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Vats H.O.,Physical Research Laboratory | Chandra S.,PPN College
Proceedings of the International Astronomical Union | Year: 2010

The coronal sidereal rotation rate as a function of latitude for each year, extending from 1992 to 2001 for soft X-ray images and from 1998-2005 for radio images are obtained. The present analysis reveals that the equatorial rotation rate of the corona is comparable to the photosphere and the chromosphere, However, at the higher latitudes, the corona rotation quite differently than the photosphere and chromosphere. The latitude differential obtained by both radio and X-ray images is quite variable throughout the period of the study. The equatorial rotation period seems to vary almost systematically with sunspot numbers which indicates its dependence on the phases of the solar activity cycle. © International Astronomical Union 2011. Source


Chandra S.,PPN College | Vats H.O.,Physical Research Laboratory | Iyer K.N.,Saurastra University
Monthly Notices of the Royal Astronomical Society | Year: 2010

The aim of this paper is to study the latitudinal variation in the solar rotation in soft X-ray corona. The time series bins are formed on different latitude regions of the solar full disc (SFD) images that extend from 80°S to 80°N. These SFD images are obtained with the soft X-ray telescope (SXT) on board the Yohkoh solar observatory. The autocorrelation analyses are performed with the time series that track the SXR flux modulations in the solar corona. Then for each year, extending from 1992 to 2001, we obtain the coronal sidereal rotation rate as a function of the latitude. The present analysis from SXR radiation reveals that: (i) the equatorial rotation rate of the corona is comparable to the rotation rate of the photosphere and the chromosphere, (ii) the differential profile with respect to the latitude varies throughout the period of the study; it was more in the year 1999 and least in 1994, and (iii) the equatorial rotation period varies systematically with sunspot numbers and indicates its dependence on the phases of the solar activity cycle. © 2010 The Authors. Journal compilation © 2010 RAS. Source


Kaur G.,Banaras Hindu University | Dwivedi Y.,Banaras Hindu University | Rai A.,PPN College | Rai S.B.,Banaras Hindu University
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy | Year: 2012

Luminescent nanophosphor of Er, Yb: Gd 2O 3 has been synthesized by well known combustion synthesis. Along with strong UV-Vis upconversion emission, sensor for temperature and magnetic field, nano-heater etc., it can also be used as luminescent ink by dispersing the nanophosphor in aqueous polyvinyl alcohol solution. The stability of the ink has been found to depend strongly on the mixing process of the phosphor powder in the polymer solution. The X-ray diffraction results and TEM images with diffraction pattern show that the particles are in cubic phase of Gd 2O 3 with particles of average size <40 nm. SEM images show flaky structure which makes it suitable for dispersing in PVA and useful for luminescent ink purposes on NIR excitation. The FT-IR and the thermal analysis support the presence of PVA. Upconversion and downconversion luminescence has also been observed with 532 nm excitation and energy transfer mechanism has been explained. The NIR pumping gives strong UC emission bands in red and green regions extending up to extreme UV (240 nm) in this host. © 2012 Elsevier B.V. All rights reserved. Source


Om Vats H.,Physical Research Laboratory | Chandra S.,PPN College
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

The solar images at 17 GHz by the Nobeyama Radio Heliograph and in X-rays by the soft X-ray telescope (SXT) on board the Yohkoh satellite have been of particular interest for the estimation of solar coronal rotation using the flux modulation approach. These studies established that the solar corona rotates differentially. The radio images estimate an equatorial rotation period lower than those estimated by the X-ray images. The latitude profiles of the coronal rotation have temporal variability. It is very interesting that the space-time plots of sidereal rotation period clearly reveal north-south (NS) asymmetry. The asymmetry appears to change its sign in odd and even activity cycles of the Sun. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source


Verma R.K.,Banaras Hindu University | Kaur G.,Banaras Hindu University | Rai A.,PPN College | Rai S.B.,Banaras Hindu University
Materials Research Bulletin | Year: 2012

A Tb 3+/Yb 3+ doped calcium aluminate phosphor has been synthesized using well known combustion synthesis. The structural characterization and morphology has been studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The characteristic luminescence of Tb 3+ in both upconversion and down-conversion modes, i.e. dual mode luminescence has been recorded on excitation with 976 nm and 266 nm radiation. The Stokes emission observed on 266 nm excitation also shows a characteristic of delayed fluorescence. The delayed fluorescence has been measured as a function of time and pump power. It has been correlated to the white light emission (broad continuum emission) from the host. The possible reason of association of electron hole trapping to the defect level as well as in crystal potential is suggested. © 2012 Elsevier Ltd. All rights reserved. Source

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