The Magnetosphere of Ganymede (Invited)

Before the 1980s who would have guessed that Jupiter's moon Ganymede was destined to become an exemplar of extremes? Titan had long been described as the largest moon in the solar system with a radius > 2800 km [e.g., Smith, 1980]. Only after Voyager 1 measured the scale of its atmosphere did Titan (radius 2575 km) cede its place as the largest moon in the solar system to Ganymede (radius 2634.1 km). Thereafter Galileo's flybys established additional extraordinary properties of Ganymede. It is the only moon with an intrinsic magnetic field, the only body in the solar system whose magnetosphere forms in a sub-Alfvénic flow, and the only body that does not rotate relative to the symmetry axis of its magnetosphere. Its magnetospheric structure is of special interest as a prototype for magnetospheres in a parameter regime not found in the solar wind. Our knowledge of its properties is based on a combination of in situ and remote sensing measurements, somewhat sketchy but most informative, supplemented by results from computer simulations. To some extent Ganymede's magnetosphere is remarkable for what it lacks. It has no bow shock, no radiation belts, and no plasmasphere. Its shape is also unique, with Alfvén wings stretched almost transverse to the upstream flow replacing tail lobes folded back in the flow direction. It is the only magnetosphere embedded within a magnetosphere, a situation that implies highly predictable and slowly changing upstream plasma and field conditions. This predictability has enabled us to characterize the properties of reconnection under known, steady upstream conditions. Ganymede's magnetosphere becomes even more interesting when compared with other planetary magnetospheres. Using Mach numbers to order magnetospheres from Ganymede to the gas giants, we learn a great deal about the physics relevant to such systems. Even the heliosphere can be fit into the picture. The IBEX spacecraft [McComas et al., 2009] measures the spatial distribution of ENAs thought to form at the heliopause. For an appropriate selection of field orientation, a magnetohydrodynamic simulation shows that the spatial distribution of plasma heated by reconnection on Ganymede's magnetopause, markedly different from the distribution of plasma heated by reconnection on Earth's magnetopause, shares many characteristics with the distribution of ENAs on the heliopause [Kivelson and Jia, 2013]. The similarity suggests that ENAs from the heliopause arise as a consequence of magnetic reconnection, and that the heliosphere may share other features with Ganymede's magnetosphere including the sub-Alfvenic speed of the plasma of the LISM within which it forms.