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T. Magara, D. W. Longcope (Montana State University)
The objective of this study is to investigate the dynamical behavior of emerging magnetic field in the solar atmosphere by means of 3-dimensional MHD numerical simulation of a buoyant magnetic flux tube. From our recent simulations, it is found that there are two kinds of evolutionary phases of emerging field lines: an expansion phase and a gradual phase. The outer field line of flux tube, which emerges earlier than the inner field line, shows a simple expansion after it comes to the solar atmosphere. On the other hand, the inner field line has a gradual phase at first, in which the field line undulates and rises gradually, and then it enters an expansion phase.
We try to understand this result by focusing on the physical process working on the individual field lines. We think that the distance between the footpoints of emerging field line plays an important role. When the outer field line emerges, its footpoint distance is almost comparable to the local value of the critical wavelength of Rayleigh-Taylor instability (\lambdaRT), although the inner field line has a much larger footpoint distance than this value when emerging. These facts cause the outer field line (emerging early) to make a simple expansion, however they undulate the inner field line (emerging late) and prevent this field line from expanding smoothly. As the height of inner field line increases, the density of the gas supported by the field line decreases because of the continuous drain of gas toward the footpoints. This leads to the increase of the local value of \lambdaRT. When this value becomes comparable to the footpoint distance, then the inner field line starts to expand rapidly.
This work is supported by AFOSR grant F49620-00-1-0128.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.