A comparison of vector magnetograms from the marshall space flight center and mees solar observatory

We compare completely independent vector magnetic field measurements from two very different polarimetric instruments. The Marshall Space Flight Center's imaging vector magnetograph is based on a birefringent filter, routinely measuring all four Stokes parameters integrated over the filter bandpass (1/8 Å) which is tunable across the Fei 5250 line in 10 mÅ steps. The Haleakala Stokes Polarimeter of the Mees Solar Observatory (MSO) is based on a spectrometer, routinely measuring all four Stokes parameters of the Fei 6302.5 line simultaneously and then spatially scanning to build up a vector magnetogram. We obtained active region magnetic field data with both the Marshall Space Flight Center (MSFC) and MSO systems on five days during June 1985. After interpolating the MSFC vector fields onto the more coarse spatial grid of MSO we make a point-by-point comparison of the two vector fields for data obtained on two of these days (June 8 and 9). From this comparison we conclude: (1) the spatially-averaged line-of-sight components agree quite well; (2) although the MSO spatial grid is coarser, the quality of the MSO image is better than that of the MSFC data because of better seeing conditions; (3) the agreement between the transverse magnitudes is affected by the poor image quality of the MSFC data; and (4) if the effects of Faraday rotation caused by including line-center linear polarization in the method of analysis are taken into account, the azimuths show good agreement within the scatter in the data caused by the averaging process.National Research Council Resident Research Associate.