Jupiter's aurora shows, among other features, a persistent, continuous oval
of luminosity that encircles each magnetic pole and maps along magnetic fi
eld lines to the middle magnetosphere (r approximate to 30 R-J). This auror
al oval is interpreted here as the ionospheric footprint of the upward Birk
eland (magnetic field aligned) current that enforces partial corotation of
magnetospheric plasma moving outward from its source (the Io plasma torus)
to its sink (the outer magnetosphere and ultimately the solar wind). A simp
lified model of this current system, based on the assumption of a constant,
uniform plasma outflow in a spin-aligned dipole magnetic field, has a maxi
mum upward Birkeland current density at an ionospheric latitude that maps t
o the middle magnetosphere. Strong upward Birkeland currents are known, fro
m terrestrial studies, to produce bright aurora.