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This report presents the results of an investigation of a proposed method for the controlled artificial generation of ultra-low-frequency (ULF) hydromagnetic waves of class Pc 1 (0.2 to 5 Hz) in the ionosphere and magnetosphere. In this method, which is called the VLF method, a large ground-based very-low-frequency (VLF) transmitter is used to stimulate the ULF waves by injecting pulses of VLF waves into the magnetosphere. A second possible method of ULF wave generation, the peninsula method, is discussed in a companion report. Combining the theoretical and experimental results obtained during this research, it is suggested that naturally-occurring repetitive VLF activity can stimulate Pc 1 pulsation events, and it is further suggested that such VLF activity may be a major source of stimulation for Pc 1 pulsations. Thus, future experiments on ULF wave generation with ground-based VLF transmitters would probably benefit greatly if they were combined with a program of simultaneous observations of naturally-occurring VLF and ULF activity.
The report describes the results of an experiment to investigate the possibility of artificially stimulating ultra-low-frequency (ULF) signals in the earth's magnetosphere by means of signals from a very-low-frequency (VLF) transmitter. A VLF transmitter system was operated near Anchorage, Alaska, in August and September, 1973. The investigator operated a low-frequency (0.1 - 45 Hz) portable magnetometer station at a site adjacent to the VLF transmitter, and also designed a schedule of modulation modes and transmission frequencies for the VLF transmitter in an effort to cause stimulation of ULF signals which would in turn be detected by the magnetometer.
During the 1973 operations of the transportable very-low-frequency (TVLF) transmitter near Anchorage, Alaska (L approx. 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 deg. 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 msec. This is ten 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 about 26 msec. An upper sideband 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. (Author).
Two methods of artificially stimulating ultra-low-frequency (ULF) waves in the ionosphere have been investigated experimentally. Radio-frequency heating of the ionospheric-dynamo-current region has been attempted using a high-frequency transmitter and has been found to be ineffective. Pulse modulation of the U.S. Navy VLF transmitter at Cutler, Maine, has been explored as a means of stimulating geomagnetic micropulsations. In a month-long experiment the transmitter was square-wave modulated at frequencies of 0.2, 1, and 5 Hz, with a network of sensitive magnetometers employed as micropulsation detectors. Though the experimental results are not statistically conclusive, micropulsations occurred on several occasions at harmonics of the transmitter modulation frequency. It is likely that ULF waves may be stimulated by this means under magnetically quiet conditions immediately following geomagnetically active days. It is concluded that practical means of emitting ULF waves for global communications are prohibitively expensive by comparison to other alternatives in the extremely-low-frequency or higher bands.
Contributed articles presented at the Workshop.
The report describes an investigation of the potential usefulness of the ultra-low-frequency (ULF) band for communications with deeply submerged reception terminals. The discussion begins with a comprehensive review of present knowledge about ULF propagation. Present understanding of ULF propagation modes (based on observations of naturally occurring geomagnetic micropulsations) is summarized, and those features which are of importance to potential communications systems are discussed. Emphasis is then placed on the study of potential ULF generation mechanisms between 0.2 and 5 Hz which would benefit from the naturally occurring magnetospheric amplification process that is observed to occur in that frequency band. Two possible source regions are considered for the artificial generation of ULF waves: the ionospheric dynamo and the magnetospheric proton belt. (Author Modified Abstract).
This report presents the results of an investigation of a proposed method for the controlled artificial generation of ultra-low-frequency (ULF) hydromagnetic waves, primarily of class Pc 1 (0.2 to 5 Hz), in the ionosphere and magnetosphere. The basis of this method, which is called the 'peninsula method' (a second possible method, the 'VLF method', is discussed in a companion report), is the passage of a ULF-modulated electric current around a relatively nonconducting peninsula in the sea or in a large saline lake to form a ULF current loop that produces a ULF magnetic field in the lower ionosphere. Provided the amplitude of the ULF magnetic field fluctuations is sufficiently large, i. e., provided the maximum magnetic moment of the peninsula current loop is greater than about 10 to the 13th power Am2, it is predicted theoretically that ULF hydromagnetic waves can be generated in a disturbed region of the lower ionosphere above the peninsula. These waves can then propagate away to large distances in the ionosphere and magnetosphere. The peninsula method is a version of a particular class of ULF wave generation methods based on the use of large ground-based ULF current systems. Compared with other possible methods of generation, these methods appear to have the advantage of reliability and versatility. However, both the construction costs and the power requirements for these systems are large.