The Very Low-Frequency Emissions Generated in the Earth's Exosphere

Naturally occurring, very low-frequency signals not associated with lightning discharges, and strongly correlated with solar activity, have been recognized nearly as long as the atmospheric whistlers which have their genesis in lightning discharges. Whereas whistlers have been satisfactorily explained, until recently these other phenomena have not. From the examination of a large quantity of high resolution spectrograms, it has been deduced that a major fraction, if not all, of these other "noises" are excited in the exosphere by streams and bunches of high-speed ionized particles precipitating into the ionized atmosphere in the presence of the earth's magnetic field. The electromagnetic waves excited are then propagated in the manner of whistlers. The excitation mechanism is similar to the operation of a traveling-wave tube. Two frequencies are simultaneously generated, and are given by: 2 [ H-c) ] Their values depend only on the three parameters: local electronic density in the exosphere, local magnetic field intensity, and particle velocity V. For a large range of conditions, corresponding to middle latitude observations, the low frequency explains many types of noise, and reduces to (V) 2 COP2 CO1 = ) _. Most of the observations require particle velocities of the order of 10,000 km/sec. The shape of the spectrum is also very sensitive to the ambient electronic density distribution. A model in which the ratio of electron density to magnetic field strength is almost constant, along a line force in most of the exosphere, seems indicated by several types of noise.