On the Origin of Differences in Helicity Parameters Derived from Data of Two Solar Magnetographs

Abstract

We analyzed how sensitivity and accuracy in solar magnetic field measurements may affect the values of mean current helicity density $h_{\mathrm{c}}$ and twist parameter $\alpha_{\mathrm{av}}$ by comparing these values obtained from two magnetographs (SMFT at Beijing and SFT at Mitaka, Tokyo). When we computed the helicity parameters from the SFT data, we replaced the values of the longitudinal field component, transverse field strength, and transverse field azimuth angle with those from the SMFT data and examined the differences. The results show that the correlation coefficient and the fraction of the data that agree in signs of $h_{\mathrm{c}}$ or $\alpha_{\mathrm{av}}$ increase when an SFT parameter is substituted by the corresponding SMFT parameter because one source of discrepancy is removed. The increase in correlation coefficient is largest when the azimuthal angles and transverse field strengths are set identical in the two instruments; the correlation coefficient of $h_{\mathrm{c}}$ ( $\alpha_{\mathrm{av}}$ ) increases from 0.74 (0.56) to 0.86 (0.78), respectively, indicating that the differences in the transverse field strength and its azimuthal angle are the largest source of discrepancy in the values of $h_{\mathrm{c}}$ or $\alpha_{\mathrm {av}}$ . We found a nonlinear relationship in the components of the magnetic field between the two instruments for some data samples; we conclude that this is due to the discrepancy in the calibration procedure between the two instruments. This nonlinearity can be another source of difference in determining helical parameters between the two instruments.