A cyclotron theory for the beaming pattern of Jupiter's decametric radio emission

A startling feature of ground-based observations of Jupiter's decametric radio emission (DAM) is the modulating effect of Io. The Voyager data showed that the DAM appears as nested arcs in the frequency-Jovian longitude plane. It is now accepted that DAM is generated in the extraordinary (x-) mode of magnetoionic theory just above the electron gyrofrequency in a region with a plasma frequency much less than this gyrofrequency. A general theory for cyclotron instabilities is currently being developed. A description is presented of some aspects of the theory from a qualitative viewpoint. It is shown that when applied to DAM the theory leads to a prediction of emission concentrated on a thin hollow cone. This makes the theory a particularly attractive candidate for the interpretation of DAM. Attention is given to the relativistic effect arising from the Doppler or resonance condition, the growth rate, the 'nonrelativistic' case, and the 'semirelativistic' approximation.