Browsing by Author "P. Pant"

Now showing 1 - 9 of 9

Application of lightning discharge generated radio atmospherics/tweeks in lower ionospheric plasma diagnostics
(Institute of Physics Publishing, 2010) A.K. Maurya; R. Singh; B. Veenadhari; P. Pant; A.K. Singh
Lightning discharges during thunderstorm are the significant natural source of electromagnetic waves. They generate electromagnetic pulses, which vary from few Hz to tens of MHz, but the maximum radiated energy is confined in extremely low (ELF: 3-3000Hz) and very low (VLF: 3-30 KHz) frequency band. These pulsed signals with frequency dispersion are known as radio atmospherics or tweeks. These waves propagate through the process of multiple reflections in the earth-ionosphere waveguide over long distances with very low attenuation (2-3 dB/1000km). Since these waves are reflected by lower boundary of ionosphere, these are used extensively for probing the D-region ionosphere. D-region is important to the space weather, as well as the submarine communication and navigational aid. In this perspective the measurement of electron density profiles of the D-region is undoubtedly of great interest to both the development of reliable models and radio wave propagation. Earlier work on the tweeks is mainly focused to the theoretical considerations related to polarization, waveform analysis, and occurrence time and propagation mechanism. In this study we investigate tweeks to determine the equivalent night time electron densities at reflection height of the D-region. Distance traveled by the VLF waves from the causative lightning discharges to the receiving station has also been calculated. Tweeks recorded at a low latitude ground station of Allahabad (Geomag. lat. 16.050 N) during the night of 23 March 2007 have been used in the present analysis. Based on the analysis of the fundamental cut-off frequency of tweeks, the estimated equivalent electron density of the D-region has been found to be in the range of ∼20 to 25 el/cm3 at ionospheric reflection height of ∼80 to 95 km respectively. Propagation distance in Earth-Ionosphere wave guide (EIWG) from causative lightning source to experimental site varies from ∼1500 to 8000 km. © 2010 IOP Publishing Ltd.
Changing picture of acute kidney injury in pregnancy: Study of 259 cases over a period of 33 years
(Medknow Publications, 2016) J. Prakash; P. Pant; S. Prakash; M. Sivasankar; R. Vohra; P.K. Doley; L. Pandey; U. Singh
The incidence of acute kidney injury (AKI) in pregnancy is declining in developing countries but still remains a major cause of maternal and fetal morbidity and mortality. The aim of the study was to analyze the changing trends in pregnancy related AKI (PR-AKI) over a period of thirty-three years. Clinical characteristics of PR-AKI with respect to incidence, etiology and fetal and maternal outcomes were compared in three study periods, namely 1982-1991,1992-2002 and 2003-2014. The incidence of PR-AKI decreased to 10.4% in 1992-2002, from 15.2% in 1982-1991, with declining trend continuing in 2003-2014 (4.68%).Postabortal AKI decreased to 1.49% in 2003-2014 from 9.4% in 1982-1991of total AKI cases.The AKI related to puerperal sepsis increased to 1.56% of all AKI cases in 2003-2014 from 1.4% in 1982-1991. Preeclampsia/eclampsia associated AKI decreased from 3.5% of total AKI cases in 1982-1991 to 0.54% in 2003-2014. Pregnancy associated - thrombotic microangiopathy and acute fatty liver of pregnancy were uncommon causes of AKI. Hyperemesis gravidarum associated AKI was not observed in our study. Incidence of renal cortical necrosis (RCN) decreased to 1.4% in 2003-2014 from 17% in 1982-1991.Maternal mortality reduced to 5.79% from initial high value 20% in 1982-1991. The progression of PR-AKI to ESRD decreased to1.4% in 2003-2014 from 6.15% in 1982-1991. The incidence of PR-AKI has decreased over last three decades, mainly due to decrease in incidence of postabortal AKI. Puerperal sepsis and obstetric hemorrhage were the major causes of PR-AKI followed by preeclampsia in late pregnancy. Maternal mortality and incidence and severity of RCN have significantly decreased in PR-AKI. The progression to CKD and ESRD has decreased in women with AKI in pregnancy in recent decade. However, the perinatal mortality did not change throughout study period.
D-region ionosphere response to the total solar eclipse of 22 July 2009 deduced from ELF-VLF tweek observations in the Indian sector
(Blackwell Publishing Ltd, 2011) Rajesh Singh; B. Veenadhari; Ajeet K. Maurya; Morris B. Cohen; Sushil Kumar; R. Selvakumaran; P. Pant; Abhay K. Singh; Umran S. Inan
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 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. Copyright 2011 by the American Geophysical Union.
Estimation of D-region electron density using tweeks measurements at Nainital and Allahabad
(2010) P. Pant; A.K. Maurya; Rajesh Singh; B. Veenadhari; A.K. Singh
Lightning generated radio atmospheric that propagates over long distances via multiple reflections through the boundaries of the Earth-ionosphere waveguide (EIWG), shows sharp dispersion near the cut-off frequency ∼1.8kHz of the EIWG. These dispersed atmospherics at lower frequency end are called as 'tweek' radio atmospherics. In order to estimate D-region electron densities at the ionospheric reflection heights we have utilized the dispersive property of tweeks observed at low latitude Indian stations Nainital (Geomag. Lat., 20.29° N) and Allahabad (Geomag. Lat., 16.05° N). Direction finding technique has also been applied to determine the source locations of causative lightning discharge of tweeks. In this perspective, the geographic locations is determined by the intersection of two circles that are drawn by taking the travelled / propagation distance by tweek atmospherics from source location to Allahabad (ALD) and Nainital (NTL) stations. These results are in good agreement with World Wide Lightning Location Network (WWLLN) data. The average D-region electron density along the propagation path varied in the range ∼20-35 el/cc at ionospheric reflection heights of 70-90km. The tweek method has unique advantage of monitoring lower boundary of the D-region over an area of several thousand of km surrounding to the receiving stations. © 2010 American Institute of Physics.
Initial results from AWESOME VLF receivers: Set up in low latitude Indian regions under IHY2007/UNBSSI program
(2010) Rajesh Singh; B. Veenadhari; M.B. Cohen; P. Pant; A.K. Singh; A.K. Maurya; P. Vohat; U.S. Inan
This article delineates preliminary results obtained from three newly set up very low frequency (VLF) radio receiver observation stations at low latitude Indian sites Allahabad (16.49°N), Nainital (20.48°N) and Varanasi (15.41°N), during 2007. These stations were set up by collaboration between the Indian Institute of Geomagnetism, Mumbai and Stanford University, USA under the International Heliophysical Year 2007/United Nations Basic Space Science Initiative (UNBSSI) program. VLF stations are part of Atmospheric Weather Electromagnetic System for Observation, Modeling and Education (AWESOME) network being operated globally to study the ionosphere and the magnetosphere with the help of electromagnetic waves in extremely low frequency (ELF) and VLF bands. The new set up of VLF receiver is a marked improvement over the traditional recording and analysis systems in India. It provides additional facilities to study the VLF phenomena at low latitudes which were not studied earlier, like direction finding of events like whistlers, emissions, etc. and D-region ionospheric perturbation caused by the geophysical phenomena such as solar flares, lightning induced electron precipitations, cosmic gamma ray flares, terrestrial gamma rays flares, geomagnetic storm effect, etc. Simultaneous observation at multiple sites will provide opportunity to study in more detail the application of ELF/VLF data as earthquake precursors. Some of the initial results obtained from the preliminary analysis are presented to show the probing potentiality of ELF/VLF waves in ionosphere/magnetosphere studies.
Intracranial Hodgkin's disease [2]
(2006) V. Gupta; J. Ghosh; P. Pant; B.D. Bhatia
[No abstract available]
Morphological features of tweeks and nighttime D region ionosphere at tweek reflection height from the observations in the low-latitude Indian sector
(Blackwell Publishing Ltd, 2012) Ajeet K. Maurya; Rajesh Singh; B. Veenadhari; Sushil Kumar; M.B. Cohen; R. Selvakumaran; P. Pant; A.K. Singh; D. Siingh; U.S. Inan
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.
Nighttime D region electron density measurements from ELF-VLF tweek radio atmospherics recorded at low latitudes
(2012) Ajeet K. Maurya; B. Veenadhari; Rajesh Singh; Sushil Kumar; M.B. Cohen; R. Selvakumaran; Sneha Gokani; P. Pant; A.K. Singh; Umran S. Inan
Dispersive atmospherics (tweeks) observed during 2010 simultaneously at two low-latitude stations, Allahabad (geomagnetic latitude, 16.05N) and Nainital (geomagnetic latitude, 20.48N), have been used to estimate the nighttime D region electron density at the ionospheric reflection height under the local nighttime propagation (21:00-02:00 LT or 15:30-20:30 UT). The analysis of simultaneously recorded tweeks at both the stations on five international quiet days during one month each from summer (June), winter (January), and equinox (March) seasons shows that the D region electron density varies 21.5-24.5 cm-3 over the ionospheric reflection height of 85-95 km. The average values of Wait lower ionospheric parameters: ionospheric reference height h and sharpness factor are almost same during winter (85.9-86.1 km, 0.51-0.52 km -1) and equinox (85.6-85.7 km, 0.54 km-1) seasons. The values of h and during summer season are about 83.5 km and 0.60 km-1 at both stations. Overall, equivalent electron density profile obtained using tweek method shows lower values of electron density by about 5-60% than those obtained using the International Reference Ionosphere (IRI-2007) model and lower/higher by 2-68% than those obtained using rocket technique. The electron density estimated using all three techniques (tweek, IRI-2007, and rocket) is consistent in the altitude range of 82-98 km. The estimated geographic locations of causative lightnings of tweeks were matched with the locations and times of lightnings detected by the World-Wide Lightning Location Network (WWLLN). The WWLLN detected about 27.5% of causative lightnings of tweeks simultaneously observed at both the stations. © 2012. American Geophysical Union. All Rights Reserved.
Superior vena cava syndrome in children
(2008) Vineeta Gupta; Srikanth R. Ambati; P. Pant; Baldev Bhatia
Superior vena cava syndrome (SVCS) is rare in childhood. 18 cases of SVCS were seen in children ranging from 3-14 years with a mean age of 8.8 years. There were 15 males and 3 female children. Diagnosis could be confirmed in 17 cases as one child succumbed to severe respiratory distress without a definitive diagnosis. The commonest cause of SVCS was lymphoma. Non-Hodgkin's lymphoma (NHL) was more common than Hodgkin's disease. In two cases the final diagnosis was tuberculosis of mediastinal lymph nodes. The diagnosis was confirmed by cervical lymph node biopsy in 6 cases, mediastinal biopsy in 6 cases and bone marrow aspiration in the remaining 5 cases. Intravenous Dexamethasone provided relief of symptoms in 13 patients. None of the children received emergency radiotherapy. Anti-tubercular treatment produced complete cure in the two patients with tubercular mediastinal lymphadenopathy. © Indian Society of Haematology & Transfusion Medicine 2008.