A short, intense current pulse (I=150 A, Δt≃0.2 μs, N≃2×${10}^{14}$ electrons) is drawn from an electrode immersed in a magnetized afterglow plasma (n≃2×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$, ${\mathit{kT}}_{\mathit{e}}$≃1.3 eV, ${\mathit{B}}_0$=10 G). The induced magnetic field B(r,t) assumes the shape of a helical double vortex which propagates along ${\mathbf{B}}_0$ through the uniform plasma as a whistler mode. The observations support a prediction of force-free (J×B+neE=0) electromagnetic fields and solitary waves. Energy and helicity are approximately conserved.

• Received 19 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2011