Spread-F and the Latitude Variation of Occurrence of Whistlers

Abstract

RECENT work of whistling atmospherics has suggested that the incidence of the phenomenon is dependent on latitude. Crouchley¹, using data from Australian and Japanese stations, demonstrated that whistler activity reaches a maximum at about 45° geomagnetic latitude, while the Stanford group² report a maximum of occurrence at 50° geomagnetic latitude. In both these reports a marked decrease in activity at lower and higher latitudes is noted. The decreasing incidence at higher latitudes has been attributed³ to decreasing thunderstorm activity, and rapidly increasing path-length and absorption with increasing latitude. However, the fall-off in activity at low latitudes cannot be so explained. Smith, Helliwell and Yabroff³ have suggested an explanation of this low-latitude fall-off in terms of whistler propagation within field-aligned ducts. They showed that the necessary ionization enhancement within the ducts for F-layer heights falls off markedly with increasing latitude and argue, therefore, that this suggests that whistler occurrence should increase with latitude, since large enhancements would be expected to occur less frequently than small enhancements. It is the purpose of this communication to examine this point in more detail.