Generation of VLF and ELF waves for active probing.

Abstract

Generation of electrical power at any frequency in this range is quite simple. Significant generation of waves at frequencies below, say, 5kHz requires antennas some kilometres in length and height above the ground plane. Antenna methods used with some success to probe the magnetosphere and ionosphere are reviewed: balloon lofted vertical monopoles (Alaska and N.Z.), a dipole over ice (Siple), borrowed power lines (arctic Norway and N.Z.), electrojet modulation (U.S.S.R. and Norway).Any such generator consists of a transmitter and an antenna. At VLF, generator design is a simpler task than at higher frequencies and for the powers required the transmitter is relatively inexpensive. The antenna required for VLF on the other hand is very large and expensive having vertical dimensions of the order of a kilometre even for non-resonant antennas and horizontal dimensions of some tens of kilometres for resonant antennas.Following the discovery (HELLIWELL, 1965) that man-made VLF transmissions could (unintentionally) trigger VLF emissions in the magnetosphere, American and Soviet groups used such existing VLF transmitter and antenna installations to do intentional active experiments. These antennas are non-resonant vertical monopoles with extensive top-loading. Their effective heights are very much less than a quarter wave length at the frequencies used and so at the feeding terminals the impedance is almost pure capacitance. A series inductor between the transmitter and antenna terminal is used to tune out this capacitance. With an effective Q of one thousand, the total resistance is reduced to perhaps only 30% above the radiation resistance and so the efficiency may be 75% or more (WATT, 1967). The disadvantage of such a high Q is that the modulation frequency can be only a few hertz. For active experiments this may not be a serious problem for amplitude modulation but it is one if frequency modulation is desired over a large range. As such VLF installations were built for military purposes the active experimenters had very limited access and could only control the On-Off modulation and not the frequency (generally above 15kHz) or the location of the antenna. None-the-less, interesting active sounding ("artificial whistlers") were done by the Stanford group and the Soviet group studied non-linear amplification effects such as modulation instability (LIKHTER et al.,