Magnetic helicity and energy of emerging solar active regions and their erruptivity

Abstract

Aims. We investigate the role of the accumulation of magnetic helicity and magnetic energy in the generation of coronal mass ejections (CMEs) from emerging solar active regions (ARs). Methods. Using vector magnetic field data obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we calculated the magnetic helicity and magnetic energy injection rates as well as the resulting accumulated budgets in 52 emerging ARs from the start time of magnetic flux emergence until they reached a heliographic longitude of 45 West (W45). Results. Seven of the ARs produced CMEs, but 45 did not. In a statistical sense, the eruptive ARs accumulate larger budgets of magnetic helicity and energy than the noneruptive ARs over intervals that start from the flux emergence start time and end (i) at the end of the flux emergence phase and (ii) when the AR produces its first CME or crosses W45, whichever occurs first. We found magnetic helicity and energy thresholds of 9×1041 Mx2 and 2×1032 erg. When these thresholds were crossed, ARs are likely to erupt. In terms of accumulated magnetic helicity and energy budgets, the segregation of the eruptive from the noneruptive ARs is violated in one case when an AR erupts early in its emergence phase and in six cases in which noneruptive ARs exhibit large magnetic helicity and energy budgets. Decay index calculations may indicate that these ARs did not erupt because the overlying magnetic field provided a stronger or more extended confinement than in eruptive ARs. Conclusions. Our results indicate that emerging ARs tend to produce CMEs when they accumulate significant budgets of both magnetic helicity and energy. Any study of their eruptive potential should consider magnetic helicity together with magnetic energy.