Ducted propagation of daytime whistlers at low latitudes as deduced from ground‐based direction findings

The purpose of the present paper is to promote our understanding of the propagation characteristics of daytime whistlers at low latitudes on the basis of the data obtained by a newly developed automatic direction finding measurement at Yamaoka (geomagnetic latitude 25°N) and the synoptic routine data from the multistation network of Kagoshima (20°), Sakushima (24°), and Moshiri (34°). Two events of daytime whistlers with extremely enhanced occurrences are found on January 16 and 15, 1982, and they are examined in detail. It is found that the ionospheric exit points determined by the direction finder at Yamaoka remain very stable for less than 2 hours, so that the diameter of their distribution is less than 60 km. This experimental fact is likely to be strongly indicative of ducted propagation. An intercomparison of the whistler spectra at the four stations has been made for these events. Then it is shown that the whistlers that emerged near Yamaoka and Sakushima tend to propagate in the earth-ionosphere waveguide toward higher latitudes, being observed at Moshiri with their frequency component cut off at frequencies below the tweek cutoff frequency, while heavily attenuated corresponding whistlers are simultaneous at Kagoshima. This phenomenon is again satisfactorily explained in terms of ducted propagation.

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