Full Stokes parameter mapping of magnetic fields in active regions has recently become possible in the mid-infrared. The cryogenic spectrometer Celeste on the McMath-Pierce telescope has produced the first 12 µm measurement of all four Stokes parameters (I, V, U, and Q) in sunspots. The magnesium (Mg I) line at 12.3 µm wavelength exhibits a large Zeeman splitting that is resolved at field strengths above a few hundred gauss. In general, fields measured with this line originate in the upper photosphere at heights above those measured in the visible and near infrared. The measurements are made by optically selecting each Stokes parameter in sequence using 1/2- and 1/4-wave plates, followed by a chopping linear polarizer.
The spectra are recorded with Celeste, a high-resolution liquid helium cooled grating spectrometer built by NASA Goddard Space Flight Center. Individual measurements record the Mg I spectrum at each point along a 2.4' slit. Data cubes of two spatial dimensions, one spectral dimension are created for each Stokes parameter by stepping the slit across the portion of the Sun being imaged.
In general, the Stokes parameter I is dominated by the solar continuum. Images in the V, U, and Q Stokes parameter show structure that is not apparent in the continuum image. The complete vector magnetic field can be constructed by combining the azimuth, elevation, and strength information contained in this type of data.
This research is a NASA-sponsored effort to exploit the 12 µm Mg I line in measuring magnetic structure. The unique infrared capability of the McMath-Pierce telescope, and its large aperture, make this work possible. This investigation is a collaboration between Goddard Space Flight Center (D. Jennings, D. Deming, G. McCabe, and T. Moran), Dickinson College (R. Boyle), and the Universidad de Monterrey, Mexico (P. Sada).
|Caption: Images in the Stokes parameters I, V, U, and Q in an isolated sunspot that was located near the center of the solar disk. The four images shown are each approximately 1.0 x 1.7 arcminutes in size. Each image is a slice of a data cube at a wavelength corresponding to a Zeeman splitting from a field of 1430 gauss. The maximum field observed in this sunspot was 1970 gauss.|