Alfvén Waves Related to Moon–Magnetosphere Interactions

The electro–dynamic interactions between moons and the magnetosphere of their host planets have been investigated since the mid-20th century, and the implication of the Alfvén waves was recognized right away. However, in the first models, Alfvén waves were only considered as current carriers. It is only after the Voyager missions that the possibility of complex reflection patterns was considered and their ability to accelerate particles became fully appreciated only recently. In this chapter, we review the history of our understanding of the various cases of moon–magnetosphere interactions in our Solar System. The presence of the largest of these moons in the stream of the magnetospheric plasma generates powerful large-scale Alfvén waves, which can break up into smaller scales, reflect off density gradients and accelerate particles, which ultimately impact the atmosphere of the planet to generate auroras and trigger radio emissions. The best known case is the Io–Jupiter interaction, since its observational signatures are the richest and most obvious. As our means of investigation have improved, signatures of similar interactions have also been discovered for the other Galilean moons, as well as for moons orbiting Saturn. Interestingly, similar interactions can occur on rare occasions between the planets themselves and the solar wind, and most likely take place in exo-planetary systems as well.