The Three-Dimensional Structure of a Sunspot Magnetic Field

Here we report on observations of the three-dimensional structure of a sunspot magnetic field from the photosphere to the chromosphere, obtained with the new visible/infrared spectro-polarimeter SPINOR. The observations, interpreted with a non-LTE modeling technique, reveal a surprisingly complex topology with areas of opposite-sign torsion, suggesting that flux-ropes of opposite helicities may coexist together in the same spot. Subject headings: line: profiles – Sun: atmosphere – Sun: magnetic fields – Sun: chromosphere The observations that we used in this work are first-light data from the new instrument SPINOR (Spectro-Polarimeter for INfrared and Optical Regions, Socas-Navarro et al. 2005). Still under development, SPINOR can already be used for high-resolution full spectropolarimetry at virtually any combination of 3 spectral regions in the 400 1000 nm range. The particular dataset that we report on was acquired on 16 June 2004 at 15:16 UT. We observed two photospheric Fe I lines (at 849.7 and 853.8 nm) and two chromospheric lines of the Ca II infrared triplet (at 849.8 and 854.2 nm) in active region NOAA 0634 at a time of particularly good seeing. The spectrograph slit was scanned over that region to construct a three-dimensional datacube, in such a way that for each (x,y,λ) point we have the four Stokes parameters I, Q, U and V . We made use of the new adaptive optics system (Ren et al. 2003) of the Dunn Solar Telescope. Combined with the excellent atmospheric conditions at the time of the observations, we achieved a spatial resolution as good as 0.6” (note that this figure varies slightly in the scanning direction due to temporal changes in the seeing conditions), which is among the best attained thus far in this kind of observations. The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation.