Some results of five years of whistler observations from Labrador to Antarctica

Some interesting curves result when whistler data are averaged over a long period of time to smooth out random short-term variations in whistler generation and propagation. Local season is such a strong factor in determining the shape of the diurnal curves of observed whistler activity that even in the case of the north-south "Whistlers-East" chain of audio-frequency receiving stations, diurnal curves of the northern- and southern-hemisphere stations tend to be similar not during the same period of the year but during the same local season at the point of observation. A basic form of the diurnal curves appears to be one which is symmetrical about local midnight with a deep minimum at local noon and relatively high nighttime activity showing maxima at about 2000, 2400 and 0400 hours local time. Depending on local season and location of the station, one or more of these peaks may be absent or enhanced. It is concluded that the shape of the diurnal curves is determined largely by the conditions of whistler propagation rather than of generation. The curves presented should therefore be useful in predicting the behavior of man-made signals propagating in the "whistler mode" through the ionosphere and the magnetosphere of the earth. Some difficulties are pointed out with the prevalent idea that the marked depression in the occurrence of whistler-mode signals in the daytime is primarily the result of absorption of these waves in passing through the D layer. On the other hand, caution is advised against neglecting ionospheric factors, other than D-region absorption, in deference to the role of field-aligned ducts of enhanced ionization in the magnetosphere. Finally, some results are presented which show that, in addition to a latitude variation of the dependence of whistler rate upon the K
p
index of geomagnetic disturbance, there is, especially at the lower latitude stations, a seasonal variation of these curves as well. Of particular interest also is the observation that, at the higher latitude stations where the maximum whistler rate obtains at low values of K
p
, the rate decreases almost linearly with greater values of K
p
.