Sheath Acceleration of Photoelectrons by Jupiter’s Satellite Io

Abstract

We are investigating a model of the influence of Jupiter’s moon Io on Jovian decametric radiation due to plasma sheaths formed around Io’s surface. With Io, we are dealing with a large, partially-conducting, and photoemitting body with a large V × B potential (~ 700000 Volts) across its diameter. A well-known model by Goldreich and Lyden-Bell (1969) assumes Jupiter’s field lines are frozen to Io, while we are proposing instead that sheaths form around Io and electrons are accelerated across these sheaths.

Two types of sheaths are considered. A Debye sheath forms around regions of Io’s surface which are negative with respect to the plasma potential while a photoelectron sheath forms where the surface potential is positive. The Debye sheath (of area A _p ) accelerates emitted photoelectrons away from the surface (with current density J _p ) while the photoelectron sheath (of area A _e ) collects an ambient electron current (J _e A _e ). The current balance is J _p A _p = J _e A _e . The boundary between the two regions has zero potential.

We estimate J _e and J _p to be 3 × 10⁻⁷ A m⁻², but both may vary considerably. The emitted particle spectrum is critically dependent on the ratio J _e /J _p . Estimates of total power available in the accelerated photoelectrons are 10¹⁰–10¹³ W, well above the 10⁷ W contained in a typical decametric burst. We have also studied the effect of Io’s orbital position on our model since decametric bursts are strongly coupled to Io’s position.

Although E is probably radial through the sheath, we believe that the electron gyroradius is small compared with sheath dimensions so that the particles emerge almost parallel to Jupiter’s field lines. Using an ‘oblique’ version of Child’s Law, we estimate the typical sheath thickness as 10–50 km. High energy (up to several hundred keV) electrons thus travel along B field lines and eventually produce the observed radio noise.