SurrvmaryIt has been shown that the detailed frequency-time shape of discrete very low frequency emissions from the exosphere can be closely fitted on the electron-cyclotron theory by suitably choosing the electron parameters. Additional support for the theory has since been obtained in the verification of its prediction. It is shown here that the fitting process can be reversed so that these parameters can be deduced from the observed spectrograms. These are: the energy and helical pitch of the emitting electrons, the field line which guides them, the instant of their passing through the equatorial plane, and the electron density of the medium. Nomograms are given allowing calculation of these with occasional use of a slide rule and simple arithmetic.
The author has suggested (Dowden 1962) that. the spectrogram (frequency v. time) shapes of V.L.F. discrete emissions can be explained as Doppler-shifted cyclotron radiation from electrons spiralling along a geomagnetic field line away from the observer. The emission frequency is given by Eidman (1958), It is reasonable to assume that y and ,8 are constant, that the electron density in the exosphere is everywhere proportional to the magnetic field, that 8 is either constant or a function of h only, and that ifo is given by the invariance of magnetic moment: sin 2 ifo/sin 2 ifoo = hlho, where subscript zero can refer to any known point such as the equatorial plane or point of minimum magnetic field. In this case the variation of frequency emitted by an electron spiralling along a field line is produced entirely by the variation of * Manuscript
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