Using relative field line helicity as an indicator for solar eruptivity

Abstract

Context. Relative field line helicity (RFLH) is a recently developed quantity that can approximate the density of relative magnetic helicity. Aims. This paper aims to determine whether RFLH can be used as an indicator of solar eruptivity. Methods. Starting from magnetographic observations from the Helioseismic and Magnetic Imager instrument on board the Solar Dynamic Observatory of a sample of seven solar active regions (ARs), that comprises over 2000 individual snapshots, we reconstruct the AR’s coronal magnetic field with a widely used non-linear force-free method. This enables us to compute the RFLH using two independent gauge conditions for the vector potentials. We focus our study on the times of strong flares in the ARs, above the M class, and in regions around the polarity inversion lines (PILs) of the magnetic field, and of RFLH. Results. We find that the temporal profiles of the relative helicity that is contained in the magnetic PIL follow those of the relative helicity that is computed by the accurate volume method for the whole AR. Additionally, the PIL relative helicity can be used to define a parameter similar to the well-known parameter R, whose high values are related with increased flaring probability. This helicity-based R-parameter correlates closely with the original parameter, showing in some cases even higher values. Additionally, it experiences more pronounced decreases during flares. This means that there exists at least one parameter deduced from RFLH that is important as a solar eruptivity indicator.