The space between the planets is filled with a solar wind of plasma that expands supersonically outward from the Sun. Most planets, including Earth and Jupiter, have an intrinsic magnetic field that deflects the solar wind, creating a comet‐shaped cavity called a magnetosphere. Earth's magnetosphere, for example, extends about 10 Earth radii in the upstream, sunward direction and hundreds of Earth radii downstream. Frictional coupling at the magnetopause drives plasma circulation, known as convection, within the magnetosphere, a dynamic process that gives rise, among other things, to visible aurora and planetary radio emissions.