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.

Characteristics of radio frequency wave propagation in bounded plasma under the various magnetic field configurations

“…9 As for basic laboratory research, which has some common and other different approaches from space plasma research ͑and thus both researches are complementary and important͒, detailed characterization of the whistler wave excited by several methods, such as the loop antennae, electrodes, and electron beams, and the role of the waves in these cases, have been clarified. 5,6,[10][11][12][13] One example of an application of the whistler wave is a helicon source. [14][15][16][17][18] The helicon wave is a bounded electromagnetic wave in the whistler wave range with both rightand left-hand circular polarization.…”

“…In whistlers, most of the experiments [10][11][12][13] have been performed so far under the conditions that the wave excited region along the radial direction was much smaller than the boundary size, using a small wave excitation source, which can be treated as an unbounded electromagnetic wave. Furthermore, wave packet excitation using a short pulse and measurement on the transient pulsed wave characteristics were done, which showed again approximately unbounded wave behavior along the axial direction.…”

Whistler wave radiation from a pulsed loop antenna located in a cylindrical duct with enhanced plasma density

“…To avoid misunderstanding, we note that for the source with time dependence (7), the condition r < t À1 0 is required in Eq. (26). For r !…”

“…Bearing this in mind, it is possible to check that no singularities of the integrands in Eq. (26) are intercepted in the derivation of Eq. (26) when passing to the limit r !…”

“…16 and 19. Over the past two decades, there has been shown a substantial degree of interest in the excitation of nonmonochromatic waves propagating in the whistler mode in a magnetoplasma. [26][27][28][29][30][31][32][33] This interest has been motivated by the importance of transient phenomena for propagation of whistler-mode waves through the magnetosphere and the ionosphere, as well as for plasma diagnostics using pulsed signals launched from antennas on spacecraft in the whistler range. However, much previous theoretical work on the subject is mainly focused on studying the fields and radiation characteristics of nonmonochromatic sources in a homogeneous magnetoplasma.…”