Stokes Profile Reconstruction with the Imaging Vector Magnetograph
We demonstrate that when appropriate account of distortions from the Earth’s atmosphere and instrumental effects are taken, an imaging polarimeter can yield Stokes Spectra which are easily suitable for quantitative polarimetry. We describe here some of the corrections which must be applied to the raw imaging data, including a correction for seeing-produced blur and spatial distortion, in order to recover accurate wavelength-sampled spectra in the Stokes [I, Q, U, V] parameters. We present the comparison between Stokes spectra from the Imaging Vector Magnetograph (“IVM”; Mickey et al. 1996) and co-temporal data from the Advanced Stokes Polarimeter (“ASP”; Lites 1996). We find that (1) the integrated polarization signals for the two instruments, both linear and circular, are well matched when the ASP data are convolved to the IVM’s spectral resolution, (2). The ratios of linear to circular polarization and of the Q, U signals are comparable between the two instruments, (3) The reconstructed [I, Q, U, V] spectra are well matched over the 6302.5A line, although (4) in dark umbral points, the IVM spectra of I are deeper than the ASP spectra, possibly due to a lack of scattered-light correction in the latter. The results demonstrate that vector polarimetry suitable for quantitative magnetic field analysis can be obtained with an imaging polarimeter.
Key wordspolarization sunspots magnetic fields instrumentation: polarimetry
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