Origin of the main auroral oval in Jupiter's coupled magnetosphere-ionosphere system

We show that the principal features of the main auroral oval in the jovian system are consistent with an origin in the magnetosphere-ionosphere coupli ng currents associated with the departure of the plasma from rigid corotati on in the middle magnetosphere, specifically with the inner region of field -aligned current directed upwards from the ionosphere to the magnetosphere. The features we refer to include its location, its continuity in local tim e, its width, and the precipitating particle energy flux and auroral lumino sity. A simple empirical model of the field and flow in the middle magnetos phere is used to estimate the field-aligned currents flowing into and out o f the equatorial current sheet associated with the breakdown of corotation. The models indicate that the current flows outwards from the ionosphere in to the current sheet through most of the middle magnetosphere. Mapped to th e ionosphere, the upward field-aligned current density is of order similar to1 muA m(-2), confined to circumpolar annular rings around each pole of la titudinal width similar to1 degrees (similar to 1000 km), centred near simi lar to 16 degrees dipole latitude. The upward current is carried principall y by downward-precipitating magnetospheric electrons from the tenuous hot p lasma which extends outside the cooler denser equatorial plasma sheet to hi gh latitudes. For reasonable observed values of the magnetospheric electron parameters it is found that such currents require the existence of field-a ligned voltages of order similar to 100 kV. The auroral primaries are thus similar to 100 keV electrons, consistent with deep penetration of the jovia n atmosphere and low-altitude auroras, as observed. The peak ionospheric en ergy flux associated with the accelerated precipitating electrons is of ord er similar to0.1-1 W m(-2), sufficient to drive a UV aurora of 1-10 MR at s imilar to 20% conversion efficiency. In addition, to produce the current, t he acceleration region must extend in altitude typically above similar to3- 4R(J). The spatially extended energetic auroral electron beams so formed ar e suggested to form a principal source of free energy for non-lo-related ra dio emissions. An important implication of the model is that the main oval auroras and radio emissions will respond principally to the dynamic pressur e of the solar wind, in the sense of anticorrelation. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.