Design concepts of EUV polarimeter

Abstract

The solar corona is a complex and dynamic region primarily controlled by the magnetic fields. Hence the routine measurement of these fields is crucial. Magnetic field information remains embedded in the polarization profiles of the spectral lines, which can be quantified through spectropolarimetry. Differential Hanle effect is a promising diagnostic tool to probe weak coronal magnetic fields and their vector properties. In a recent study we have found several combinations of spectral lines located in the wavelength range 100 to 1210 ˚A suitable for the above purpose whose critical Hanle field lies in the range of 0.01-200 Gauss. Since the background emission from the Sun’s underlying layers, such as the photosphere and the chromosphere, is almost negligible at these short wavelengths, the coronagraphic technique is not required to study the solar corona, thereby enabling both on-disk and off-limb measurements. Yet there is no EUV polarimeter built for solar observations. In this context we explored a few polarimeter concepts based on the configurations of two and three reflecting mirrors. The performances of these configurations are studied in terms of their throughput and polarizing power using Zemax software in the wavelength range 740-780 ˚A. They are also compared with different coating materials namely, BaF2, MgF2 and LiF. It is found that BaF2 has significantly higher throughput and polarizing power in the above stated wavelength range.