Fig. 1: Characterization of the instability.

a Time series of the imposed magnetic field Bz(t) at Rm = 3 and B0 = 0.2. b–d Corresponding time series of the measured radial magnetic field Br(t) (b), the spectrogram of Br(t) (c) and the instability amplitude χ(t) (d) in the midplane. The horizontal lines from top to bottom in (c) represent f1, f12, and f13, respectively. The solid line in (d) shows an exponential fit, \(\chi (t) \sim \exp (\gamma t)\) to the data points with γ = 1.8 s−1. The vertical line through the four panels indicates the time when the instability starts to grow.