How to identify important magnetic helicity locations in solar active regions

Magnetic helicity is a physical quantity of great importance in the study of magnetized plasmas as it is conserved in ideal magneto-hydrodynamics and slowly deteriorating in non-ideal conditions such as magnetic reconnection. A meaningful way of defining a density for helicity is with field line helicity, which, in solar conditions, is expressed by relative field line helicity (RFLH). Here, we study in detail the behaviour of RFLH in the large, well-studied, eruptive solar active region (AR) 11158. The computation of RFLH and of all other quantities of interest is based on a high-quality non-linear force-free reconstruction of the AR coronal magnetic field, and on the recent developments in its computational methodology. The derived photospheric morphology of RFLH is very different than that of the magnetic field or the electrical current, and also manages to depict the large decrease in the value of helicity during an X-class flare of the AR. Moreover, the area of the RFLH decrease coincides with the location of the magnetic structure that later erupted, the flux rope. Based on these results we review the necessary steps one needs to follow in order to identify the locations in an AR where magnetic helicity is more important. This task can provide crucial information for the conditions of an AR, especially during eruptive events.