Dynamics are investigated for buoyant flux ropes in a gravitationally stratified solar atmosphere using realistic chromosphere and corona parameters. Two-dimensional magnetohydrodynamic simulations of horizontally oriented flux ropes are performed under the approximation that the flux rope remains parallel to the solar surface as it rises. As expected, this motion tends to halt at the chromosphere-corona boundary, where the buoyancy force drops significantly. It is shown that fragmentation of the rising flux rope into multiple flux bundles can be reduced if the flux-rope field lines are sufficiently twisted and that this is consistent with previous results. It is also shown that a weak, vertically oriented ambient magnetic field, (Ba/Btube)2 1, can prevent fragmentation of an untwisted flux tube. A vertical field of sufficient strength, but still "weak," can direct the upward buoyancy-driven flow so that the untwisted flux tube penetrates the chromosphere-corona boundary.
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