Propagation characteristics of low‐latitude whistlers

Abstract

Whistler observations at Okinawa (geomagnetic latitude 15.3°N), Japan, have been carried out as one of the IMS projects since October 1974. Whistlers and their echoes following Eckersley's law have been observed mainly during winter night. The diurnal variation of whistler occurrence at Okinawa has no evening peak, while that at mid‐latitudes has an evident peak. This may be due to the evening D region absorption resulting from the late sunset in the subtropical region, a lack of field‐aligned ducts, and the whistler absorption by the ion‐electron Coulomb collision in the ionospheric equatorial anomaly. The whistler dispersion at Okinawa varies, on the average, from 20 s 1/2 in the evening to 9 s 1/2 in the dawn and has little seasonal variation. Observed dispersions fit well‐computed ones along the International Geomagnetic Reference Field (IGRF) line passing through Okinawa for the Chapman distribution of the electron density. The IGRF line enters into an active area of thunderstorms east of Java island. The whistler activity at Okinawa is less than about one fifth of that at Kagoshima (20.7°N) and increases with increasing local geomagnetic activity. Most durations of whistler activities (>5/min) at Okinawa are no more than 2 hours which are close to theoretical lifetimes of field‐aligned irregularities over Okinawa. Whistlers at Okinawa are associated with weak ionospheric irregularities of f o E s =2.0‐2.4 MHz. These suggest ducted propagation of low‐latitude whistlers along field‐aligned irregularities. The ducted propagation of low‐latitude whistlers and duct formation models which function effectively at night and in equatorial region are discussed, especially on the ionization enhancement factor for trapping whistlers in field‐aligned irregularity and the electrostatic field in the irregularity. Some results of direction finding of whistlers at Okinawa support the ducted propagation of low‐latitude whistlers.