Pulsed currents carried by whistlers. Part I: Excitation by magnetic antennas

Abstract

Time-varying plasma currents associated with low-frequency whistlers have been investigated experimentally. Pulsed currents are induced in the uniform, boundary-free interior of a large laboratory plasma by means of insulated magnetic antennas. The time-varying magnetic field is measured in three dimensions and the current density is calculated from R∇×B(r,t)=μ0J, where J includes the displacement current density. Typical fields B(r,t) and J(r,t) induced by a magnetic loop antenna show three-dimensional helices due to linked toroidal and solenoidal field topologies. Constant amplitude and phase surfaces assume conical shapes since the propagation speed along B0 is higher than oblique to B0. The wave vector is highly oblique to B0 while the energy flow is mainly along B0. The electric field in the wave packet contains both inductive and space-charge contributions, the latter arising from the different dynamics of electrons and ions as explained by physical arguments. The dominant electric field in a whistler packet is a radial space-charge field. Neither the field topology nor the propagation characteristics are sensitive to the induced magnetic field amplitude up to Bwave≲B0. The results are relevant to both the basic properties of whistlers and to applications such as large loop antennas and electrodynamic tethers in space plasmas.