Velocity-Resolved Multi-Scale Imaging of Na Escape from Io

Abstract:

Multi-scale measurements from McDonald Observatory of sodium escaping Io in Jan 2012 and Nov 2013 detail the interaction between the atmosphere and the plasma torus. An image-slicer is used to probe Io’s neutral Na clouds on scales of several Io radii with a velocity resolution of 3 km/s. The spatial and Doppler structure of two distinct clouds is measured: a bright low-velocity (< 10 km/s) cloud due to atmospheric sputtering and a narrow high-velocity (10 – 90 km/s) cloud thought to originate from dissociative recombination of NaCl⁺. The high-velocity component populates a diffuse nebula extending beyond 300 Jovian radii, which is the detection limit with concurrent wide-field coronagraphic imaging. Activity in this distant cloud was remarkably quiescent with stable emissions of 15 Rayleighs at 100 Jovian radii to the east and west, falling off slightly faster than 1/r with distance.