Unstable coronal loops: Numerical simulations with predicted observationalsignatures

We present numerical studies of the nonlinear, resistive magnetohydrodynami c (MHD) evolution of coronal loops. For these simulations we assume that th e loops carry no net current, as might be expected if the loop had evolved because of vortex flows. Furthermore, the initial equilibrium is taken to b e a cylindrical flux tube with line-tied ends. For a given amount of twist in the magnetic field, it is well known that once such a loop exceeds a cri tical length it becomes unstable to ideal MHD instabilities. The early evol ution of these instabilities generates large current concentrations. First we show that these current concentrations are consistent with the formation of a current sheet. Magnetic reconnection can only occur in the vicinity o f these current concentrations, and we therefore couple the resistivity to the local current density. This has the advantage of avoiding resistive dif fusion in regions where it should be negligible. We demonstrate the importa nce of this procedure by comparison with simulations based on a uniform res istivity. From-our numerical experiments we are able to estimate some obser vational signatures for unstable coronal loops. These signatures include th e timescale of the loop brightening, the temperature increase, and the ener gy released and the predicted observable how speeds. Finally, we discuss to what extent these observational signatures are consistent with the propert ies of transient brightening loops.