Two-step Emergence of the Magnetic Flux Sheet from the Solar Convection Zone

TL;DR

This study uses 2D MHD simulations to explore how magnetic flux sheets rise through the solar convection zone and photosphere, revealing a two-step emergence process crucial for active region formation.

Contribution

It introduces a two-step emergence model based on simulations, identifying conditions under which magnetic flux rises to form active regions.

Findings

Flux sheets decelerate beneath the photosphere due to plasma pile-up.

Local Parker instability triggers further emergence into the corona.

Active region fluxes likely undergo the two-step emergence process.

Abstract

We perform two-dimensional MHD simulations on the solar flux emergence. We set the initial magnetic flux sheet at z=-20,000 km in the convection zone. The flux sheet rises through the convective layer due to the Parker instability, however, decelerates beneath the photosphere because the plasma on the flux sheet piles up owing to the convectively stable photosphere above. Meanwhile, the flux sheet becomes locally unstable to the Parker instability within the photosphere, and the further evolution to the corona occurs (two-step emergence model). We carry out a parameter survey to investigate the condition for this two-step model. We find that magnetic fluxes which form active regions are likely to have undergone the two-step emergence. The condition for the two-step emergence is 10^21 - 10^22 Mx with 10^4 G at z=-20,000 km in the convection zone.