New horizons in planetary magnetospheres

The magnetospheres of Mercury, the Earth and Jupiter provide an especially good comparison of the processes that control the behavior of magnetospheres. The Mercurian magnetosphere is the smallest. Its field lines are anchored in the electrically conducting interior of Mercury and not in a conducting ionosphere. The Earth’s magnetosphere is over an order of magnitude larger than Mercury’s and its ionosphere is dynamically important. Also significant are that synchronous orbit occurs relatively far out in the magnetosphere and that the solar wind is responsible for the energization of processes therein. Jupiter’s magnetosphere is another two orders of magnitude larger. Its synchronous orbit, where gravity and centrifugal force balance, lies deep inside the magnetosphere, about 2% of the way to the magnetopause. Most importantly the jovian magnetosphere has a strong source of ions deep in the magnetosphere, but well outside of synchronous orbit, that are accelerated to high velocities. The energy of these accelerated ions and their centrifugal force are sufficient to drastically alter the behavior of the jovian magnetosphere so that it behaves much differently than the terrestrial magnetosphere. In this review, we examine what we know about each of these magnetospheres in the areas in which their contrasts reveal the underlying controlling factors of magnetospheric behavior.