## Appendix A: model equation for fast waves propagating vertically in a cold coronal arcadeLet us consider the ideal magnetohydrodynamic equations for a cold
plasma (Eqs. [1]-[4]). To make analytical progress we introduce a
moving coordinate frame which follows (with the speed of the linear
wave, ) vertically propagating nonlinear waves.
In this frame, plasma quantities depend on the spatial coordinates
where is a small parameter (which measures
the weakness of nonlinearity) and
() corresponds to up-going (down-going)
propagating waves. It is noteworthy that the case of a horizontally
propagating wave can be treated analogously. The only difference would
be to replace All plasma variables are expanded around the unperturbed state (8), (11) as follows This expansion means that we are concerned with weakly nonlinear waves. Our approach consists of a first order improvement over the linear theory, so it is not correct for strongly (large amplitude) nonlinear waves. Substituting (A1) and (A2) into the system of MHD equations (with
A compatibility condition at leads to the equation where the coefficient is given by Transforming this equation back into the original we obtain the final form of the model equation for in the weakly nonlinear regime. This equation can be simplified by changing to the dependent variable (cf. Eq. [17]), already used in obtaining solutions to the linear problem, with We now restrict ourselves to the case , which corresponds to a constant Alfvén speed with height. Moreover, since the initial impulse is launched at and only vertical propagation is being considered, we also have . It is convenient to use dimensionless coordinates and introduce the normalised velocities and In dimensionless coordinates, equation (A9) can then be rewritten as follows This equation is used in Sect. 7.4 to explain the basic features of the nonlinear waves observed in the numerical calculations. Finally, it is worth mentioning that equation (A13) is less general than the nonlinear Klein-Gordon equation that was recently derived by Nakariakov & Oraevsky (1995) and Nakariakov et al. (1997) for the case of fast wave propagation along a smoothed interface. We believe a similar analysis can be done for the arcade, although it can be more difficult to obtain the most important nonlinear terms here. © European Southern Observatory (ESO) 1998 Online publication: January 16, 1998 |