NONLINEAR EXCITATION OF GLOBAL MODES AND HEATING IN RANDOMLY DRIVEN CORONAL LOOPS

We solve the nonlinear three-dimensional MHD equations for fully compr essible, low-beta, resistive plasma to model resonant Alfven wave heat ing of a coronal loop. Alfven waves are driven in the loop by a (pseud o)random time-dependent forcing with a bounded amplitude. We find that global modes are excited and resonantly heat the loop in the nonlinea r regime in three dimensions. Resonant heating occurs in several narro w layers accompanied by high velocity and magnetic field shear. The na rrow dissipation layers are affected by the self-consistent velocity s hear and are carried around by the flow. Consequently, the topology of the perpendicular magnetic field and the ohmic heating regions differ s significantly from the linear or single-frequency driver regimes, an d the heating is spread more uniformly inside the loop. The heating ra te varies significantly on a timescale of one to several global mode p eriods. We conclude that, in solar active regions, random field-line m otions can excite global mode oscillations and resonantly heat the loo ps with a time-varying heating rate.