  Context. Relative field line helicity (RFLH) is a recently developed quantity
which 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 onboard the Solar Dynamic
Observatory of a sample of seven solar active regions (ARs), which 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
RFLH using two independent gauge conditions for the vector potentials. We focus
our study around 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 (Schrijver 2007), whose high values are
related with increased flaring probability. This helicity-based R-parameter
correlates well with the original one, showing in some cases even higher
values, and additionally, it experiences more pronounced decreases during
flares. This means that there exists at least one parameter deduced from RFLH,
that has important value as a solar eruptivity indicator.
