Comparison analysis of current helicity inferred from several solar vector magnetographs

Since the 1990s, great progress has been made in study of current helicity in solar photosphere. The statistical results of large samples reveal an important trend of current helicity distribution: active regions in solar surface mainly show negative sign of current helicity in northern hemisphere and positive in southern hemisphere. In this paper. using vector magnetograms from different vector magnetographs, we analyzed the variation of current helicity density h(c) with magnetic field and discussed the reasons for differences among different instruments. We divided longitudinal magnetic field B-z and total magnetic field B into six intervals and calculated h(c) in each interval. The statistical study show that: for B-z was less than 600 G, there was a good consistency of h(c) inferred from different data sets, and h(c) obeyed the hemispheric sign rule (HSR); for B-z was larger than 600 G, the discrepancy among different data sets increased and k disobey the HSR in some B-z intervals; the HSR was better followed in southern hemisphere. The variation trend of h(c) with B was similar to that of B. Using the same method, we studied individual active region and analyzed the influence of magnetic optical effect on helicity calculation. Through the above analysis, we conclude that the calculated helicity parameters are basically believable when B-z is less than 600 G. The real reasons for the discrepancy among different instruments when B-z is larger than 600 G need to be further studied.