Measurement of solar vector magnetic fields using Kodaikanal tower telescope

Abstract

A Stokes polarimeter was built at the Kodaikanal Tower Telescope to study the vector field map of active regions like sunspot using the well known Fe I lines 6301.5 Å and 6302.5 Å lines. The Mueller matrix of the polarimeter was obtained using simple laboratory and field tests. The instrumental polarisation due to the oblique reflections are removed before the observed Stokes profiles were analysed. The advanced Stokes Polarimeter data analysis program (ASP code) was used to invert the corrected Stokes profiles to calculate the vector magnetic field parameters of NOAA 8951. The maximum field strength obtained at the umbra of the sunspot was 2500 Gauss. The variation of the field strength, the line-of-sight angle and the azimuthal angle along a cut which passes through the umbra resembles that of a simple round sunspot. The accuracy in the calculation of the vector field is high in the umbra and low in the outer edge of the penumbra of the observed sunspot. The range of errors in the calculation of the field strength, line-of-sight inclination angle, and the azimuthal angle of the magnetic vector are 20-100 Gauss, 1-5O, and 12-20O respectively. We also find a good fit for the observed Stokes profiles of the Ti I line at 6303.78 Å with a synthetic profile produced using the vector field parameters derived from the Fe I 6301.5 Å and 6302.5 Å lines.