The role of cancelling magnetic fields in the buildup to erupting filaments and flares

Abstract

We present a scenario for understanding the role of cancelling magnetic fields in the build-up to eruptive solar flares. The key intermediate step in this scenario involves the formation of a filament magnetic field in the corona above a photospheric polarity inversion where cancelling magnetic fields are observed. The formation of a filament magnetic field is accomplished in several recent models by first interpreting the cancelling fields as a visible effect of a slow, steady magnetic reconnection. This reconnection results in a reconfiguring of the magnetic field; line-of-sight pairs of closely-spaced opposite-polarity fields disappear from the photosphere thereby accounting for the cancellation; simultaneously the horizontal component is increased in the corona above the polarity inversion. The new and increasing horizontal component is synonymous with the building of a magnetic field where mass can accumulate to forma filament. If the magnetic reconnection continues for a sufficient length of time, the changing equilibrium between the growing filament magnetic field and the overlying, coronal magnetic field will result in a very slow, simultaneous ascent of both the filament magnetic field and the overlying coronal magnetic field with greater motion in the outer, weaker coronal field. This upward stretching of the magnetic fields eventually results in a closer spacing of oppositely-directed corona) magnetic fields (resembling a tangential discontinuity) beneath the filament. As depicted in some flare models, magnetic reconnection then suddenly occurs in the corona beneath the filament; flare loops form in the lower part of the reconnected field and a corona) mass ejection and erupting filament comprise the upper part of the reconnected field. To illustrate the observable phases of this scenario, we describe the build-up to two simple eruptive flares in a small active region.