Maximum scales for preserving flux tube content in radial diffusion driven by interchange motions

Abstract

Plasma in the inner magnetosphere of Jupiter is commonly assumed to be transported radially by fluctuating interchange motions, carrying, on the average, high‐density flux tubes outward and low‐density flux tubes inward. On a coarse‐grained view, this is a diffusion process; viewed on a sufficiently fine scale, high‐density outward‐diffusing tubes and low‐density inward‐diffusing tubes might be observable. However, the required fine scale is much smaller than the characteristic size of circulating eddies of the interchange motion; as a consequence of repeated bifurcations of the flow as the eddy pattern changes randomly, only flux tubes of cross‐sectional dimension smaller than a typical eddy size by the square of (eddy size divided by diffusion distance) have a reasonable chance of preserving their initial plasma content. Energy‐dependent guiding center drifts will randomize and smooth out density variations even on this fine scale if the plasma energy spectrum is wide enough; from a simple model of gradient drifts, the required energy width is proportional to the magnetic flux through a typical eddy divided by the diffusion time. The absence of fine‐grained density fluctuations in the Voyager plasma observations can be explained by such smoothing from gradient drifts, given the observed typical ion thermal energy spread of some 40 eV and the assumption that the typical eddy size is no larger than about 0.7 Jovian radii.