We study the emergence of magnetic fields in the solar atmosphere with a focus on photospheric and coronal activities produced by flux emergence. These activities are examined using a three-dimensional MHD simulation of an emerging flux tube. The simulation has been performed for a highly twisted flux tube (highly twisted case) and a weakly twisted one (weakly twisted case). The emerging flux tube globally forms a bipolar region associated with a flow in the photosphere. We decompose the flow into several fundamental flow components, such as rotation, expansion/contraction, and distortion. The analysis shows that as the emerging field becomes vertical, a torsional flow appears in a polarity region with intense flux, and the polarity region apparently rotates opposite to the torsional flow. We also find that the evolution of the photospheric neutral line in the highly twisted case is different from that in the weakly twisted case. We then study the coronal structure of an emerging field by focusing on the distribution of current density at the chromospheric footpoint of emerging field lines. Field lines with high current density at their footpoints display a distinct shape in both highly twisted and weakly twisted cases: the highly twisted flux tube produces the confined, sigmoidal structure of coronal loops, while the expanded loop structure appears in the weakly twisted case. Based on these results we discuss the mechanism for producing several solar phenomena, such as filament channels, flux ropes, and sigmoids.
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