Ducted whistler-mode propagation in the magnetosphere; A half-gyrofrequency upper intensity cutoff and some associated wave growth phenomena

A large class of ducted magnetospheric whistlers exhibits an upper intensity cutoff at which the wave amplitude decreases by > 10db within a fraction of a kHz. A preliminary study, based in large part on whistlers recorded at Eights, Antarctica (L ∼ 4), indicates that the ratio of cutoff frequency to whistler nose frequency ƒco/ƒn is invariant with respect to path equatorial radius, local time, and magnetic activity. Measurements on 541 whistler components propagating in the plasmasphere were such that ƒco/ƒn = 1.33 ± 0.08 for about 70% of the cases. The corresponding result for ƒco/ƒHo, the ratio of cutoff frequency to minimum path gyrofrequency, is ƒco/ƒHo = 0.51 ± 0.03. (To relate ƒn and ƒHo, the approximate relation ƒn = 0.38 ƒHo was used, from Angerami's work on the diffusive-equilibrium model of the distribution of thermal ionization along the field lines in the plasmasphere.) The observed behavior of ƒco/ƒHo is apparently controlled by a propagation effect and not by a resonant particle damping process, the propagation effect probably being the half-gyrofrequency upper limit on ducted whistler-mode propagation in tubes of enhanced ionization predicted by Smith. Spectrographic VLF records exhibit much evidence of wave growth in the 0.4–0.5 ƒHo band, an example being the stimulation of noise at ∼0.5 ƒHo by both whistlers and a low-power (∼100 watts) controlled source. A whistler-triggering event may include: an intensity maximum above the whistler nose, followed by a slight dispersion anomaly and the cutoff; a quasi-constant triggered noise tone with bandwidth 1 sec, and intensity comparable to that of the associated whistler. The noise tone itself does not usually exhibit dispersive whistler-mode echoing, nor does it exhibit an intensity cutoff as it rises in frequency through the 0.5 ƒHo level. Earlier observations by Kimura of triggering of magnetospheric noise by ∼1-sec, 10.2-kHz pulses from the Omega transmitter at Forest Port, New York, have been extended to show that the triggering is confined in space to a range of paths with minimum gyrofrequency within a few per cent of twice the transmitted frequency. A similar tendency is evident in noise produced by high-power (∼106 watts) transmitters. A crude ampligram analysis of the cutoff effect shows that in a typical example the whistler wave intensity drops roughly 20 db within a time interval <13 msec and a frequency interval <50 Hz.