ELECTRONMAGNETOHYDRODYNAMIC RESPONSE OF A PLASMA TO AN EXTERNAL CURRENT PULSE

In this paper we examine the dynamic response of a magnetoplasma to an external time-dependent current source in the context of electronmagn etohydrodynamics (EMHD). A combined analytic and numerical technique i s developed to address this problem. The set of cold electron plasma a nd Maxwell's equations are first solved analytically in the (k,omega) space. Inverse Laplace and three-dimensional complex Fast Fourier Tran sform techniques are used subsequently to numerically transform the ra diation fields and plasma currents from the (k,omega) space to the (r, t) space. The results show that the electron plasma responds to a time -varying current source imposed across the magnetic field by exciting whistler/helicon waves and forming an expanding local current loop, dr iven by field-aligned plasma currents. The current loop consists of tw o antiparallel field-aligned current channels concentrated at the ends of the imposed current and a cross-field Hall current region connecti ng these channels. The characteristics of the current closure region a re determined by the background plasma density, the magnetic field, an d the time scale of the current source. The results are applied to the ionospheric generation of extremely low-frequency (ELF) and very low- frequency (VLF) radiation using amplitude modulated high-frequency hea ting. It is found that contrary to previous suggestions the dominant r adiating moment of the ELF/VLF ionospheric source is an equivalent hor izontal magnetic dipole. (C) 1996 American Institute of Physics.