Magnetic field evolution starting from a superposition of the symmetric first harmonic and a zero net-flux finite perturbation which does not add any extra internal null. The panels show four selected instants (top: initial state; bottom: asymptotic evolution). Note the different ranges of the ordinate axes in the different panels. The evolving self-similar first symmetric harmonic is shown as a dashed line in red. The net magnetic flux is Φ_tot = 7.7 × 10⁻⁶; it does not change during the time evolution. The base-10 logarithm of the diffusive time is indicated. Vertical dashed lines mark locations of the internal zero (blue) and outer front (red) of the solution. An associated animation shows intermediate stages including the solution (upper panel) and the corresponding diffusive flux (lower panel); the associated movie is available online.