Light Bridge in a Developing Active Region. II. Numerical Simulation of Flux Emergence and Light Bridge Formation

TL;DR

This paper uses numerical simulations to explore how light bridges form in sunspot regions, revealing the role of convective upflows and magnetic shear in creating structures conducive to magnetic reconnection and activity events.

Contribution

It provides a detailed physical model of light bridge formation through radiative magnetohydrodynamics simulations, linking observational features with magnetic and velocity structures.

Findings

Convective upflows transport horizontal magnetic fields to the surface.

Magnetic shear creates a cusp-shaped current layer above the bridge.

Simulation results align with observational data, supporting the proposed formation mechanism.

Abstract

Light bridges, the bright structure dividing umbrae in sunspot regions, show various activity events. In Paper I, we reported on analysis of multi-wavelength observations of a light bridge in a developing active region (AR) and concluded that the activity events are caused by magnetic reconnection driven by magnetconvective evolution. The aim of this second paper is to investigate the detailed magnetic and velocity structures and the formation mechanism of light bridges. For this purpose, we analyze numerical simulation data from a radiative magnetohydrodynamics model of an emerging AR. We find that a weakly-magnetized plasma upflow in the near-surface layers of the convection zone is entrained between the emerging magnetic bundles that appear as pores at the solar surface. This convective upflow continuously transports horizontal fields to the surface layer and creates a light bridge structure. Due to the magnetic shear between the horizontal fields of the bridge and the vertical fields of the ambient pores, an elongated cusp-shaped current layer is formed above the bridge, which may be favorable for magnetic reconnection. The striking correspondence between the observational results of Paper I and the numerical results of this paper provides a consistent physical picture of light bridges. The dynamic activity phenomena occur as a natural result of the bridge formation and its convective nature, which has much in common with those of umbral dots and penumbral filaments.