VERY FAST OPENING OF A 3-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 plas ma and having its footpoints twisted by slow photospheric motions. Suc h a process has been simulated with a recently developed magnetohydrod ynamic code. In the particular calculation reported here, the system o ccupies a large cubic box. The field is initially potential, being gen erated by an underlying horizontal dipole, and it is twisted by two vo rtices 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-st atically through a sequence of force-free configurations. Thus, it ent ers a dynamical phase during which it suffers a very fast expansion, c losely approaching after some finite time a semiopen configuration. Th e energy increases monotonically during all the evolution, and it tend s to a limit, which is equal to about 80% of the energy of the totally open field associated with B-z.