Selected stages in the evolution of the numerical solution of the ambipolar diffusion equation with initial condition having the shape of the seventh symmetric self-similar harmonic and with the numerical mesh containing 2048 grid points. Red: Exact solution for the seventh self-similar harmonic. Black: Actual numerical solution. Cyan (only in the three lowermost panels): exact solution for the first self-similar harmonic that coincides with the numerical solution at the final time calculated in the figure. Note the very different ranges of the ordinate axes in the different panels. Vertical dashed lines: Locations of the internal zeros of the numerical solution (blue); outer front of the exact 7th symmetric self-similar harmonic (red). The total (or net) magnetic flux is −7.4 × 10⁻⁸. An associated animation is provided that shows intermediate stages including the solution (top) and the corresponding diffusive flux (bottom); the associated movie is available online.