ON THE DOPPLER SHIFT AND ASYMMETRY OF STOKES PROFILES OF PHOTOSPHERIC Fe I AND CHROMOSPHERIC Mg I LINES

We analyzed the full Stokes spectra using simultaneous measurements of the photospheric (Fe I 630.15 and 630.25 nm) and chromospheric (Mg I b 2 517.27 nm) lines. The data were obtained with the High Altitude Observatory/National Solar Observatory (HAO/NSO) advanced Stokes polarimeter, about a near disk center sunspot region, NOAA AR 9661. We compare the characteristics of the Stokes profiles in terms of Doppler shifts and asymmetries among the three spectral lines, helping us to better understand the chromospheric lines and the magnetic and flow fields in different magnetic regions. The main results are: (1) for the penumbral area observed by the photospheric Fe I lines, Doppler velocities derived from Stokes I (ν i) are very close to those derived from linear polarization profiles (νlp) but significantly different from those derived from Stokes V profiles (νzc), thus providing direct and strong evidence that the penumbral Evershed flows are magnetized and mainly carried by the horizontal magnetic component; (2) the rudimentary inverse Evershed effect observed by the Mg I b 2 line provides qualitative evidence on its formation height that is around or just above the temperature minimum region; (3) νzc and νlp in the penumbrae and νzc in the pores generally approach their ν i observed by the chromospheric Mg I line, which is not the case for the photospheric Fe I lines; (4) the outer penumbrae and pores show similar Stokes V asymmetry behavior that tend to change from positive values in the photosphere (Fe I lines) to negative values in the low chromosphere (Mg I line); (5) the Stokes V profiles in plage regions are highly asymmetric in the photosphere and more symmetric in the low chromosphere; and (6) strong redshifts and large asymmetries are found around the magnetic polarity inversion line within the common penumbra of the δ spot. We offer explanations or speculations to the observed discrepancies between the photospheric and chromospheric lines in terms of the three-dimensional structure of the magnetic and velocity fields. This study thus emphasizes the importance of spectropolarimetry using chromospheric lines.