On the Estimate of Magnetic Non-potentiality of Sunspots Derived Using Hinode SOT/SP Observations: Effect of Polarimetric Noise

Abstract

The accuracy of Milne-Eddington (ME) inversions, used to retrieve the magnetic field vector, depends upon the signal-to-noise ratio (S/N) of the spectro-polarimetric observations. The S/N in real observations varies from pixel to pixel; therefore the accuracy of the field vector also varies over the map. The aim of this work is to study the effect of polarimetric noise on the inference of the magnetic field vector and the magnetic non-potentiality of a real sunspot. To this end, we use the Hinode SOT/SP vector magnetogram of a real sunspot NOAA 10933 as an input to generate synthetic Stokes profiles under ME model assumptions. We then add normally distributed polarimetric noise of the level 0.5% of continuum intensity to these synthetic profiles and invert them again using the ME code. This process is repeated 100 times with different realizations of noise. It is found that within most of the sunspot areas (>90% area) the spread in the (1) field strength is less than 8 G, (2) field inclination is less than 1°, and (3) field azimuth is less than 5°. Further, we determine the uncertainty in the magnetic non-potentiality of a sunspot as determined by the force-free parameter α _g and spatially averaged signed shear angle (SASSA). It is found that for the sunspot studied here these parameters are α _g = -3.5 ± 0.37(×10^-9 m^-1) and SASSA = -1.68 ± 0fdg014. This suggests that the SASSA is a less dispersed non-potentiality parameter as compared to α _g . Further, we examine the effect of increasing noise levels, viz. 0.01%, 0.1%, 0.5%, and 1% of continuum intensity, and find that SASSA is less vulnerable to noise as compared to the α _g parameter.