Rapid whistler‐mode wave growth resulting from frequency‐time curvature

Abstract

The rapid (<50 ms) temporal growth of ducted whistlers is simulated using controlled injection of VLF pulses from the Siple Station, Antarctica transmitter. The results show that, when the frequency‐time function of the injected pulse has a positive slope and negative curvature, producing a kind of “chirp” such that it approximates the f(t) shape of a lightning‐generated whistler at frequencies above the ‘nose’ frequency, growth up to a saturation level (20–30 dB) commonly occurs within <50 ms as opposed to 200–300 ms that is required for monochromatic input signals. The phenomenon is explained in terms of second‐order‐resonance theory [Helliwell, 1967; Carlson et al., 1985; Chang et al., 1983] where the frequency variation of the pulse matches the changing cold plasma parameters, facilitating enhanced resonance interactions over extended portions of the field line.Magnetospheric whistler‐mode signals that originate in ground sources (e.g. lightning, VLF transmitters) often stimulate or trigger nonlinear responses in the form of amplified signals, narrowband variable frequency emissions and complex sidebands [Helliwell, 1988]. One such effect commonly associated with nose whistlers is the growth of the whistler in the upper part (above the ‘nose’) of its frequency range and the associated emission triggering that tends to occur at the whistler's upper cutoff frequency (usually at 0.5 fH, where fH is the equatorial electron gyrofrequency). An example of the dynamic spectrum of such an event is shown in Figure 1a where the growth of the whistler is represented by the darkening and broadening of the upper segment of the trace. The whistler triggered emission is, in essence, a narrowband oscillation of slowly‐varying center frequency. What makes this phenomenon remarkable is the relatively short time (∼50 ms) of growth (∼20dB) of the whistler compared with the time (200–400 ms) required for a monochromatic signal under comparable conditions to exhibit the same growth [Helliwell, 1988]. Since the triggered emissions associated with whistlers and constant frequency signals (such as those from VLF transmitters) are comparable in their intensities (as observed on the ground or on satellites) and spectral characteristics, one might expect the mechanisms of their generation to be the same. On the other hand, since the typical peak intensities of whistlers excited by lightning impulses are likely to exceed the signal level injected from ground‐based transmitters that have been used in such experiments, one might simply attribute the observed difference in behavior to unknown nonlinear effects (none is suggested here) related to the high intensity of the input signals. In this paper, we describe a new experiment where the upper part (i.e. frequencies above the nose frequency [Helliwell, 1965]) of a one‐hop nose whistler is simulated using the Siple Station experimental VLF transmitter [Helliwell, 1988] to reproduce two‐hop nose whistlers that may exhibit the rapid growth described above (e.g. Figure 1a).