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 fra gment K. The emissions were observed 47-57 minutes after the impact at a location similar to 12 degrees south of the (southern-hemisphere) i mpact 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 o f the impact plume. The infalling plume material, moving primarily sou thward, 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 alig ned) current sheets connecting to Pedersen currents in the conjugate n orthern ionosphere. We expect, by analogy with terrestrial aurora, tha t the upward Birkeland current sheets require magnetic-field-aligned e lectrostatic potential drops which in turn accelerate the primary elec trons that are responsible for auroral excitation. Other SL-9 fragment s of similar size probably produced similar auroral emissions that wer e not observed, either because the viewing geometry was unfavorable or because uv observations were not made at those times.