Io's oxygen source: Determination from ground‐based observations and implications for the plasma torus

Abstract

From modeling analysis of two different ground‐based observations for Io's neutral oxygen cloud in [O I] 6300 Å emission, new exobase atomic oxygen source rates produced by the composite effects of ion sputtering (represented by an incomplete collisional cascade distribution) have been determined to be 1.27 × 1028 and 1.47 × 1028 atoms s−1, ∼4 times larger than earlier estimates. Implications for local heating and escaping energy rates of neutrals at Io and for ion pickup energy‐input rates to the plasma torus produced by this larger oxygen cascade source and also by other atomic oxygen and sulfur source processes (approximately scaled from the oxygen cascade source) are discussed. The estimated power to sustain these O and S neutral sources is 0.84 × 1012 W, including 0.88 × 1011 for nonescape atmospheric heating. The total ion pickup input power to the plasma torus is estimated to be in the range ∼1–2 × 1012 W with an “outer source region” (neutral clouds above Io's exobase) to “inner source region” (charge exchange below Io's exobase) torus power input ratio of ∼1:1–2. A thin “power ribbon” created by the “inner source region” may provide a rationale for the observed ultraviolet “flashes and sparkles” of the S++ plasma torus ribbon and the very abrupt variations of [O I] 6300 Å emission observed at Io. The physical picture presented provides a viable solution to the mass loading and energy crisis of the plasma torus with the future determination of refined source rates important in establishing its validity.