The propagation step is carried out by an explicit Euler scheme. To overcome the severe stability restriction using an explicit scheme, a multi-sequence integration scheme is employed following the idea by Vilsmeier and Hänel (1995). Here, the maximum time step is increased by locally employing a sequence of integration steps, where the number of levels (steps) may vary from element to element.
The maximum number of levels (or time steps) in the propagation (transport) calculation is 32.