Three-dimensional plasma simulation of Io's interaction with the Io plasmatorus: Asymmetric plasma flow

A three-dimensional, stationary, two-fluid plasma model for electrons and o ne ion species was developed to understand the local interaction of Io's at mosphere with the Io plasma torus and the formation of Io's ionosphere. Our model calculates, self-consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a gi ven neutral atmosphere and imposed Io plasma torus conditions but assumes f or the magnetic field the constant homogeneous Jovian field. With only phot oionization in a pure SO2 atmosphere it is impossible to correctly model th e plasma measurements by the Galileo spacecraft. With collisional ionizatio n and photoionization the observations can be successfully modeled when the neutral atmospheric column density is N-col = 6 x 10(20) m(-2) and the atm ospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy fo r the maintenance of the collisional ionization process and thus the format ion of Io's ionosphere. Anisotropic conductivity is shown numerically as we ll as analytically to be essential to understand the convection patterns an d current systems across Io. The electric field is very greatly reduced, be cause the ionospheric conductances far exceed the Alfven conductance Sigma( A), and also strongly twisted owing to the Hall effect. We find that the el ectric field is twisted by an analytic angle tan Theta(twist) = Sigma(2)/(S igma(1) + 2 Sigma(A)) from the anti-Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Sig ma(1) and Sigma(2) within a circle encompassing Io's ionosphere. Because th e electron velocity is approximately equal to the E x B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, w ith E and B the electric and magnetic fields, respectively. Since Sigma(1) similar to Sigma(2), the electron how is strongly asymmetric during convect ion across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupite r when passing Io. Large electric currents flow in Io's ionosphere owing to these substantially different flow patterns for electrons and ions, and ou r calculations predict that a total electric current of 5 million A was car ried in each Alfven wing during the Galileo flyby. We also find a total Jou le heating rate dissipated in Io's ionosphere of P = 4.2 x 10(11) W.