This paper is intended to study the evolution of a magnetic flux tube that rises from the upper convection zone to the solar atmosphere by means of a 2.5-dimensional MHD simulation with the focus on the cross section of the flux tube. A cylindrical flux tube placed horizontally in the convection zone starts rising by magnetic buoyancy. When the top of the tube reaches the photosphere, the cross section of the tube changes from the circular shape to horizontally extended shape, forming a magnetic layer under the contact surface between the tube and the photosphere. As the plasma inside that magnetic layer is squeezed out to both sides of the layer, the contact surface is locally subject to the Rayleigh-Taylor instability because the lighter magnetic layer is overlain by the heavier photospheric layer. The wavelength of the undulating magnetic layer at the contact surface increases as the flattening of the tube proceeds, and after it becomes longer than the critical wavelength for the Rayleigh-Taylor instability, the tube can emerge through the photosphere. The emergence part of the tube starts expanding into the atmosphere if it has a sufficiently strong magnetic pressure compared to the surrounding gas pressure. We find that this expansion process is characterized by a self-similar behavior, that is, both the plasma and the magnetic field have a steady distribution in the expanding area. On the basis of those results, we try to clarify several important features of emerging flux tubes expected from observations. We focus on two solar phenomena, the birth of emerging flux tubes and the formation of filaments, and discuss the physical processes related to these phenomena.
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