The Evolution of the Twist Shear and Dip Shear During X-class Flare of 2006 December 13: Hinode Observations

Abstract

The non-potentiality of solar magnetic fields is traditionally measured in terms of a magnetic shear angle, i.e., the angle between the observed and potential field azimuths. Here, we introduce another measure of the shear that has not been previously studied in solar active regions, i.e., the one that is associated with the inclination angle of the magnetic field. This form of the shear, which we call "dip shear," can be calculated by taking the difference between the observed and the potential field inclination. In this Letter, we study the evolution of the dip shear as well as the conventional twist shear in a δ-sunspot using high-resolution vector magnetograms from the Hinode space mission. We monitor these shears in a penumbral region located close to a flaring site during 2006 December 12 and 13. It is found that (1) the penumbral area close to the flaring site shows a high value of the twist shear and dip shear as compared with other parts of the penumbra, (2) after the flare, the value of the dip shear drops in this region while the twist shear tends to increase, (3) the dip shear and twist shear are correlated such that pixels with a large twist shear also tend to exhibit a large dip shear, and (4) the correlation between the twist shear and dip shear is tighter after the flare. The present study suggests that monitoring the twist shear alone during the flare is not sufficient, but we need to monitor it together with the dip shear.