Theory and observations of electrostatic ion waves in the cold Io torus

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Abstract
A study of the ELF plasma wave environment of the cold Io torus in Jupiter's magnetosphere is made. Voyager 1 data are presented which show three distinct types of electrostatic ion waves occurring there: the Buchsbaum ion‐ion mode just below the proton cyclotron frequency fcp, hydrogen Bernstein modes at (n+½)fcp, and lower hybrid waves near fLHR. The presence of these waves at their characteristic frequencies is consistent with a predominantly heavy ion plasma composed of singly ionized sulfur and oxygen ions along with a small admixture of protons. The hydrogen Bernstein modes are tightly confined to the magnetic equator, occurring within ±4° of it, while the Buchsbaum mode is localized to the dense heavy ion plasma of the cold torus near the centrifugal equator. A general theory for excitation of the waves based on the ion pickup process is developed. Newly ionized S+ and O+ ions created out of the extended neutral clouds of Io form a ring distribution in velocity space which is unstable to a large variety of plasma wave modes. The particular emissions found here are readily excited given the assumption that the heavy ions are unmagnetized. This condition allows for the propagation of strictly perpendicular wave modes free of electron Landau damping while sustaining large growth rates due to the pickup ions.