Astron. Astrophys. 321, 935-944 (1997)

Appendix A: the numerical method

In this appendix, some details are presented about the numerical code that produced the results shown in the present paper. The semi-implicit predictor-corrector scheme is not new and has been applied to numerical simulations of three-dimensional time-dependent MHD phenomena before. In fact, our code is based on the code Biskamp & Welter:1987 used to study sawtooth disruptions in tokamaks. Yet, many variations on the semi-implicit algorithm are possible and we modified the scheme used by Biskamp and Welter. In this appendix, only the main features of the algorithm we used are given. More detailed information on the semi-implicit method can be found elsewhere (see e.g. Harned & Kerner 1985 , Harned & Kerner 1986 , Harned & Schnack 1986 , Lerbinger & Luciani 1991).

A.1. Spatial discretisation

In the - and the z -direction, the plasma response to the external driving can be expected to be relatively smooth so that global expansion functions can be used to discretize these directions. The physical variables f are expanded in a Fourier series:

The nonlinear terms in the MHD equations then need careful treatment because these terms are products in real space, which lead to convolution sums in Fourier space. These convolution sums are a CPU time consuming operation which is not vectorizable.

For the spatial discretization of the radial direction, on the other hand, finite differences are used. Due to the occurence of resonances, the solutions can become extremely localized in the radial direction excluding the possibility of using global expansion functions for the spatial discretization in this direction. Two staggered meshes are used. This is necessary to obtain a correct representation of the linear MHD spectrum and to satisfy certain vector identities identically, e.g. , in finite different form so that can be implemented as an initial condition on the magnetic field. The quantities , , , and are defined on the `integer' mesh , where is the number of radial mesh points. The quantities , , , , and P are defined on the `half-integer' mesh , with . Quantities which are defined on the integer (half-integer) mesh are calculated, if needed, on the half-integer (integer) mesh by simple averages, e.g. . Two `virtual' points and are introduced to calculate such averages at the boundaries and , respectively. These additional mesh points are defined by

These two additional mesh points are used to impose regularity conditions in and boundary conditions in for the variables which are defined on the half-integer mesh (see section A.3). The two staggered meshes do not need to be uniform. The mesh points can be accumulated whenever this is necessary, e.g. at the resonant layer and/or at the plasma boundary.

The semi-implicit predictor-corrector scheme of Harned & Kerner 1985 is used. This scheme avoids both the overly restrictive CFL condition for the compressional fast magnetosonic waves on the time step in explicit schemes and the complexity and computational intensity of implicit schemes by treating the fast magnetosonic waves, and only these, implicitely in the linear phase. This is accomplished by adding and subtracting, at different time steps, a simple approximation of the term that yields the fast magnetosonic waves in the momentum equation. We use the same `semi-implicit' term as Harned and Kerner which involves only the velocity components in the plane:

where is a constant. Notice that this term does not affect the solution as . However, the algorithm we use differs somewhat from the algorithm used by Harned & Kerner 1985 and by Biskamp & Welter 1987: the stabilizing semi-implicit term is applied both in the predictor step and in the corrector step and the Crank-Nicolson-type advance is applied to both V and B . The scheme then reads:

predictor step:

corrector step:

where , , and denote the ideal parts of the right-hand-sides of the MHD Eqs. (1), (2), and (3), respectively. A semi-implicit resistive advance of B and P is performed after the corrector step. Hence, our scheme looks formally much like the scheme used by Lerbinger & Luciani 1991 except for the semi-implicit term itself: Lerbinger and Luciani treat all waves implicitely in the linear phase whereas we used the semi-implicit term of Harned and Kerner which is chosen to make only the fastest time scale implicit in the linear phase and leaving the rest explicit. Lerbinger & Luciani 1991 remarked that the application of the stabilizing semi-implicit term both at the predictor and corrector levels greatly improves the nonlinear stability of the code. If is chosen to be 0.5, the method is second order accurate in time, with the exception of the fast magnetosonic modes which remain first order accurate due to the semi-implicit terms. However, in practice typically or 0.52 is chosen to add some numerical damping in order to reduce numerical noise. The simplicity of the semi-implicit term (A3) with the constant coefficient avoids the problem of convolutions coupling the individual mode equations. As a result, after discretization, a simple tridiagonal system has to be solved in every predictor and corrector step. This is the only overhead compared to an explicit scheme. Yet, the saving of CPU time accomplished by this simple operation can be very high, up to a factor 1000, due to the fact that the CFL condition on the fast magnetosonic waves is avoided and replaced by the CFL condition imposed by the second-shortest time scale in the problem, viz. the Alfvén time scale (Harned & Kerner 1985).

A.3. Boundary conditions

When the physical variables are expanded in a Fourier series according to Eq.  A1, regularity conditions and single-valuedness at the origin yield analytic conditions for the various m. In particular, the conditions applied at for V , B , and P are displayed in Table 1, where the prime denotes the partial derivative with respect to r. At the loop surface , the same boundary conditions are used as for a plasma cylinder surrounded by a perfectly conducting wall, except for the radial component of the velocity field. The incidence of the external wave is numerically simulated by imposing a harmonic oscillation with given frequency and amplitude on the radial velocity component at the loop surface. This yields

at for all modes, except for the driven mode, where

at . Here, A denotes the amplitude of the incident wave and the driving frequency.

Table 1. The boundary conditions at for different mode numbers.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998