A Study of Preflare Solar Coronal Magnetic Fields: Magnetic Energy and Helicity

Solar flares fall into two types, with eruptive ones associated with coronal mass ejection (CME) and confined ones without CME. To explore whether there are preflare conditions in terms of magnetic energy and helicity that can effectively determine the types of flares, here we analyzed a suite of related parameters of the reconstructed preflare coronal magnetic field of major solar flares, either eruptive or confined, from 2011 to 2017 near the solar disk center. The investigated parameters include extensive-type quantities such as the total magnetic energy E _T , the potential energy E _P , the free energy E _F , the relative helicity H _R , and the nonpotential helicity H _J , as well as the intensive-type indices E _F /E _P , ∣H _J /H _R ∣, ∣ H R / ϕ ′ 2 ∣ , and ∣ H J / ϕ ′ 2 ∣ , where ϕ ′ is half of the total unsigned magnetic flux. We have the following key findings: (1) None of the extensive parameters can effectively distinguish the eruptive and confined potential of the preflare coronal fields, though the confined events have averagely larger values. (2) All the intensive parameters have significantly larger average and median values for eruptive flares than the confined events, which indicates that the field for eruptive flares has overall higher degree of nonpotentiality and complexity than that of the confined flares. (3) The energy ratio E _F /E _P and the normalized nonpotential helicity ∣ H J / ϕ ′ 2 ∣ , which are strongly correlated with each other, have among the highest capability of distinguishing the fields that possibly produce a major eruptive or confined flare, as over 75% of all the events are successfully discriminated between eruptive and confined flares by using critical values of E _F /E _P ≥ 0.27 and ∣ H J / ϕ ′ 2 ∣ ≥ 0.009 .