New description of Io's cold plasma torus

Despite more than 25 years of study of the Io plasma torus, its generation, dynamics, and even its spatial structure are still poorly understood, especially in the case of the inner, cold region of the torus. To remedy this lack, we analyzed ground-based coronagraphic images of the torus in S+ 6371 angstrom emission. We derived cold torus properties by modeling and removing these images' inherent line-of-sight integration and atmospheric blurring, using new deconvolution techniques, obtaining high-spatial-resolution estimates of the three-dimensional (3-D) S+ distributions. From these 3-D distributions, we discovered that the cold torus is washer-shaped, with a roughly constant vertical thickness <= 0.25 Jovian radius (R-J), and a radial width that varies from 0.6 to 0.9 R-J. The cold torus is separated by a 0.1-0.2 R-J-wide low-density region, or gap,'' from the ribbon'' region which lies just outside it. The small, approximately constant washer height implies an ion parallel temperature (T-parallel to) of similar to 3eV, compared with a ribbon T-parallel to that varies from about 20 to 50 eV as a function of Jovian magnetic longitude (lambda(III)). The washer has a distinct inner edge, not seen before, whose jovicentric distance varies with lambda(III) so as to create the variable cold torus width. Thus this inner edge is concentric with neither Jupiter nor the rest of the torus. We also confirm the existence of a tilt between the midplanes of the ribbon and cold torus, with an orientation that cannot be produced by the magnetic mirror force acting on ion temperature anisotropy. The structure and composition of the gap and cold torus are best explained by a model in which a small amount of warm S+ plasma diffuses inwards while radiatively cooling. While still warm, its distribution over a large scale height keeps its density small, forming the gap. After sufficient cooling, it collapses to the centrifugal equator, where its higher density and continued inward diffusion make it more visible as the cold torus washer. However, its low electron temperature (probably <= T-parallel to) must be kept from further decline by a hitherto-unsuspected energy source that powers the observed visible wavelength radiation from the cold torus and fluctuates on timescales less than the plasma diffusion time. The formation of the abrupt cold torus inner edge might indicate the loss there of either this energy source or the plasma itself.