K. Sankarasubramanian

GLOBAL TWIST OF SUNSPOT MAGNETIC FIELDS OBTAINED FROM HIGH-RESOLUTION VECTOR MAGNETOGRAMS

The presence of fine structures in sunspot vector magnetic fields has been confirmed from Hinode as well as other earlier observations. We studied 43 sunspots based on the data sets taken from ASP/DLSP, Hinode (SOT/SP), and SVM (USO). In this Letter, (1) we introduce the concept of signed shear angle (SSA) for sunspots and establish its importance for non-force-free fields. (2) We find that the sign of global α (force-free parameter) is well correlated with that of the global SSA and the photospheric chirality of sunspots. (3) Local α patches of opposite signs are present in the umbra of each sunspot. The amplitude of the spatial variation of local α in the umbra is typically of the order of the global α of the sunspot. (4) We find that the local α is distributed as alternately positive and negative filaments in the penumbra. The amplitude of azimuthal variation of the local α in the penumbra is approximately an order of magnitude larger than that in the umbra. The contributions of the local positive and negative currents and α in the penumbra cancel each other giving almost no contribution for their global values for the whole sunspot. (5) Arc-like structures (partial rings) with a sign opposite to that of the dominant sign of α of the umbral region are seen at the umbral-penumbral boundaries of some sunspots. (6) Most of the sunspots studied belong to the minimum epoch of the 23rd solar cycle and do not follow the so-called hemispheric helicity rule.

Line Shape Changes and Doppler Measurements in Solar Active Regions. I. A Method for Correcting Dopplergrams from SOHO MDI

We present a method to correct MDI high-resolution Dopplergrams in active regions for systematic observational errors due to the spectral line shape changes. We use a measurement campaign with the Advanced Stokes Polarimeter to investigate the influence of line shape changes in active regions on MDI Dopplergram calibration. An estimate of the width of the Ni I λ6768 absorption line used by MDI to measure velocity provides a linear correction to the standard calibration. This correction can be largely explained by the Zeeman broadening of the absorption line.

In-orbit Calibrations of the Ultraviolet Imaging Telescope

The Ultra-Violet Imaging Telescope (UVIT) is one of the payloads in ASTROSAT, the first Indian Space Observatory. The UVIT instrument has two 375 mm telescopes: one for the far-ultraviolet (FUV) channel (1300–1800 A), and the other for the near-ultraviolet (NUV) channel (2000–3000 A) and the visible (VIS) channel (3200–5500 A). UVIT is primarily designed for simultaneous imaging in the two ultraviolet channels with spatial resolution better than 18, along with provisions for slit-less spectroscopy in the NUV and FUV channels. The results of in-orbit calibrations of UVIT are presented in this paper.

In-orbit performance of UVIT and first results

The performance of the ultraviolet telescope (UVIT) on-board AstroSat is reported. The performance in orbit is also compared with estimates made from the calibrations done on the ground. The sensitivity is found to be within

Helicity at Photospheric and Chromospheric Heights

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A HOT COMPANION TO A BLUE STRAGGLER IN NGC 188 AS REVEALED BY THE ULTRA-VIOLET IMAGING TELESCOPE (UVIT) ON ASTROSAT

∼15% of the estimates, and the spatial resolution in the NUV is found to exceed significantly the design value of

The Horizontal Branch Population of NGC 1851 as Revealed by the Ultraviolet Imaging Telescope (UVIT)

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