propagation at the low latitudes. A total of 864 whistlers were observed at Allahabad, India (Geomag. lat. 16.05°N; Geomag. long. 155.34°E; L = 1.08) during the night of 26 January 2011. Using GLD360 network data, we show the occurrence of thunderstorm activity between 200 and 450 km from the conjugate point of Allahabad. We also report the distribution of peak currents of whistler-producing lightning, which demonstrate a cutoff at 30 kA.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Coronal mass ejection–driven shocks and the associated sudden commencements/sudden impulses (2012)
    Wiley
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    Publication Date: 2012-04-11
    Description: Interplanetary (IP) shocks are mainly responsible for the sudden compression of the magnetosphere, causing storm sudden commencement (SC) and sudden impulses (SIs) which are detected by ground-based magnetometers. On the basis of the list of 222 IP shocks compiled by Gopalswamy et al. (2010), we have investigated the dependence of SC/SIs amplitudes on the speed of the coronal mass ejections (CMEs) that drive the shocks near the Sun as well as in the interplanetary medium. We find that about 91% of the IP shocks were associated with SC/SIs. The average speed of the SC/SI-associated CMEs is 1015 km/s, which is almost a factor of 2 higher than the general CME speed. When the shocks were grouped according to their ability to produce type II radio burst in the interplanetary medium, we find that the radio-loud (RL) shocks produce a much larger SC/SI amplitude (average ∼32 nT) compared to the radio-quiet (RQ) shocks (average ∼19 nT). Clearly, RL shocks are more effective in producing SC/SIs than the RQ shocks. We also divided the IP shocks according to the type of IP counterpart of interplanetary CMEs (ICMEs): magnetic clouds (MCs) and nonmagnetic clouds. We find that the MC-associated shock speeds are better correlated with SC/SI amplitudes than those associated with non-MC ejecta. The SC/SI amplitudes are also higher for MCs than ejecta. Our results show that RL and RQ type of shocks are important parameters in producing the SC/SI amplitude.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Morphological features of tweeks and nighttime D region ionosphere at tweek reflection height from the observations in the low-latitude Indian sector (2012)
    Wiley
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    Publication Date: 2012-05-02
    Description: 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.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 10
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    Low-mid latitude D-region ionospheric perturbations associated with 22 July 2009 Total Solar Eclipse: Wave-like signatures inferred from VLF observations (2014)
    Wiley
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    Publication Date: 2014-09-17
    Description: 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, 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 minutes at stations under total eclipse and of period ~30-80 minutes at stations under partial eclipse (~85-54 % totality) with delay times between ~50-100 minutes 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 wave induced D-region ionospheric perturbations.
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    Topics: Geosciences , Physics
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