High frequency whistler excitations in spheromaks

Using a loop antenna with axial magnetic field opposing the ambient field ($B_{0}\sim$ 7~G) nonlinear whistler modes are generated which create magnetic null points in the total field. The time scale of such spheromak-like fields is imposed by the oscillating antenna current ($< 0.1$ MHz $\ll \omega_{ce}$). However, within the propagating whistler spheromaks, high frequency ($\sim 10$ MHz), small-amplitude magnetic oscillations are excited spontaneously above a certain threshold. Their space-time dependence and parameter scaling is measured in order to determine the mechanism for this magnetic instability.