Nonlinear standing Alfvén wave current system at Io: Theory

Abstract

We present a nonlinear analytical model of the Alfv6n current tubes continuing the currents through Io (or rather its ionosphere) generated by the unipolar inductor effect due to Io's motion relative to the magnetospheric plasma. We thereby extend the linear work by Drell et al. (1965) to the fully nonlinear, sub-Alfv6nic situation also including flow which is not perpendicular to the background magnetic field. The following principal results have been obtained: (1) The portion of the currents feeding Io is aligned with the Affv•n characteristics at an angle 0A --tan -z MA to the magnetic field for the special case of perpendicular flow where M• is the Affv•n Mach number. (2) The Alfv6n tubes act like an external conductance • --l/(goV•(l + M• 2 + 2M• sin 8) z/2) where VA is the Affv•n speed and 0 the angular deviation from perpendicular flow towards the direction of Alfv•n wave propagation. Hence the Jovian ionospheric conductivity is not necessary for current closure. (3) In addition, the Alfv6n tubes may be reflected from either the torus boundary or 'the Jovian ionosphere. The efficiency of the resulting interaction with these boundaries varies with Io position. The interaction is particularly strong at extreme magnetic latitudes, thereby suggesting a mechanism for the Io control of decametric emissions. (4) The reflected Alfv6n waves may heat both the torus plasma and the Jovian ionosphere as well as produce increased diffusion of high-energy particles in the torus. (5) From the point of view of the electrodynamic interaction, Io is unique among the Jovian satellites for several reasons: these include its ionosphere arising from ionized volcanic gases, a high external Alfv6nic conductance •, and a high corotational voltage in addition to the interaction phenomenon with a boundary. (6) We find that Amalthea is probably strongly coupled to Juoiter's ionosphere while me outer Galilean satellites may occasio•y experience super-Alfv6nic conditions.

Alfvén wings and electromagnetic induction in the interiors: Europa and Callisto

“…We also note that the exact shape is not even elliptical but is a more complex one. As shown in a previous paper [Neubauer, 1980] The additional inertial currents can be neglected, because they are smaller than the currents above by the factor MA.…”

In situ measurements of the plasma bulk velocity near the Io flux tube

“…Under the assumption that lo is a perfect conductor, we find the radius of the flux tube to be ~1.25 R-p » implying that the region of high conductivity is the ionosphere of lo, rather than the satellite itself. For this case, the total current flowing through (or around) lo is given by (see Neubauer, 1980) …”

“…Neubauer (1980) found that an exact solution of the nonlinear MHD equations can be used to describe in detail the magnetic, electric, and velocity fields in the vicinity of ID'S Alfven wing. Analysis by showed that the Voyager magnetometer data are consistent with this theory.…”

Galileo plasma observations at Europa: Ion energy spectra and moments

“…Recent radio occultation experiments gained with the Galileo spacecraft show Europa to have a substantial ionosphere [Kliore et al, 1997]. This conducting ionosphere may carry currents in a system similar to the Alfv6n-wing system at Io [Neubauer, 1980]. These currents and the associated magnetic fields would tend to shield the surface and the atmosphere from the magnetospheric plasmas.…”