FOCUSING OF NONDUCTED WHISTLERS BY THE EQUATORIAL ANOMALY

Impulsive ELF/VLF electric field bursts observed by the vector electri c field instrument (VEFI) on the Dynamics Explorer 2 (DE 2) satellite on almost every crossing of the geomagnetic equator in the evening hou rs are interpreted as originating in lightning discharges. These signa ls that peak in intensity near the magnetic equator are observed withi n 5-20 degrees latitude of the geomagnetic equator at altitudes of 300 -500 km with amplitudes of the order of similar to mV/m in the 512- to 1024-Hz frequency band of the VEFI instrument. Whistler-mode ELF/VLF wave propagation through a horizontally stratified ionosphere predicts strong attenuation of subionospheric signals reaching the equator at low altitudes. However, ray tracing analysis shows that the presence o f the equatorial density anomaly, commonly observed in the upper ionos phere during evening hours, leads to the focusing of the wave energy f rom lightning near the geomagnetic equator at low altitudes, thus acco unting for all observed aspects of the phenomenon. The observations pr esented here indicate that during certain hours in the evening, almost all the energy input from lightning discharges entering the ionospher e at <30 degrees latitude remains confined to a small region (in altit ude and latitude) near the geomagnetic equator. The net wideband elect ric field, extrapolated from the observed electric field values in the 512- to 1024-Hz band, can be similar to 10 mV/m or higher. These stro ng electric fields generated in the ionosphere by lightning at local e vening times may be important for the equatorial electrodynamics of th e ionosphere.