THEORY AND OBSERVATIONS OF ELECTROMAGNETIC ION-CYCLOTRON WAVES IN SATURNS INNER MAGNETOSPHERE

High-resolution Voyager 1 magnetic field observations of Saturn's inner magnetosphere are examined for the presence of ultralow frequency waves. Quasi-circular left-hand polarized transverse oscillations are found in the near-equatorial region of 5-7 R(S) with a wave period approximately 10 s and peak amplitude approximately 2 nT. The wave is identified as the electromagnetic oxygen cyclotron mode occurring at a frequency just below the O+ ion cyclotron frequency. The properties of the waves are very similar to those found by Smith and Tsurutani (1983) using Pioneer 11 data. A theoretical model of wave excitation based on gyroresonant coupling through a temperature anisotropy of O+ pickup ions is developed which accounts for the principal features of the wave spectrum. The theory and observations form the basis for the hypothesis that wave-particle interactions provide a level of scattering commensurate with the weak pitch angle diffusion regime but nonetheless one that regulates and maintains a constant thermal anisotropy of ions along the magnetic field. Arguments are also presented that O+ was the dominant thermal ion of the Dione-Tethys plasma torus at the time of the Pioneer 11 encounter the year previous to the Voyager 1 measurements.