INFERENCE OF EUROPAS CONDUCTANCE FROM THE GALILEO ENERGETIC PARTICLESDETECTOR

On Galileo's first approach to Europa on December 19, 1996, the Energe tic Particles Detector (EPD) observed decreases in the count rates of low-energy ions in Europa's plasma wake. We understand these decreases to be ''microsignatures'' of Europa, that is, the signatures of recen t satellite absorptions of energetic ions. Using a previously develope d simulation of count rates measured by a detector, we compare simulat ed data with measurements from Europa's immediate and distant wakes. W e find that if Europa is considered to be an inert, solid body absorbe r of charged particles, we cannot account for the depth of observed mi crosignatures using our model. Instead,.if Europa, its ionosphere, or its magnetosphere is sufficiently conducting that Jovian field lines b ecome ''hung up'' on the moon with contact times of the order of the b ounce time of energetic ions, then we can explain the decreases in the data using our model. For a conducting Europa, our simulation gives t he best order-of-magnitude agreement with a global radial diffusion of the order of D-LL = 10(-10)L(4). Simulation results put a lower limit of Sigma(E) approximate to 8 mho on the global conductance of Europa and/or its environs, similar to 50% the value for Io. Finally, higher- energy microsignatures in the EPD data can be understood when we inclu de the effects of a bend in magnetic field lines near Europa that caus es a local curvature drift perpendicular to the direction of plasma mo tion.