T. Zic, B. Vrsnak, M. Temmer
Hvar Observatory, Faculty of Geodesy, Kaciceva 26, HR-10000 Zagreb,
Croatia
We consider expanding cylindrical and spherical structures as drivers of MHD shock waves. It is assumed that the source surface accelerates over a certain time interval to achieve a particular maximum velocity. Such an expansion creates a large amplitude simple wave in the ambient plasma. Due to the nonlinear evolution of the wave front, its profile steepens and after a certain time/distance a discontinuity forms, marking the onset of the shock formation. We investigate how the time/distance needed for the shock formation depends on various input parameters: the acceleration phase duration, maximum expansion velocity (defining also acceleration), Alfven velocity (defining also Mach number), initial size of the piston. The model is an extension of the 1-dimensional model by Vrsnak and Lulic; [2000]; the basic difference lies in the fact that in the 3D evolution, a decrease of the wave amplitude with distance must be taken into account. We present basic results, focusing on the timing of the shock formation in the low versus high plasma beta environment. Implications for the coronal conditions are discussed.