Very fast opening of a three-dimensional twisted magnetic flux tube

This Letter is devoted to the still open problem of the evolution of a three-dimensional coronal flux tube embedded in a low-beta ideal plasma and having its footpoints twisted by slow photospheric motions. Such a process has been simulated with a recently developed magnetohydrodynamic code. In the particular calculation reported here, the system occupies a large cubic box. The field is initially potential, being generated by an underlying horizontal dipole, and it is twisted by two vortices located on the lower face {z = 0} of the box, on both sides of the neutral line. In a first phase, the field roughly evolves quasi-statically through a sequence of force-free configurations. Thus, it enters a dynamical phase during which it suffers a very fast expansion, closely approaching after some finite time a semiopen configuration. The energy increases monotonically during all the evolution, and it tends to a limit, which is equal to about 80% of the energy of the totally open field associated with B_z.