  We have performed a three-dimensional magnetohydrodynamic simulation to study
the emergence of a twisted magnetic flux tube from -20,000 km of the solar
convection zone to the corona through the photosphere and the chromosphere. The
middle part of the initial tube is endowed with a density deficit to instigate
a buoyant emergence. As the tube approaches the surface, it extends
horizontally and makes a flat magnetic structure due to the photosphere ahead
of the tube. Further emergence to the corona breaks out via the
interchange-mode instability of the photospheric fields, and eventually several
magnetic domes build up above the surface. What is new in this
three-dimensional experiment is, multiple separation events of the vertical
magnetic elements are observed in the photospheric magnetogram, and they
reflect the interchange instability. Separated elements are found to gather at
the edges of the active region. These gathered elements then show shearing
motions. These characteristics are highly reminiscent of active region
observations. On the basis of the simulation results above, we propose a
theoretical picture of the flux emergence and the formation of an active region
that explains the observational features, such as multiple separations of
faculae and the shearing motion.
