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The statistical study on the field intensity of daytime whistlers at low (geomag. lat. 25°) and middle (35°) latitudes has been made, based on a lot of data obtained by the field-analysis direction finding system based on the simultaneous measurement of two horizontal magnetic field components and one vertical electric field component. At low latitude, the maximum absolute intensity is estimated to be 250 μV/m, while the corresponding maximum intensity at middle latitude amounts to 600 μV/m, being about 2.4 times that at low latitude. This latitudinal variation of daytime intensity is interpreted in terms of the joint influence of (a) source activity, (b) magnetospheric propagation effect and (c) ionospheric transmission loss. As the result, it is found that whistlers at each station are attributed to ducted propagation in the magnetosphere and have exited the ionosphere close to each observing station, as determined by the direction finding results. Furthermore, the cloud distributions observed by meteorological satellites have yielded that the duct entrance point of whistlers at each station fall within the active thunderstorm region. Hence, we can conclude that daytime whistlers are originated in the active thunderstorms in each conjugate region, are trapped in field-aligned ducts and followed by nearly the vertical exit from the ionosphere at each station. Finally the latitudinal difference of the intensity is satisfactorily interpreted in terms of the difference in the ionospheric transmission loss on the assumption of the same source intensity at each conjugate point and of no amplification in the magnetosphere.
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