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Multi-Height Spectropolarimetry of Sunspots with FIRS and IBIS

The effects of radiative transfer prevent the characterization of the magnetic field at a single geometric height in the photosphere of a sunspot. Therefore, a full 3D characterization of the magnetic field is necessary to understand many properties of sunspots, such as the true state of hydrostatic equilibrium. Many current and proposed solar spectropolarimeters are capable of taking near-simultaneous observations at multiple wavelengths. Combining these rich datasets provides a welcome problem to the community. We present joint observations of the magnetically sensitive photospheric Fe I lines at 6302 and 15650 A taken with the Facility Infrared Spectropolarimeter (FIRS); and the chromospheric Ca II line at 8542 A taken with the Interferometric Bidimensional Spectrometer (IBIS); both instruments operated at the Dunn Solar Telescope. These wavelengths allow us to probe the magnetic field over a broad range of heights, from the bottom of the photosphere to the chromosphere. We investigate the magnetic field topologies of two sunspots of different magnetic complexity from the same active region. The Interferometric Bidimensional Spectrometer (IBIS) is a F a bry-P e rot i magingspectropolarimeter (Cavallini, 2006) which builds up spectra by stepping the narrow filter bandpass in the wavelength dimension. IBIS has an 80” diameter field of view which is masked to 40” in one direction when the polarimeters are in use, with 0.16”/pixel sampling. The wavelength tuning was set up to observe a 2.5 A bandpass around the chromospheric Ca II line at 8542 A sampled with 0.08 A steps in scans lasting ~36 sec. IBIS was run continuously during the FIRS scans, so we have selected only the first scan for this analysis. The FIRS slit was oriented in a direction perpendicular to the horizon so that the displacement due to atmospheric refraction was in the direction along the slit. The figure to the left shows the relative size and alignment of the FIRS and IBIS fields on the active region. Reduction Data reduction for FIRS data was carried out as in Jaeggli (2012). All infrared images were first corrected for the detector linearity. Darks and line-subtracted flat-fields were applied to the observations. The observed polarization states were demodulated to obtain the Stokes vector. The spectra from the four slits were geometrically corrected, and the two beams were combined to reduce spurious signals from seeing effects. Instrumental polarization cross-talk was corrected via the Kuhn Method (Kuhn, 1994) based on the expected polarization properties of the Stokes line profiles in the sunspots. The telluric lines were removed from the 6302 A spectra. IBIS data reduction was somewhat more complicated. Apart from the standard dark and flat-field corrections, the IBIS observations were speckle reconstructed to account for image motion due to seeing, and a polarization correction based on the telescope geometry and polarization calibration was applied to the Stokes spectra. The IBIS spectra were resampled to be linear in wavelength.