P. Venkatakrishnan

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

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.

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

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.

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

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.

Structure and Evolution of the Transition Region Network Observed in he ii λ304

The length scale and life time of the transition region network cells were studied using He ii λ304 filtergrams. The temporal structure function was calculated from spatially aligned He ii λ304 images. The estimated life time of the network cell was about 27 hr. We compared this life time with the life time of photospheric magnetic network and of the extrapolated magnetic network. The spatial structure function was calculated from the He ii λ304 filtergrams. The calculated spatial structure function saturates at ∼ 25 000 km. The transition region network elements are bigger in size than the photospheric magnetic network element. The magnetic network element equals the size of the He ii λ304 network element when the photospheric magnetic field is extrapolated to a height of 3000 km above the photosphere where the magnetic fluxes are deployed. The derived value of the diffusion speed of the network elements was 0.098 km s−1.

Acoustic Power Absorption and its Relation to Vector Magnetic Field of a Sunspot

The distribution of acoustic power over sunspots shows an enhanced absorption near the umbra – penumbra boundary. Previous studies revealed that the region of enhanced absorption coincides with the region of strongest transverse potential field. The aim of this paper is to i) utilize the high-resolution vector magnetograms derived using Hinode SOT/SP observations and study the relationship between the vector magnetic field and power absorption and ii) study the variation of power absorption in sunspot penumbrae due to the presence of spine-like radial structures.It is found that i) both potential and observed transverse fields peak at a similar radial distance from the center of the sunspot, and ii) the magnitude of the transverse field, derived from Hinode observations, is much larger than the potential transverse field derived from SOHO/MDI longitudinal-field observations. In the penumbra, the radial structures called spines (intra-spines) have stronger (weaker) field strength and are more vertical (horizontal). The absorption of acoustic power in the spine and intra-spine shows different behavior, with the absorption being larger in the spine as compared to the intra-spine.

Development of solar scintillometer

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.

Stokes polarimetry at the Kodaikanal tower tunnel telescope

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.

Time-Dependent Interaction of Granules with Magnetic Flux Tubes

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.