NUMERICAL SIMULATIONS OF IMPULSIVELY GENERATED MHD WAVES IN A POTENTIAL CORONAL ARCADE

Impulsively generated waves in coronal arcades are simulated numerical ly by an application of nonlinear ideal magnetohydrodynamic (MHD) equa tions. The simulations sire performed in the (x, z)-plane on a non-uni form Cartesian mesh. In this geometry the magnetic field can be expres sed in terms of the vector potential. The governing equations, which a re applied in the limit of low plasma-beta, are solved by a Aux correc ted transport method. The model excludes the Alfven waves and, since t he slow mode is absent in the cold plasma limit, the excited disturban ces are fast magnetosonic waves. Numerical results show that for short times after the impulse is launched (i. e., in the linear regime), on ly motions normal to the equilibrium magnetic field get propagated awa y from the position of the initial displacement and that any velocity parallel to the unperturbed magnetic field lines remains essentially u nchanged in time. In the nonlinear regime there is conversion between normal and. parallel flow and the two velocity components propagate fr om the site of the initial impulse. In addition, nonlinearities that a re built in the MHD equations modify the shape and speed of the propag ating wavefront, an effect that becomes most noticeable where the wave amplitude is larger The effect of nonlinearity on down-going perturba tions is to speed up positive wave amplitudes and to slow down negativ e wave amplitudes (positive and negative refers to die sign of the nor mal velocity component). On the contrary, up-going positive and negati ve waves are slowed down and speeded up, respectively. Impulsively gen erated waves exhibit temporal signatures with characteristic time scal es of the order of 10 s. Similar scales have been recently reported in radio observations. microwaves, and hard X-rays.