Top panel: curves of normalised j_z recorded at the null for fixed resistivity η = 10⁻⁴ and perturbation amplitudes j₀ = 0.01 (black), 0.05 (blue), 0.1 (green), 0.2 (red). It is visually clear that the larger amplitude perturbations form the initial current sheet more rapidly (t_c occurs earlier, the initial maxima) and subsequently associated with higher frequency oscillations, both in terms of the reversal periods and the frequency of the secondary (same signed) peaks. Bottom panel: normalised Fourier power spectra distribution indicating the dominant period. There is a clear decrease in the dominant period (associated with the main reversal cycle) as perturbation energy increases. Additionally, we note that as the amplitude is increased, spectral power in the lower-period (higher frequency) oscillations is also increased.