Observations of electron gyroharmonic waves and the structure of the Io torus

Narrow‐banded emissions were observed by the planetary radio astronomy experiment on the Voyager 1 spacecraft as it traversed the Io plasma torus. These waves occur between harmonics of the electron gyrofrequency and are the Jovian analogue of electrostatic emissions observed and theoretically studied for the terrestrial magnetosphere, The observed frequencies always include the component near f uhr , the upper hybrid resonant frequency, but the distribution of the other observed emissions varies in a systematic way with position in the torus. A detailed discussion of the observations is presented. A refined model of the electron density variation, based on identification of the f uhr line, is also included. Spectra of the observed waves are analyzed in terms of the linear instability of an electron distribution function consisting of isotropic cold electrons and hot loss cone electrons. The positioning of the observed auxiliary harmonics with respect to f uhr is shown to be an indicator of the cold to hot temperature ratio T C /T H . It is concluded that this ratio increases systematically by an overall factor of perhaps 4 or 5 between the inner (L ∽5 R J ) and outer (L ∽9 R J ) portions of the torus. Other relevant plasma and spectroscopic data are discussed.