H$$\alpha $$ full line spectropolarimetry as diagnostics of chromospheric magnetic field

Abstract

Analysis of spectropolarimetric observations of two circular sunspots located close to disk centre in H $${\alpha }$$ (6562.8 A) and Fe i (6569.22 A) is presented in this paper. The corresponding active region numbers are NOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The vector magnetic field at the photosphere is derived through inversion of Stokes profiles of Fe i under Milne–Eddington atmospheric model. The chromospheric vector magnetic field is derived from H $${\alpha }$$ Stokes profiles under weak-field approximation. Azimuthally averaged magnetic field as a function of radial distance from the centre of sunspot at the photosphere and chromosphere are studied. At the photosphere, the radial variation shows a well known behaviour that the total field and the line-of-sight (LOS) component monotonically decrease from centre to the edge of the sunspot and the transverse component initially increases, reaches a maximum close to half the sunspot radius and then decreases. LOS and the transverse components become equal close to half the sunspot radius consistent with the earlier findings. At the chromosphere, all the components of the magnetic field decrease with the sunspot radius. However, the LOS component decreases monotonically whereas the transverse component decreases monotonically up to about 0.6 times the sunspot radius after which it reaches a constant value. Azimuthally averaged magnetic field gradient from photosphere to chromosphere is also presented here.