Origin of the main auroral oval in Jupiterʹs 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 coupling currents associated with the departure of the plasma from rigid corotation 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 time, its width, and the precipitating particle energy flux and auroral luminosity. A simple empirical model of the field and flow in the middle magnetosphere is used to estimate the field-aligned currents flowing into and out of the equatorial current sheet associated with the breakdown of corotation. The models indicate that the current flows outwards from the ionosphere into the current sheet through most of the middle magnetosphere. Mapped to the ionosphere, the upward field-aligned current density is of order ∼1 μA m−2, confined to circumpolar annular rings around each pole of latitudinal width ∼1° (∼1000 km), centred near ∼16° dipole latitude. The upward current is carried principally by downward-precipitating magnetospheric electrons from the tenuous hot plasma which extends outside the cooler denser equatorial plasma sheet to high latitudes. For reasonable observed values of the magnetospheric electron parameters it is found that such currents require the existence of field-aligned voltages of order ∼100 kV. The auroral primaries are thus ∼100 keV electrons, consistent with deep penetration of the jovian atmosphere and low-altitude auroras, as observed. The peak ionospheric energy flux associated with the accelerated precipitating electrons is of order ∼0.1–1 W m−2, sufficient to drive a UV aurora of 1–10 MR at ∼20% conversion efficiency. In addition, to produce the current, the acceleration region must extend in altitude typically above ∼3–4RJ. The spatially extended energetic auroral electron beams so formed are suggested to form a principal source of free energy for non-Io-related radio emissions. An important implication of the model is that the main oval auroras and radio emissions will respond principally to the dynamic pressure of the solar wind, in the sense of anticorrelation.