Helicity computation using observations from two different polarimetric instruments

We compare vector magnetograms of active region NOAA 5747 observed by two very different polarimetric instruments: the imaging vector magnetograph of Huairou Solar Observing Station (HSOS) and the Haleakala Stokes Polarimeter of Mees Solar Observatory (MSO). Unlike previous comparative studies, we concentrate our attention on differences in observations and data reduction techniques that can affect the helicity computation. Overall, we find a qualitative agreement between the HSOS and MSO vector magnetograms. The HSOS data show slightly higher field strength, but the distribution of inclination angles is similar in measurements from the two instruments. There is a systematic difference (up to similar to 20 degrees) in the azimuths of transverse fields, which is roughly proportional to the longitudinal field strength. We estimate that Faraday rotation in the HSOS magnetograms contributes similar to 12 degrees in the azimuth difference if possible sources of error are taken into account. Next, we apply two independent methods to both data sets to resolve 180 degrees azimuth ambiguity and to compute two helicity measures - the force-free field parameter alpha(best) and the current helicity fractional imbalance rho(h). The methods agree reasonably well in sign and value of the helicity measures, but the HSOS magnetograms show systematically smaller values of rho(h) and alpha(best) in agreement with an expected contribution of Faraday rotation. Finally, we discuss the role of Faraday rotation in computation of alpha(b)est and rho(h) and conclude that it does not affect the strength of the hemispheric helicity rule. The strength of the rule appears to be related to a helicity parameter: alpha(best) shows weaker hemispheric asymmetry than rho(h).