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Context. Magnetic helicity is a physical quantity of great importance in thestudy of astrophysical and natural plasmas. Although a density for helicitycannot be defined, a good proxy for it is field line helicity. The appropriatequantity for use in solar conditions is relative field line helicity (RFLH).Aims. This work aims to study in detail the behaviour of RFLH, for the firsttime, in a solar active region (AR). Methods. The target active region is thelarge, eruptive AR 11158. In order to compute RFLH and all other quantities ofinterest we use a non-linear force-free reconstruction of the AR coronalmagnetic field of excelent quality. Results. We find that the photosphericmorphology of RFLH is quite different than that of the magnetic field or of theelectrical current, and this is not sensitive to the chosen gauge in thecomputation of RFLH. The value of helicity experiences a large decrease, 25% ofits pre-flare value, during an X-class flare of the AR, a change that is alsodepicted in the photospheric morphology of RFLH. Moreover, the area of thischange coincides with the area that encompasses the flux rope, the magneticstructure that later erupted. Conclusions. The use of RFLH can provideimportant information about the value and location of the magnetic helicityexpelled from the solar atmosphere during eruptive events.

Relative field line helicity of a large eruptive solar active region