Direction findings of medium‐latitude whistlers and their propagation characteristics

Abstract

The propagation characteristics of medium‐latitude whistlers are studied on the basis of measurements with a new direction finder at Moshiri (geomag. lat. 34.5°N; L=1.6) during January and February 1978. Detailed analyses are made for two selected events: one with the highest daytime activity on January 22 and the other with the highest nighttime activity on February 20. Daytime whistlers are of multi‐flash or of an isolated type, and the dispersion remains constant during the event. The measured polarization is, on the average, close to exactly circular, indicating a negligible effect of multi‐paths in the earth‐ionosphere waveguide, implying that the ionospheric exit points are determined accurately. The characteristics on the direction finding results are compared with the properties expected for ducted and nonducted pro‐longitudinal mode propagation and it is concluded that the whistlers may be attributed to ducted progagation. Just a single duct is present during the event. The dimension at the lower boundary of the ionosphere, illuminated by the duct which probably terminated at the F2, is found to be ∼100 km. The small apparent movement of the exit regions may be due to a temporal fluctuation of the propagation conditions between the duct endpoint and the lower boundary of the ionosphere. For the nighttime whistlers there are four distinct dispersions, but we concentrate ourselves on the main dispersion which remained constant during the event. The observed polarization is less circular than for the case of daytime whistlers for the measured incident angle ≲60°. For larger incident angles, 60° ≲i ≲75°, we find a great variability in the measured polarization. These indicate that multi‐path propagation may have a significant effect on the propagation in the waveguide at night. Correspondingly, there is a great scatter in the estimated exit points during each time interval. The exit region also shows a large apparent drift. This may be interpreted in terms of a joint effect of the presence of four ducts at the nearly same L shell and the temporal variation of the activity of the lightning sources at the feet of the four ducts in the conjugate zone. The scale of nighttime ducts is difficult to determine due to the great scatter of the data.