Radio signals in the 1.5-to 16-kHz range transmitted from Siple Station, Antarctica (L = 4), are used to control wave-particle interactions in the magnetosphere. Observations at the conjugate point show signal growth and triggered emissions including risers, failers, and hooks. Growth rates of the order of 100 dB/s and total gains up to 30 dB are observed. Triggered emissions may be cut off or have their frequency rate of change altered by Siple pulses. Similar effects occur in association with harmonics from the Canadian power system. The observed temporal growth is predicted by a recently proposed feedback model of cyclotron interaction. Plasma instability theories predicting only spatial growth are not in accord with these observations. Possible applications include study of nonlinear plasma instability phenomena, diagnostic measurements of energetic particles trapped in the magnetosphere, modification of the ionosphere through control of precipitation, and VLF communication through the magnetosphere. This is a report of the first results of a new experiment to control wave-particle interactions in the magnetosphere, using VLF waves. Coherent whistler mode signals were injected into the magnetosphere in the 1.5-to 16.0-kHz range from a 100-kW transmitter located at Siple Station, Antarctica. The output signals were observed at the Siple conjugate point near Roberval, Quebec. The signals followed field-aligned ducts near L = 4 (see sketch in Figure la). Among the new results is the observation that the signal received at Roberval generally shows exponential growth as a function of time, with growth rates of the order of 100 dB/s. Total power gains of up to 3 orders of magnitude (30 dB) have been observed. Following the exponential growth phase, emissions at new frequencies are usually triggered, in the form of narrow band rising or falling tones. A by-product of these investigations is the finding that VLF radiation at harmonics of the commercial power system appears to be present in the magnetosphere, modifying the spectrum of the triggered emissions.In the following section of this paper we review briefly the previotis experimental work on VLF wave injection into the magnetosphere. In the next three sections we describe the siting of Siple Station, the apparatus, and some of the first experimental results. In the last section we discuss these results and some possible applications. PREVIOUS WORK Man-made whistler mode signals were first detected atCape Horn from Station NSS on 15.5 kHz, located at Annapolis, Maryland [Helliwell and Gehrels, 1958]. Later discrete VLF emissions were observed to be triggered by Morse code transmissions from Stations NPG on 18.6 kHz and NAA on 14.7 kHz [Helliwell et al., 1964]. Nearly all triggering was caused by the Morse dashes. When the dots did trigger, the resulting emissions were always weak falling tones [Lasch, 1969]. Emission triggering by the long (• 1 s) pulses from a relatively low power (• 100 W) Omega transmitter operating on 10.2 kHz was also observed [Kimura, 1968...

Sudden changes in the amplitude of long‐distance subionospheric VLF transmissions have been found at night in association with whistlers. Both increases and decreases in signal strength have been observed, depending on signal frequency and orientation of the receiving antenna. Sample observations at Eights Station in Antarctica of station NSS (Annapolis, Maryland) on 22.3 kHz showed increases in signal strength that averaged 3 db, with rise times of about 2 sec and durations of about 30 sec. Coincident with every rise was a midlatitude (L ≈ 2.5) whistler originating in the northern hemisphere. To explain this association, it is suggested that the whistler dumps energetic (30–300 kev) electrons into the D region. The resulting ionization then alters the properties of the earth‐ionosphere wave guide. The mechanism of precipitation is thought to be pitch angle scattering of trapped electrons that resonate with the magnetic field of the whistler wave near the magnetic equator.

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In a recent experiment, discrete VLF emissions from the magnetosphere were triggered by a transmitter at Siple Station in Antarctica. Spectrograms of these signals as received at the conjugate point, Roberval, Quebec, showed changes in slope, entrainments, and cutoffs at frequencies (several kilohertz) close to the harmonic induction lines from the local 60‐Hz power system. This observation led to the suggestion that harmonic radiation from the power system enters the magnetosphere and interacts with the triggered emissions. New evidence supporting this suggestion has been found in spectrograms of simultaneous recordings made at Roberval and at Siple Station in Antarctica. It is shown that line radiation, near harmonics of 60 Hz, travels along the earth's magnetic field in the whistler mode and is received in the conjugate hemisphere at Siple Station. Echoing of the line radiation between Siple and Roberval is often observed. The magnetospheric lines are usually shifted in frequency by 20–30 Hz with respect to the adjacent induction line, but their spacings are near 120 Hz. They may trigger and cut off emissions as do signals from VLF transmitters. Occasionally, magnetospheric lines are seen with spacings of only 20–30 Hz. This smaller frequency separation and the frequency shift of other lines spaced 120 Hz apart are related to the positive frequency offset of emissions triggered by VLF signals from the Omega navigation transmitters. Harmonic lines of reasonable amplitude (∼10−3 γ) are shown to enhance significantly the precipitation of 2‐keV electrons over the eastern parts of the American continents near L ∼ 4. Some mid‐latitude hiss bands appear to consist of sets of magnetospheric lines and their associated triggered emissions.