• P. Venkatakrishnan

      Articles written in Journal of Astrophysics and Astronomy

    • Time-dependent interaction of granules with magnetic flux tubes

      P. Venkatakrishnan S. S. Hasan

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      The time-dependent interaction of the granulation velocity field with a magnetic flux tube is investigated here. It is seen that when a magnetic field line is displaced normal to itself so as to simulate thebuffeting action of granules, a flow of gas is initiated along the field. By choosing a lateral velocity field which is consistent with observations of granules, it is found that the resulting gas motion is a downward flow with a velocity compatible with the observed downflow in isolated photospheric flux tubes. It is therefore proposed that the observed photospheric downflow is a manifestation of the interaction of granules with flux tubes.

    • Nonlinear development of convective instability within slender flux tubes. I. Adiabatic flow

      P. Venkatakrishnan

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      The nonlinear development of convective instability within slender flux tubes is studied using the method of characteristics. It is seen that the initial magnetic field influences the development of the instability. The asymptotic state of the unstable tube depends on the boundary conditions. Flux tubes subjected to ’open’ boundary conditions show a better evidence for field amplification than those subjected to ’closed’ boundary conditions. In either case, convective instability results in the generation of significant gas flow within slender flux tubes.

    • Nonlinear development of convective instability within slender flux tubes. II. The effect of radiative heat transport

      P. Venkatakrishnan

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      Inclusion of radiative heat transport in the energy equation for a slender flux tube leads to oscillations of the tube. The amplitude of the oscillations depends on the radius of the tube when lateral heat exchange alone is considered. Longitudinal heat transport has a greater influence on the evolution of the instability than lateral heat exchange for the particular value of tube radius considered in the calculation. Heat transport is seen to reduce the efficiency of concentration of magnetic fields by convective collapse in the case of polytropic tubes.

    • Is a sunspot in static or dynamic equilibrium?

      P. Venkatakrishnan

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    • Stokes polarimetry at the Kodaikanal tower tunnel telescope

      K. Sankarasubramanian G. Srinivasulu A. V. Ananth P. Venkatakrishnan

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      A Stokes Polarimeter has been developed using a masked CCD arrangement for the measurement of the vector magnetic field of sunspots. Charge shifting within the CCD is used to record near simultaneous orthogonal polarisation. The testing of the Stokes Polarimeter and the behavior of the integrated system combined with the Kodaikanal tower tunnel telescope will be discussed.

    • Design and status of Solar Vector Magnetograph (SVM-I) at Udaipur solar observatory

      Sanjay Gosain P. Venkatakrishnan K. Venugopalan

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      We present the status of the instrument called Solar Vector Magnetograph Phase-I (SVM-I) currently being developed at Udaipur Solar Observatory. SVM-I is an instrument which aims to determine the magnetic field vector in the solar atmosphere by measuring Zeeman induced polarization across the spectral line. The instrument is currently in a preliminary development stage, with all components under an evaluation process. The integration of components is being done. The integrated performance of the system on a tracking mount and its control software is being tested. Some test observations of sunspots has been carried out. In this paper we give a technical description of the hardware and software elements of the instrument and present the polarized images obtained during test observations.

    • Development of solar scintillometer

      Sudhir Kumar Gupta Shibu K. Mathew P. Venkatakrishnan

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      The index of scintillation measurement is a good parameter to compare different sites for image quality or ‘seeing’. We have developed a scintillometer, which is deployed on the high resolution SPAR telescope in the island site of Udaipur Solar Observatory, for the site characterization to specify the proposed MAST (Multi Application Solar Telescope). The scintillometer consists of a miniature telescope, termed as micro telescope (4 mm aperture, 15 mm focal length) mounted on a drive which tracks the Sun continuously, associated amplifiers and a data acquisition system. A photodiode is used as the detector. The telescope along with detector was obtained from National Solar Observatory (NSO), and is similar to the one used for Advanced Technology Solar Telescope (ATST) site survey. At USO we developed the amplifier and data acquisition system for the scintillometer. A 24-bit analog to digital converter based system was designed, assembled, tested and used as the data acquisition system (DAS). In this paper, we discuss the instrumentation and present the initial results.

    • Editorial

      Rajmal Jain P. Venkatakrishnan Judith T. Karpen

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    • Software for Interactively Visualizing Solar Vector Magnetograms of Udaipur Solar Observatory

      Sanjay Gosain Sanjiv Tiwari Jayant Joshi P. Venkatakrishnan

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      The Solar Vector Magnetograph (SVM) at Udaipur Solar Observatory saw its first light in April 2005. The retrieval of vector fields from the imaging spectro-polarimetric observational data requires a substantial amount of computer post-processing. The GUI-based data reduction and analysis software have been developed to make the data processing pipeline user-friendly and less time-consuming. In this paper we describe these software packages.

    • Development of a Low-order Adaptive Optics System at Udaipur Solar Observatory

      A. R. Bayanna B. Kumar R. E. Louis P. Venkatakrishnan S. K. Mathew

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      A low-order Adaptive Optics (AO) system is being developed at the Udaipur Solar Observatory and we present in this paper the status of the project, which includes the image stabilization system and calibration of wavefront sensor and deformable mirror. The image stabilization system comprises of a piezo driven tip-tilt mirror, a high speed camera (955 fps), a frame grabber system for sensing the overall tilt and a Linux based Intel Pentium 4 control computer with Red Hat Linux OS. The system operates under PID control. In the closed loop, an rms image motion of 0.1–0.2 arcsec was observed with the improvement factor varying from 10–20 depending on the external conditions. Error rejection bandwidth of the system at 0 dB is 80–100 Hz. In addition to that, we report the on-going efforts in the calibration of lenslet array and deformable mirror for sensing and correcting the local tilt of the wavefront.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

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