Linear and nonlinear amplification in the magnetosphere during a 6.6‐kHz transmission

Abstract

Reception at Dunedin of the magnetospheric signal at 6.6 kHz transmitted from Anchorage, Alaska, showed both linear and nonlinear amplification during an event lasting some 20 min near local midnight. Linear amplification of the transmitter signal was ∼20 dB. Natural whistlers were also amplified but often at frequencies sharply limited to those from the transmitter frequency upward. Nonlinear amplification (NLA) produced a signal positively offset from the transmitter frequency by 20–150 Hz at amplitudes over 40 dB above the unamplified transmitter signal. This signal appeared as a largely self‐sustaining embryo emission (EE) under the control of the transmitter signal. The phase of this EE signal in each NLA event was tracked with respect to a recorded phase reference. These phase studies showed that the accumulating phase of the offset EE signal is frequently interrupted by negative phase steps (‘N events’) which tend to reduce the offset frequency. Five of the NLA events during key‐down transmission were quenched by whistlers which themselves triggered free emissions at ½ƒBO. The theory of nonlinear wave‐wave interaction between the transmitter or input wave (IW) and the embryo emission is developed to explain these features. It is shown that coupling depends on the offset frequency δƒ and the ‘control frequency’ Fc: for δƒ > Fc the emission is effectively free; for δƒ < Fc, EE is controlled by IW. Curiously, Fc is determined by the EE amplitude (Bw) as Fc ∞ BW1/2, and is almost independent of IW amplitude. This control applies whether the emission was originally generated by IW or captured by it. Fc is determined from Bw measurement to be 60–120 Hz, which fits the observed behavior quite well. For δƒ < Fc a fraction of the phase‐bunched electrons are trapped by IW as they are detrapped by EE in the growth region. This superimposes a strong component oscillating in phase which can produce N events, effectively phase locking the low‐amplitude end of EE to IW. Amplitude fluctuations, δƒ, and Fc are interrelated in a complicated way which gives rise to short‐term instabilities and somewhat longer‐term stabilizing influences.

Magnetospheric VLF line radiation observed at Halley, Antarctica

“…Perhaps selective amplifi cation of lines from two or more uncoupled sys tems, followed by line frequency drifting takes place. According to DOWDEN et al (1978), a relatively weak input signal such as an unamplified Power Line Harmonic can act as a source of embryonic emission under favourable conditions. The input wave will control the embryo emission as long as the frequency offset of of the embryo emission from the input wave is less than F e , the control frequency, where, , we obtain an upwards frequency drift rate of "'10 Hz/min, which is in rough agreement with our measure ments.…”

“…Results from controlled experiments, in which VLF signals transmitted from the ground are used to stimulate equatorial wave generation out to several earth radii, have provided new information concerning the wave/particle and wave/wave interactions which govern wave growth (HELLIWELL and KATSUFRAKIS, 1974;McPHERSON et al, 1974;KOONS and DAZEY, 1974;DOWDEN et al, 1978). These results have given a stimulus to new theoretical work (HELLIWELL and CRYSTAL, 1973;NUNN, 1974;DOWDEN et al, 1978), which in turn has led to a greater un derstanding of the role that relatively weak coherent waves such as PLHR can play in the growth phase of VLF emissions.…”

“…These results have given a stimulus to new theoretical work (HELLIWELL and CRYSTAL, 1973;NUNN, 1974;DOWDEN et al, 1978), which in turn has led to a greater un derstanding of the role that relatively weak coherent waves such as PLHR can play in the growth phase of VLF emissions.…”

Physical mechanisms of man-made influences on the magnetosphere

“…Their ionospheric perturbations have mainly been observed in-situ by satellites. It includes: triggering of new waves (Dowden et al, 1978;Bell et al, 1983;Titova et al, 1984;Bell, 1985;Groves et al, 1988;Bell andNgo, 1988, 1990;Sotnikov et al, 1991), ionospheric heating (Inan, 1990;Bell et al, 1991;Dowden and Adams, 1992;Inan et al, 1992;, wave-electron interactions (Inan et al, 1978;Inan et al, 1982Inan et al, , 1984Inan et al, , 1985Dowden, 1985), and particle precipitation (Koons et al, 1981;Vampola, 1987Vampola, , 1990. Moreover, magnetospheric amplification of VLF pulses through wave-electron interaction is enhanced during magnetic storms (Smith and Clilverd, 1991).…”

Generation of VLF and ELF waves for active probing.

“…Such a system was operated at two different sites in Alaska in 1971and 1973respectively, in New Zealand in 1975 and, without the balloon and winch system, in Norway in as discussed below. Active experiments using the TVLF system so placed and a receiver at the conjugate studied linear and non-linear amplification in the magnetosphere including modulation instabilities, artificial triggering of emissions, quiet bands, and the phase variation of trapped emissions relative to the transmitter signal (DOWDEN et al, 1978). Although the TVLF is still the only transportable station in use for active experiments, it is costly to transport and labour intensive and therefore costly to operate.…”