Simulation of the Formation of a Solar Active Region

Abstract

We present a radiative magnetohydrodynamics simulation of the formation of an active region (AR) on the solar surface. The simulation models the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere. The rise of the magnetic plasma in the convection zone is accompanied by predominantly horizontal expansion. Such an expansion leads to a scaling relation between the plasma density and the magnetic field strength such that B ∝ 1/2 . The emergence of magnetic flux into the photosphere appears as a complex magnetic pattern, which results from the interaction of the rising magnetic field with the turbulent convective flows. Small-scale magnetic elements at the surface first appear, followed by their gradual coalescence into larger magnetic concentrations, which eventually results in the formation of a pair of opposite polarity spots. Although the mean flow pattern in the vicinity of the developing spots is directed radially outward, correlations between the magnetic field and velocity field fluctuations allow the spots to accumulate flux. Such correlations result from the Lorentz-force-driven, counterstreaming motion of opposite polarity fragments. The formation of the simulated AR is accompanied by transient light bridges between umbrae and umbral dots. Together with recent sunspot modeling, this work highlights the common magnetoconvective origin of umbral dots, light bridges, and penumbral filaments.

Evolution of Active Regions

“…The undulatory serpentine flux takes on the global Ω-loop shape of the active region only after successive reconnection events. This flux emergence scenario is also found in the 3D radiative numerical simulation of Cheung et al (2010).…”

“…The top portion of the Ω-loop becomes denser than its surroundings as it approaches the top of the convective zone, its rise stops, and magnetic flux piles up under the photosphere. Here, the flux takes on an undulatory (serpentine) shape due to the magnetic Rayleigh-Taylor instability (Parker, 1966) and the effects of convective flows (Cheung et al, 2008(Cheung et al, , 2010.…”

“…However, the serpentine flux may carry a significant amount of plasma (Cheung et al, 2010) which will drain into the U-loop sections of the serpentine field and which must be unloaded from the field lines before a buoyant emergence into the solar atmosphere can resume following the subphotospheric pile-up. A data driven 3D numerical simulation of the emergence of a serpentine field shows that electric currents build up along the separatix surface that forms along the field, and photospheric shearing motions lead to magnetic reconnection in the bald patch sections (Pariat et al, 2009).…”

What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating?

“…However, one can couple a flux emergence model forming an active region [96,97] that covers the solar interior and the near-surface magneto-convection to a coronal model [68]. Some results of this model have already been discussed in §4f.…”

Magneto-convection

“…(a) Magnetic flux emergence As mentioned already, there are two types of flux emergence simulations: those that start with a coherent, twisted horizontal flux tube [9,[54][55][56][57] and those that start from a minimally structured, uniform, untwisted, horizontal field advected by inflows into the domain at the bottom boundary [10,58]. These two very different approaches show many robust, common features.…”