Hα StokesV/IFeatures Observed in a Solar Active Region

Abstract

AbstractWe carried out an imaging polarimetry observation of active region NOAA 10596 at 9–11 wavelengths around the $\mathrm{H}\alpha$ line with a ferroelectric liquid-crystal polarimeter during 2004 April 20–26. The latitude of the region was $\mathrm{S8}^{\circ}$, and it passed the central meridian on April 23. On the basis of this observation, the Stokes $V/I$ features of the active region in $\mathrm{H}\alpha$, which are expected to indicate the chromospheric longitudinal magnetic field, were revealed as follows: (1) There is persistent weakening (sometimes reversal) of the Stokes $V/I$ signal at the center of the umbra. This is not due to the actual weakening of the magnetic field strength, but probably due to the peculiarity of the atmospheric structure of the umbral chromosphere. (2) The peak wavelengths of the Stokes $V/I$ profile of the plages and that of the sunspot are systematically different from each other and, therefore, it is difficult to apply the weak-field approximation to estimate the chromospheric magnetic field strength. Both of these results mean that an estimation of the chromospheric magnetic field on the basis of the $\mathrm{H}\alpha$ polarimetry is not straightforward, and that an inversion considering the atmospheric structure in the chromosphere is essentially important.