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Solar coronal loops observed in soft X-rays often show twisted structures. Sometimes, magnetic loops unwind after the topological change of magnetic field lines (McAllister et al. 1992). Since twisted flux tubes have more free energy than untwisted loop, when they emerge into the corona and interact with other loop or overlying magnetic fields, the release of magnetic energy will lead to energetic events such as flares, jets, and prominence eruptions.
We performed three-dimensional nonlinear MHD simulation of the evolution of twisted flux tubes. The initial state consists of hydrostatic atmosphere with a cold dense layer and a hot corona. In the cold layer, magnetic fields are concentrated in the horizontal flux tube. Fluid rotation at both ends of the flux tube is continuously applied. Such twisting motion generates torsional Alfv\'en waves which propagate along the flux tube. After several rotations, the flux tube becomes kink unstable around the midpoint where the magnetic twists are accumulated. The flux tube is deformed into a super-coil structure. Later, twisted magnetic loops rise by the magnetic force and the buoyancy force created by sliding the gas along the flux tube. The length of each rising loop is about $10H$ in the cold layer, where $H$ is the scale height. We will further study the subsequent evolution of twisted flux tubes in the corona, and their interaction with overlying magnetic fields.
