Io, a jovian unipolar inductor

Abstract

We show that To may be considered to be a unipolar generator which develops an emf of 7 X 1O~ volts across its radial diameter (as seen from a coordinate frame fixed to Jupiter). This voltage difference is transmitted along the magnetic flux tube which passes through Jo. The induced charge separation on the surface of the flux tube causes the plasma within it to rotate with Jo's orbital angular velocity. A current of about 106 amp is driven across each foot of the flux tube in the Jovian ionosphere. The current flows up along the half-surface of the flux tube which faces Jupiter, crosses the magnetic field in Jo, and then flows down along the opposite half of the flux tube's surface. Because the number density of charge carriers in the magnetosphere is low (we use fi = 0.5 cm3 in our calculations) the current must be carried by keV electrons which are electrostatically accelerated at lo and at the top of Jupiter's ionosphere. We argue that beam instabilities in the current sheets are responsible for the Jo-induced decametric bursts. The geometry of the beaming of the bursts strongly suggests coherent cyclotron radiation as the emission mechanism. In addition, we are led to predict that Europa should modulate decametric bursts whose max- imum frequencies are below 8 Mc/s. Bigg was the first to demonstrate that the position of lo in its orbit was strongly correlated with the reception of Jovian decametric bursts. Previously it had been established that the detection of the bursts was influenced by the position of Jupiter's magnetic dipole. Our aim is to explain the origin of To's great influence. I. THE INTERACTION OF 10 WITH JUPITER'S MAGNETOSPHER