Ionospheric dynamo theory for production of far ultraviolet emissions on Uranus

A model is presented to explain diffuse FUV emissions from the outer planets, specifically Uranus, in excess of those diffuse emissions that are currently explainable by scattering of sunlight and/or excitation by photoelectrons. These electroglow emissions in H Ly α and H₂ bands, which occur in the sunlit hemisphere slightly above the homopause, appear to require particle excitation in the 10- to 50-eV range. We propose an in situ mechanism for accelerating photoelectrons (and ions) involving neutral wind dynamo generation of field-aligned currents analogous to what occurs in the Earth's equatorial E and F regions. Sufficiently strong field-aligned currents are found in the model calculation for Uranus to produce a potential drop of ~100 eV for greater between the F peak and homopause, concentrated at lower altitudes, and capable in principle of accelerating photoelectrons (and ions) to the 10- to 50-eV energies required to explain the observed emissions. The fact that the excitation and ionization cross sections are larger than elastic scattering cross sections in an H₂ atmosphere at these energies makes in situ acceleration feasible for the production of UV on the outer planets.