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We present the new results of the two-dimensional numerical experiments onthe cross-sectional evolution of a twisted magnetic flux tube rising from thedeeper solar convection zone (-20,000 km) to the corona through the surface.The initial depth is ten times deeper than most of previous calculationsfocusing on the flux emergence from the uppermost convection zone. We find thatthe evolution is illustrated by the two-step process described below: theinitial tube rises due to its buoyancy, subject to aerodynamic drag due to theexternal flow. Because of the azimuthal component of the magnetic field, thetube maintains its coherency and does not deform to become a vortex roll pair.When the flux tube approaches the photosphere and expands sufficiently, theplasma on the rising tube accumulates to suppress the tube's emergence.Therefore, the flux decelerates and extends horizontally beneath the surface.This new finding owes to our large scale simulation calculating simultaneouslythe dynamics within the interior as well as above the surface. As the magneticpressure gradient increases around the surface, magnetic buoyancy instabilityis triggered locally and, as a result, the flux rises further into the solarcorona. We also find that the deceleration occurs at a higher altitude than inour previous experiment using magnetic flux sheets (Toriumi and Yokoyama). Byconducting parametric studies, we investigate the conditions for the two-stepemergence of the rising flux tube: field strength > 1.5x10^4 G and the twist >5.0x10^-4 km^-1 at -20,000 km depth.

Numerical Experiments on the Two-step Emergence of Twisted Magnetic Flux Tubes in the Sun