Delta-sunspot Formation in Realistic Magnetic Flux Emergence Simulations

Abstract:

Observations revealed that the strongest solar flares tend to occur in complex-shaped active regions called delta-sunspots, often associated with sheared polarity inversion lines (PILs) in between. The formation of delta-spots is, however, not understood well because we cannot probe the subsurface layer with direct optical observations. To this end, we perform a series of flux emergence simulations with using the radiative magnetohydrodynamics code R2D2. This code solves thermal convection of various scales from 100 Mm sized cells to surface granules at the same time. We set a computational box that stretches down to -140 Mm, which is deeper than any previous simulations of this kind, and place a magnetic flux tube at -17 Mm without any artificial triggering of buoyant emergence. It is found that the flux tube is elevated by large-scale convective upflows at two segments and, as a result, a pair of emerging bipolar spots appear on the photosphere. As the emergence continues, the spots of opposite polarities collide against each other and eventually form strongly-packed delta-spots. Each spot shows rotating motion that is driven by the Lorentz force, and between the positive and negative polarities, strongly sheared PILs are created. Above the PIL, a helical flux rope is produced. All these structures are the key elements of flare-productive active regions. Moreover, around the PILs, we detect super-equipartition magnetic fields (exceeding 6000 G), which are produced by the shear motion between the delta-spots. These results indicate that the strong coupling between emerging magnetic flux and background turbulence is a key to generate active regions that are prone to major flares.