I recommend reading this paper, it basically describes the Io-Jupiter interaction as a unipolar inductor.
IO-JUPITER SYSTEM: A UNIQUE CASE OF MOON-PLANET INTERACTION
I think papers like this show how stubborn mainstream astrophysicists are in clinging to the dogma of gravitational tidal heating.
Io and Jupiter constitute a moon-planet system that is unique in our solar system. Io is the most volcanically
active planetary body, while Jupiter is the first among the planets in terms of size, mass, magnetic field strength, spin rate, and volume of the magnetosphere. That Io is electrodynamically linked to Jupiter is known for nearly four decades from the radio emissions. Io influences Jupiter by supplying heavy ions to its magnetosphere, which dominates its energetic and dynamics. Jupiter influences Io by tidally heating its interior, which in turn drives the volcanic activity on Io.
An analogy of the Io-Jupiter system is now being applied to the astrophysical and cosmic objects. A conducting body traversing a magnetic field produces an induced electric field. When the circuit is closed, a current will set up, resulting in resistive dissipation. The Jupiter-Io system therefore operates as a unipolar inductor. Another potential cosmic unipolar inductor could be a planet orbiting around a magnetic white dwarf. These systems have a similar configuration, with the differences being their orbital period and separation, the masses and radii of the two components, and the magnetic moment of the magnetic body.
In fact the generation of electric current between the magnetic object and non-magnetic body result in heating of the atmosphere/surface of the magnetic object where the current touches it and would result in production of emission in the polar region of the magnetic object. This could be one way of revealing a planet around a white dwarf that is difficult to detect otherwise.
Why just the polar regions? Why not along certain latitude bands as well? The voltage potential is about 400kV with a current of about 106
A...that's 400,000 volts and 1,000,000 amps.
This article contains a pretty big clue as to the volcano's being discharge events:
Io's Alien Volcanoes
"The biggest mystery about Io's volcanoes is why they're so hot," says Bill Smythe, a co-investigator on JPL's NIMS team. "At 1800 K, the vents are about 1/3 the temperature of the surface of the sun! Billions of years ago basaltic lava on Earth was about that hot, but now -- thanks to mixing in subduction zones -- terrestrial basalts have a lower melting point. Lavas we see now on Earth are about 300 K cooler than they used to be. It's very surprising to see lava flows on Io as hot as these ancient flows on Earth. Why? Simply because Io's soil has been reworked many, many times, so the melting temperature should be lower for the same reason that Earth's basalts melt at a lower temperature. It's a real mystery."
I'm pretty sure the plumes move around a bit also.
Funny how the solar systems most electrically dynamic moons happen to have volcanic plumes as well.