RAY-TRACING STUDIES OF THE ARRIVAL DIRECTION AND THE IONOSPHERIC PENETRATION OF WHISTLERS AT LOW LATITUDES

Direction-finding measurements of whistlers on the ground at low latitudes reveal that whistlers arrive from narrow regions of the zenith of only a few tens of kilometers in diameter. Using an appropriate geomagnetic field model and an ionospheric model including a field-aligned duct, the characteristics of the arrival direction of low-latitude whistlers are studied. Wave normal directions at the duct exit are calculated for whistler waves initially injected vertically upward at the conjugate duct entrance. Vertically downward wave normals appear at the duct exit quasi-periodically for the injection positions at the duct entrance. Thus the whistler duct operates to direct the wave normal of whistlers at the duct exit vertically downward and make ionospheric penetration easier. The mechanism of this penetration is investigated comparing the observed characteristics of the arrival direction with the ray-tracing calculations. The whistler energy emitted from the duct exit reaches the bottom of the ionosphere without spreading and then is radiated into free space from there. The heights of the conjugate duct ends at low latitudes are concluded to extend downward in the bottomside ionosphere for most ground whistlers. These results may be significant also for the study of ground whistlers at higher latitudes