We discuss spectropolarimetric measurements of photospheric (Fe I 630.25 nm) and chromospheric (Ca II 854.21 nm) spectral lines. Our long-term goal is to diagnose properties of the magnetic field near the base of the corona. We compare ground-based two-dimensional spectropolarimetric measurements with (almost) simultaneous space-based slit spectropolarimetry. The ground-based observations were obtained May 20, 2008, with IBIS in spectropolarimetric mode, The space observations were obtained with the Spectro-Polarimeter aboard the HINODE satellite. The agreement between the near-simultaneous co-spatial IBIS and HINODE Stokes-V profiles at 630.25 nm is excellent, with V/I amplitudes compatible with to within 1 %. IBIS QU measurements are affected by residual crosstalk from V, arising from calibration inaccuracies, not from any inherent limitation of imaging spectroscopy. We use a PCA analysis to quantify the detected cross talk. Chromospheric magnetic fields are difficult to constrain by polarization of Ca II lines alone. However, we demonstrate that high cadence, high angular resolution monochromatic images of fibrils in Ca II and H-alpha, can be used to improve the magnetic field constraints, under conditions of high electrical conductivity. Such work is possible only with time series datasets from two-dimensional spectroscopic instruments under conditions of good seeing.
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