The propagation mechanism of low latitude daytime whistlers is investigated on the basis of ground measurements made continuously during daytime in North India at Jammu (geomag. lat. 22â—¦26/ N; L = 1.17). On February 14, 1998 extremely small dispersion (ESD) whistlers with dispersion varying from 5â€“10 sec1/2 in surprisingly large numbers were recorded at Jammu during daytime in the late afternoon. The results of a study of the characteristics of ESD whistlers are presented and the discussion indicates that ESD whistlers recorded are the VLF waves radiated from the return stroke of the lightning discharge launched at the ionosphere with different initial wave normal angles, propagated upwards under either quasi-longitudinal conditions or pro-longitudinal whistler mode, turned around at different heights due to quasi-transverse propagation and received at Jammu with the dispersion of the order of 5â€“10 sec1/2. The validity of this suggestion has been tested by performing actual ray-tracing computations in the presence of equatorial anomaly model.
Keyword : Dispersion, magnetosphere, whistlers
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