Observation and Modeling of the Circular Polarization of the Cr i Magnetic-field-induced Transition at 533.03 nm

Abstract

We study the circular polarization of the magnetic-field-induced transition (MIT) between the 3d 5(6 S)4d 7 D 2 and 3d 5(6 S)4p 7 P 4° states of Cr i at 533.03 nm (wavelength in air). The fractional circular polarization V/I of this spectral line resulting from the solution of the radiation transfer problem in a sunspot model permeated by a homogeneous magnetic field of 3 kG shows amplitudes of about 2%. Spectropolarimetric observations of two sunspots were obtained with the Zurich Imaging Polarimeter-3 at the Istituto ricerche solari Aldo e Cele Daccò observatory in Locarno, Switzerland. The observed V/I profiles show approximately antisymmetrical shapes with an amplitude of around 0.1% and 0.2% for the two sunspots. The center of this profile coincides with the wavelengths predicted for the abovementioned MIT. We apply an inversion code to the spectropolarimetric data of the Cr i-permitted lines at 532.91 and 532.98 nm, as well as to the MIT line at 533.03 nm, to infer a stratification of the emitting atmosphere. We compare the V/I profiles synthesized in the inferred atmosphere models with the observations, showing that the observed signal likely corresponds to the MIT line.