Two-dimensional magnetohydrodynamic model of emerging magnetic flux in the solar atmosphere

Abstract

A two-dimensional magnetohydrodynamic code is used to study the nonlinear evolution of the undular mode of the magnetic buoyancy instability (the Parker instability) in an isolated horizontal magnetic flux embedded in a two-temperature layered atmosphere (solar corona-chromosphere/photosphere). The numerical results are analyzed in detail and compared with observed data for emerging magnetic flux in the solar atmosphere. The flux sheet with 0 ~ 1 is initially located at the bottom of the photosphere. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of enhanced magnetic buoyancy. This expanding magnetic flux is identified as the emerging magnetic flux. In the nonlinear regime of the instability, the expansion of the magnetic loop (arch) shows self-similar behavior: the rise velocity of a magnetic loop and the local Alfvén speed at the top of the loop increase linearly with height as long as the magnetic pressure of the loop is larger than the coronal total pressure. The rise velocity of the magnetic oop in the high chromosphere (h-4000-6000 km) is 10-15 km s" 1 , and the velocity of downflow along the loop is 30-50 km s , both of which are consistent with observed values for arch filament systems. Numerical results also explain the small rise velocity of emerging magnetic flux in the photosphere, and strong down-drafts near photospheric pores. The effects of interaction and reconnection with overlying magnetic fields in the network or in a preexisting active region are also investigated.