MIDLATITUDE JOVIAN AURORA PRODUCED BY THE IMPACT OF COMET SHOEMAKER-LEVY-9

We propose a theoretical interpretation of the midlatitude ultraviolet auroral emissions in Jupiter's atmosphere that were observed with the Hubble Space Telescope after the impact of comet Shoemaker-Levy-9 fragment K. The emissions were observed 47-57 minutes after the impact at a location similar to 12 degrees south of the (southern-hemisphere) impact site and, with brighter intensity, at the magnetically conjugate site in the northern hemisphere. Consistent with these observations, we hypothesize that the aurora is driven by the atmospheric re-entry of the impact plume. The infalling plume material, moving primarily southward, acts as a snowplow, driving a divergent pair of east-west flow jets in the upper atmosphere. These jets drive polarization currents in the dynamo layer which are closed by Birkeland (magnetic field aligned) current sheets connecting to Pedersen currents in the conjugate northern ionosphere. We expect, by analogy with terrestrial aurora, that the upward Birkeland current sheets require magnetic-field-aligned electrostatic potential drops which in turn accelerate the primary electrons that are responsible for auroral excitation. Other SL-9 fragments of similar size probably produced similar auroral emissions that were not observed, either because the viewing geometry was unfavorable or because uv observations were not made at those times.