A weakly nonlinear Alfvenic pulse in a transversely inhomogeneous medium

The interaction of a weakly nonlinear Alfvenic pulse with an Alfven speed i nhomogeneity in the direction perpendicular to the magnetic field is invest igated. Identical to the phase mixing experienced by a harmonic Alfven wave , sharp transverse gradients are generated in the pulse by the inhomogeneit y. In the initial stage of the evolution of an initially plane Alfvenic pul se, the transverse gradients efficiently generate transversely propagating fast magnetoacoustic waves. However, high resolution full MHD numerical sim ulations of the developed stage of the pulse evolution show that the genera tion saturates due to destructive wave interference. It is shown that the w eakly non-linear description of the generated fast magnetoacoustic wave is well described by the driven wave equation proposed in Nakariakov et al. (1 997), and a simple numerical code (2D MacCromack), which solves it with min imal CPU resources, produces identical results to those obtained from the f ull MHD code (Lare2d, Arber et al. 2001). A parametric study of the phenome non is undertaken, showing that, contrary to one's expectations, steeper in homogeneities of the Alfven speed do not produce higher saturation levels o f the fast wave generation. There is a certain optimal gradient of the inho mogeneity that ensures the maximal efficiency of the fast wave generation.