Pulsed currents carried by whistlers. II. Excitation by biased electrodes

The transport of time-dependent current between electrodes in contact with a large laboratory magnetoplasma is examined experimentally. Single electrodes biased with respect to the chamber wall or pairs of electrically floating electrodes are used to produce pulsed currents (ω_ci≪2π/Δt≪ω_ce). The associated magnetic field vector, B(r,t), is measured in space and time, and the total current density is calculated from J(r,t)=∇×B(r,t)/μ₀. The current front is found to propagate at a characteristic wave speed, which does not depend on current amplitude or polarity. The transient current spreads across B₀ within a conical region, which depends on source geometry and plasma parameters. It is shown by Fourier transforming B(r,t) into B(k,ω) that the transient fields consist of a spectrum of oblique low-frequency whistler waves. In Fourier space, the inductive and space charge electric fields are calculated from Faraday's law and the assumption that E_tot=E_ind+E_sc along B₀ is negligible. Inverse transforming yields E(r,t). The transient wave fields (B,J,E) exhibit multiple induction effects and the formation of space charges. The results are relevant to pulsed Langmuir probes, beams, and antennas as well as moving steady-state magnetic/current sources such as particle collectors on spacecraft and magnetized asteroids (e.g., Gaspra).