APPARENT AND REAL FINE-STRUCTURES IN MAGNETOSPHERIC ULF OR VLF EMISSIONS

A theoretical study is presented of the apparent frequency-time structures which should result from different frequency analysis techniques applied to two kinds of signals. Signals of type I consist of two or more gliding tones (whose frequency varies linearly with time) closely spaced in time. Type II signals consist of two or more quasi-monochromatic emissions of finite duration, slightly displaced in frequency and time. It is shown that when very narrow or very wide filter bandwidths are used, similar frequency-time patterns may be obtained with both signals. These patterns are described, as a function of the characteristics of both the signal and the filter. A procedure is suggested which consists of analyzing the same signal with different filter bandwidths and which enables one to distinguish between type I and type II signals in most cases. The validity of the theory is checked on computer-simulated wave forms. Application is also made to natural magnetospheric emissions recorded both on the ground and on board satellites.