Determination by triangulation of the regions where whistlers emerge from the ionosphere

Observations made on 10 July 1972, using four goniometer (direction-finding) VLF receivers established in eastern Canada, have been analyzed. The regions where multicomponent, two-hop whisters emerge from the ionosphere into the earth-ionosphere waveguide have been found by triangulation, for the first time. For recordings made before the total solar eclipse, the L-values of the whistler exit points determined by triangulation are consistently lower, by about 0.7 of an L-value, than those determined from their nose frequencies. This discrepancy cannot be fully accounted for by the effect of the ring current. In order to explain it, it is postulated that on this occasion the electron-density trough in the topside ionosphere lies at a lower L-value than that of the plasma-pause. Ray-tracing studies show that, on the high-latitude side of the trough, the waves are refracted to higher L-values before becoming ducted. Thus the multigoniometer technique is proving to be a powerful method of studying both temporal and spatial variations of the ionospheric and magnetospheric plasmas. Networks of goniometers should be established during IMS