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.

Narrow‐band VLF signals with a frequency separation of 100–130 Hz have been detected by a receiver aboard the S3‐3 satellite. The observations were made at L = 2.9 at an altitude of 5700 km. Satellite nadir was 45°N and 151°E. The radiation has the same characteristics as those reported for ground‐based observations of magnetospheric lines resulting from the nonlinear amplification of power line radiation. As is seen in ground‐based observations, the lines are not exact harmonics of the power system frequency, nor are they spaced at exactly 2 or 3 times that frequency. The frequencies of the three dominant lines were typically 7364, 7494, and 7598 Hz. During the time period of these observations the transportable very low frequency (TVLF) transmitter was performing magnetospheric wave injection experiments from a site in Central Otago, New Zealand. The modulation was 0.5 Hz frequency shift keying between 7350 and 8780 Hz. The narrow‐band signals detected by the S3‐3 satellite were observed in the 250‐Hz band above the lower frequency. The narrow‐band magnetospheric lines were apparently observed because power line harmonic radiation was amplified to detectable levels by a nonlinear interaction involving the TVLF signal. The most likely sources of the power line radiation are the 50‐Hz power grids in Tasmania, southeastern Australia, or New Zealand.

A high impedance system has been developed to make direct measurements of the atmospheric potential difference up to several thousand feet. A tethered balloon flown from Wallops Island, Virginia was used to loft a high voltage, insulated wire and a conducting collector in a test flight to 550 meters for two days of experiments in October 1980. The balloon was equipped with a payload to measure exact altitude, wind speed and direction, and other meteorological parameters. Electric potentials of 170,000 volts at 550 meters were measured. The collected currents which could be drawn through the wire by grounding the lower end were in the 10 microamp range indicating a system impedance of about 1010 ohms. This paper will describe the apparatus and details of these measurements.

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Interactions in the magnetosphere between man-made whistler mode waves and electrons can produce either an enhancement of the wave or the generation of waves at a different frequency. Furthermore, natural emissions can be frequency shifted or modified by a nearby transmitter signal. Injection of extremely low frequency waves into the outer magnetosphere was initiated in 1978 inNorway by both the University of Paris and The Aerospace Corporation. The antenna is a 10.6-km power line tuned to the transmitted frequency. The University of Paris used a 1-kW transmitter to drive the antenna with a maximum current of 8 A. The Aerospace Corporation used the transportable very low frequency transmitter with a typical antenna current between 20 and 45 A. Experiments were conducted at times when the GEOS 2 and SCATHA satellites were near the magnetic meridian of the transmitter. The transmissions consisted of either a keyed fixed frequency or continuous waves swept in frequency. Emissions correlated with the transmissions were detected by the satellite receivers on several dates. Although the satellites were within a few degrees of the magnetic meridian of the transmitter, emissions influenced by the transmissions are seen during only a small fraction (<10%) of the total transmission time. With the first type of transmission, signals similar to power line harmonic radiation were recorded. Artificially stimulated emissions are also likely to have been triggered. Both types of transmissions triggered or enhanced hiss at a constant frequency during fixed-frequency transmissions and at a variable frequency during a swept-frequency transmission. During sweptfrequency transmission there occurred two examples of natural emissions shifting in frequency by the man-made signal. Electron cyclotron harmonic emissions also appear to have been shifted in frequency by the man-made signal. EXPERIMENTATION AND SIGNAL PROCESSING In 1975 it was proposed to try to transmit man-made extremely low frequency (ELF) signals to GEES. An antenna was to be built in Sweden, but because of the price of this operation it was decided to use a power line as an antenna. A first line was tested in Norway near Andenes, in March 1978. Transmissions to GEeS 1 were attempted. However, the use of this line brought important perturbations to the Norwegian power distribution network, so that it was necessary to stop the experiment. Another power line was found near Sortland, and The Aerospace Corporation's transportable very low frequency (TVLF) transmitter [Koons and Dazey, 1975] was operated in October 1978 for transmissions to GEeS 2. An example of the signals received will be presented later. During on-off keyed transmissions at 1525 Hz, strong interference with the telephone network took place, and these transmissions also had to stop. A third and last power line was proposed by Norway in 1979 near Kafjord (69.4øN, 20.9øE). It runs 10.6 km up a mountainside and can be used all day long without any interference on the telephone network. Transmissions took place ...

During the 1973 operations of the transportable very low frequency transmitter near Anchorage, Alaska (L ∼ 4), an experiment was performed to determine the effect of controlled phase changes of the transmitted wave on the magnetospherically propagated signal received in the conjugate region. At periodic intervals the phase of the driving voltage was changed (essentially instantaneously) by 180°. The amplitude of the 6.6‐kHz signal detected in the conjugate region went to zero and recovered with a characteristic time constant of 33 ms. This is 10 times longer than the antenna current response time and is in fact comparable with characteristic electron interaction times with whistler mode waves. Between the times at which the phase reversals occurred the received signal was amplitude modulated. The period of the modulation was ∼26 ms. An upper side band was present in the spectrum while these pulsations were occurring. These characteristic times are in general agreement with theoretical predictions of bandwidths, growth rates, and particle‐trapping frequencies for whistler instabilities in the magnetosphere. Data obtained from the controlled transmissions and from lightning‐generated whistlers propagating in the same duct were combined to determine the plasma and wave parameters at the geomagnetic equator. Of particular interest is the level at which the magnetic field of the wave saturated. During the time period for which the data were analyzed this was found to be 3.5 pT (mγ).

A very low frequency (VLF) magnetospheric transmission experiment was conducted between Port Heiden, Alaska, and Dunedin, New Zealand, during August 1972. The transmitter was located at Port Heiden (56.93øN, 201.42øE), and the receiver was located at Dunedin (45.87øS, 170.22øE).A mobile VLF transmitter developed by the U.S. Navy for shore-to-submarine communication was used for the experiment in Port Heiden. The unique feature of this transmitter is the antenna, which is a conducting cable suspended by a helium-filled balloon. The original system was designed to operate with the balloon at altitudes up to 3000 m. The size of the balloon used in this experiment was limited by available resources, and the maximum altitudes possible were between 1200 and 1500 m. Vertical radiators of these dimensions are required because at 10 kHz, the wavelength is approximately 30 km so that antennas of sensible dimensions are much shorter than a wavelength and hence are poor radiators. Conjugate VLF transmissions have been conducted or are