Pulsed currents carried by whistlers. III. Magnetic fields and currents excited by an electrode

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Abstract
Detailed measurements and analysis of electromagnetic fields asociated with pulsed plasma currents are reported. The objective is to demonstrate the properties of plasma currents in the electron magnetohydrodynamic regime and their relation to low frequency whistler waves. Short current pulses (fce−1≪Δt≪fci−1) are injected from an electrode into a large, uniform magnetoplasma. The dynamic fields, B(r,t), are measured with probes in three-dimensional space and time, and are observed to propagate as wave packets predominantly along the guide magnetic field, B0. Four-dimensional fast Fourier transformation of B(r,t) to B(k,ω) verifies that the wave fields fall on the dispersion surface of low-frequency oblique whistlers. The magnetic field topology of the packets consists of linked toroidal and solenoidal contributions in force-free configurations. The wave magnetic helicity is obtained quantitatively. Similarly, the topology of the current density field, J=∇×B/μ0, is explained by its components, characteristic field lines, and helicity, ∫J⋅BdV. It is shown, both theoretically and experimentally, that A, B, and J are nearly parallel to one another. Field energy and helicity decay at the same rate without change in topology.