A comparative study of solar flux emergence and eruptivity in simulations of horizontal versus toroidal magnetic fields

Abstract

Context: Magnetic flux emergence is a fundamental driver of eruptive activity in the solar atmosphere. While many numerical studies employed idealized horizontal flux tubes, toroidal tubes provide a more realistic geometry for finite emerging loops with anchored footpoints. Aims: We compare the evolution and eruptive capability of horizontal and toroidal flux tubes under identical initial parameters. Methods: We performed 3D resistive magnetohydrodynamic (MHD) simulations of the emerging magnetic flux structures to evaluate their respective dynamics Results: Although the toroidal tube emerges later than in the horizontal case, it produces a higher frequency of eruption-driven jets (four versus two) because the supply of coronal axial flux is sustained. In contrast, the horizontal tube injects magnetic flux and energy more impulsively, driving stronger but less persistent activity and then rapidly stagnating when its atmospheric axial-flux reservoir is depleted. Free magnetic energy builds up after emergence and is released in discrete drops associated with eruptions. The toroidal case exhibits a quasi-cyclic buildup and release pattern, whereas the horizontal case relaxes to a lower-activity state after its early eruptions. The temporal evolution of relative magnetic helicity mirrors the free-energy evolution. Helicity increases with the stressing and twisting of the coronal field during emergence, peaks near eruptive episodes, and decreases as eruptions remove twisted flux, with the toroidal tube maintaining a more persistent helicity budget that supports recurrent events. Conclusions: Initial flux-tube geometry strongly controls the coronal flux budget and the storage and release of free energy and helicity, and therefore, the frequency and longevity of eruptive phenomena in emergence-driven active regions.