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

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