Temporal evolution of the (a) toroidal and (b) poloidal magnetic energies for the azimuthal modes m = 0 to m = 6 in the unstratified run TU0. The dashed black lines show exponential fits to the energy evolution of the m = 1 mode within the time interval $6.5\le (t-t_1){\overline{\omega }}_{\mathrm{A}1}\le 10.5$. These fits are used to determine the instability growth rates σ discussed in the text.