Magnetic Helicity and Free Magnetic Energy as Tools to Probe Eruptions in two Differently Evolving Solar Active Regions

Using vector magnetograms from the HMI/SDO and a magnetic connectivity-based method, we calculate the instantaneous relative magnetic helicity and free magnetic energy budgets for several days in two solar active regions (ARs), AR11890 and AR11618, both with complex photospheric magnetic field configurations. The ARs produced several major eruptive flares while their photospheric magnetic field exhibited primarily flux decay in AR11890 and primarily flux emergence in AR11618. Throughout much of their evolution both ARs featured substantial budgets of free magnetic energy and of both positive and negative helicity. In fact, the imbalance between the signed components of their helicity was as low as in the quiet Sun and their net helicity eventually changed sign 14-19 hours after their last major flare. Despite such incoherence, the eruptions occurred at times of net helicity peaks that were co-temporal with peaks in the free magnetic energy. The losses associated with the eruptive flares in the normalized free magnetic energy were in the range 10-60%. For the helicity, changes ranged from 25% to the removal of the entire excess helicity of the prevailing sign, leading a roughly zero net helicity, but with significant equal and opposite budgets of both helicity senses. The removal of the slowly varying background component of the free energy and helicity timeseries revealed that all eruption-related peaks of both quantities exceeded the 2$\sigma$ levels of their detrended timeseries. There was no eruption when only one or none of these quantities exceeded its 2$\sigma$ level. Our results indicate that differently evolving ARs may produce major eruptive flares even when, in addition to the accumulation of significant free magnetic energy budgets, they accumulate large amounts of both negative and positive helicity without a strong dominance of one handedness over the other.Comment: 17 pages, 11 figures, accepted for publication in Astronomy & Astrophysics (abbreviated abstract