Radial diffusion in Io's torus: Some implications from Voyager I

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Abstract
Data from several Voyager 1 experiments are used to determine the magnitude and L dependence of the radial diffusion coefficient for low‐energy charged particles outside of Io's orbit under steady‐state conditions. The extreme ultraviolet observations near 685Å are inverted to produce an ion density profile for L > 6. This normalized ion profile, as well as the (equatorial) electron density profile estimated from the planetary radio astronomy (PRA) observations falls off as L−5. Such a density gradient would make possible centrifugally driven cross‐L diffusion outside of Io's orbit without ruling out the presence of an atmospherically driven mechanism. A lower limit for the radial diffusion coefficient DLL is 1.5 × 10−10 L5 RJ² sec−1, yielding a characteristic diffusion time from 6RJ to 7RJ of less than 10 days, much shorter than previously anticipated. Steady‐state diffusion is not a good assumption inside of Io's orbit, where the particle densities decrease sharply from 6RJ to 5RJ; the diffusion time in that region is probably longer than outside of Io's orbit.