The Roles of Networks of VLF Direction-Finding Receivers during the IMS

Abstract

A simple model of the distribution of electric fields and plasma dynamics in the magnetospheric equatorial plane is first reviewed. The significance of the plasma-pause in this context is mentioned. A discussion of the power, and limitations, of the normal whistler method for studying this region of space plasma leads on to a consideration of the roles that can be played by networks of VLF direction-finding (goniometer) receivers.

Some results are presented of an experiment conducted in Eastern Canada during July 1972 using four goniometer receivers. The regions where two-hop whistlers emerged into the Earth-ionosphere waveguide were obtained by triangulation. For nearly 2 h on this occasion, the L-values of the whistlers’ exit points were systematically ~ 0.6 less than the L-values determined from the whistlers’ nose frequencies. This result can be understood once the ray paths of unducted whistler-mode signals have been computed in a model of the topside ionospheric plasma having concentration gradients with latitude at constant altitude. The relative frequency of occurrence of this result is not known.

Preliminary results of a similar experiment carried out in Alaska during March 1974 following the injection of barium up to geomagnetic field lines are also presented. Discrete ELF and VLF emissions were observed some 90 min after the release; these may be due to the stimulation of the electron cyclotron instability via the enhancement of the thermal plasma density.

The significance of such experiments, which need to be performed with simultaneous topside ionospheric soundings, is discussed. Field-aligned ducts of interest, such as those on which wave-particle interactions are taking place or within which a certain satellite exists, may then be localised in space. Motions of the ionospheric exit points may be studied directly; by comparing these with those deduced from changes of the whistlers’ nose frequencies with time, the decoupling of the ionosphere from the magnetosphere may be investigated. Some suggestions are made for co-ordinated ground-based, balloon, rocket and satellite IMS projects in which networks of VLF direction-finding receivers play a key role.

On leave from Dept. of Physics, University of Southampton, UK.