Chapter 9 - Comparative planetary ring currents

Abstract

A significant test of our understanding about the formation and removal of Earth’s ring current populations is how predictive that understanding is when applied to other systems. With varying levels of fidelity, measurements and analyses have been performed on the physics of ring currents for all of the strongly magnetized planets of the solar system, specifically Earth, Jupiter, Saturn, Uranus, and Neptune. We discuss what is known about the similarities and differences between the physical processes that create, transport, and destroy the ring current populations within these systems. Of particular importance is the comparison between Earth and Jupiter. Both of these systems are extremely active and energetic in their own ways, but for very different reasons. Earth is the prototypical solar-wind driven magnetosphere, and solar wind dynamics modulate and buffet its ring current populations. Jupiter is the prototypical rotationally driven magnetosphere with much of the dynamics of the ring current populations driven by the urgent need of plasmas coming ultimately from the volcanoes of the moon Io to find their ways out of the system. Saturn is also a highly dynamic system from a combination of solar wind and rotational effects. But because Saturn’s magnetosphere is more neutral-gas dominated than any of the other systems, its ring current and radiation populations are far more inhibited than they otherwise would be. Uranus and Neptune represent a fascinating contrast. Simple parametric analysis suggest that both systems are rotationally driven, but because of the unique spin axis orientation of Uranus, lying as it does close to the ecliptic plane of planetary orbits, the solar wind can have a far greater influence than it otherwise would have. Neptune, without a substantial power source, by contrast, has the quietest and least energetic magnetosphere and ring current populations of the solar system.