On the propagation of lf whistler-mode waves deduced from conjugate measurements at low latitudes

Whistler-mode signals transmitted from three Decca stations (Biei, L = 1.54, ƒ c = 85.725 kHz; Akkeshi, 1.51, 114.300 kHz; Wakkanai, 1.61, 128.588 kHz), Japan, were measured at multiple observing points in the magnetic conjugate area around Birdsville ( L = 1.55), Australia. The whistler-mode signals observed in the conjugate area indicated a remarkable frequency dependence of the occurrence ; the signals were usually observed at 85.725 kHz at the multiple points including the conjugate point of the transmitter ( ʌ = ƒ c/ƒ Heq = 0.32 ), and not detected at 114.300 kHz (0.40), and they were not identified at 128.588 kHz around the conjugate point (0.55). No detection at 114.300 kHz may be due to the heavy attenuation of signals while penetrating the ionosphere down to the observing point (Birdsville) with wave normals at large angles with respect to the magnetic field. However, such a difference of occurrence between 85.725 and 128.588 kHz can not be understood from calculation of the transmission loss in the lower ionosphere by means of a full wave treatment, and it has demonstrated the ducted propagation in field-aligned enhancements of electron density. Also, direction finding results at 85.725 kHz may support the ducted propagation, indicating ionospheric exit almost parallel to the magnetic field through the observing point. At magnetically severely disturbed times, the whistler-mode signals appeared almost continuously during the night-time, and they were intensified by more than 20 dB. Such an intensity increase may be due to interactions with energetic electrons of 50–100 keV.