Goniometric direction finding for low‐latitude whistlers and their propagation mechanism

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Abstract
In order to study the propagation mechanism of low‐latitude whistlers, their ionospheric exit points have been determined by a triangulation of the goniometer‐polarimeter systems at the two stations of Kagoshima (geomagnetic coordinates 20°24′N, 197°53′E; L=1.22) and Ohgata (22°06′N, 199°51′E; L=1.25), supplemented by the routine observation at Sakushima (24°08′N, 203°19′E; L=1.28). Detailed analyses were made for January 23, 1978, when the whistler activity was most enhanced during one month’s measurement. The difference in temporal variations of the occurrence rate at the three stations and the temporal variation of the single‐valued dispersion enabled us to subdivide the daytime whistlers into three characteristic time intervals and the nighttime ones into two intervals. The characteristics during the whole intervals were found to be satisfactorily accounted for in terms of ducted propagation. The azimuth measurements have indicated that there is present a discontinuity in the characteristics between each two intervals, indicative of the different duct group for different time intervals; in other words, indicative of the identity and persistency of ducts for each period. Then we obtained the ‘apparent lifetime’ of ducts of 1–2 hours. During the last interval for the daytime whistlers, a few ducts were found to be aligned on a certain latitude, which showed a temporal movement, or first increased and then decreased. The simultaneous presence of a few ducts at the same latitude is also seen at night.