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.