Drift resonance and stability of the Io plasma torus

The observed local time asymmetry of the Io plasma torus is generally attri buted to the presence of a persistent dawn-to-dusk electric field in the Jo vian magnetosphere. The local time asymmetry is modulated at the System 3 r otation period of Jupiter's magnetic field, suggesting that the dawn-to-dus k electric field may be similarly modulated. We argue that such a System 3 modulation would have a profound disruptive effect on the observed torus st ructure if the torus were to corotate at exactly the System 3 rate: the tor us would be a resonantly forced harmonic oscillator, and would disintegrate in a few rotation periods, contrary to observations. This destabilizing ef fect is independent of, and in addition to, the more familiar effect of the centrifugal interchange instability, which is also capable of disrupting t he torus in a few rotation periods in the absence of other effects. We conc lude that the observed (few percent) corotation lag of the torus is essenti al to preserving the observed long-lived torus structure by detuning the re sonant frequency (the torus drift frequency) relative to the forcing freque ncy (System 3). A possible outcome of this confinement mechanism is a resid ual radial oscillation of the torus at the beat period (similar to 10 days) between System 3 and the torus drift period.