SOLAR MAGNETIC-FIELD STUDIES USING THE 12-MICRON EMISSION-LINES .2. STOKES PROFILES AND VECTOR FIELD SAMPLES IN SUNSPOTS

This paper reports a polarimetric study of the extremely Zeeman-sensit ive 12.32 mum neutral magnesium (Mg I) emission line from sunspots. A single blocked impurity band (BIB) detector in a cryogenic grating pos tdisperser was used to limit the McMath Fourier transform spectrometer (FTS) bandpass and obtain high signal/noise spectra at 0.005 cm-1 spe ctral resolution with 4.5'' spatial resolution. A polarization analyze r preceded the FTS and consisted of an antireflection coated CdS 1/4 w aveplate, a thin film Ge linear polarizer, and a second 1/4 waveplate. The instrument polarization introduced by the McMath telescope is sho wn to be negligible for the purpose of 12 mum polarimetry. Stokes I, Q , U, V profiles were generated, with a time resolution of 12 minutes, by subtracting successive interferograms. Within the sunspot the Zeema n triplet was fully resolved. Vector fields were derived by nonlinear least squares fits of the Seares formulae to the observed Stokes profi les. The adequacy of the fit procedure was established by comparison t o complete Stokes radiative transfer calculations, based on recent NLT E line formation results for the 12.32 mum transition. The visually sy mmetric sunspot in the region NOAA/USAF group 5754 was observed over t he period 1989 October 23-28 and the complex sunspot region NOAA/USAF group 5014 over the period 1988 May 10-15. The 5754 sunspot field stre ngth varied from 2050 G in the umbra to 650 G at the outer penumbral e dge, with the field extending well beyond the photometric boundary of the sunspot. Vector magnetograms obtained by the Haleakala Stokes pola rimeter, operating at 6302.5 angstrom, show an umbral field strength w hich is larger by 400 G. The magnetic data indicate the height of form ation for the Mg I line to be approximately 600 km above tau = 1 for t he 6302.5 angstrom line. Unusual penumbral field inclinations are seen in the 12 mum data, and these may represent the magnetic signatures o f X-ray loops which have recently been observed to originate in the pe numbra. The ratio of the polarized 12 mum emission (Q2 + U2 + V2)1/2 t o the total intensity (I) is found to be unity within the umbra and mo st of the penumbra, decreasing to 0.9 at the outer edge of the penumbr a. This implies that the sunspot magnetic field at the 12 mum height i s not filamentary in the sense of containing field-free regions, nor i s there cancellation of field over any spatial scale in the beam area. We reiterate our previous conclusion that the widths of the 12 mum si gma-components are caused by spatially unresolved magnetic field stren gth variations of order +/-250 G.