The Smallest Magnetosphere as a Model for the Largest

Images of the heliopause obtained from energetic neutral atom (ENA) imaging from the IBEX spacecraft reveal a belt of strong emission whose location on the plane of the sky and whose orientation and symmetry differ from expectations based on early models of the interaction of the solar wind with the local interstellar medium (LISM). The symmetry center is not at the nose of the heliosphere, defined by the direction of motion of the Sun relative to the LISM. Furthermore, the extent of the region of intense emission is larger than anticipated. The authors here suggest that the structure of the mini-magnetosphere of Ganymede, a magnetized obstacle embedded in the submagnetosonic flow of the plasma of Jupiter's magnetosphere, may closely parallel the structure of the heliosphere and that lessons can be drawn from the analogy. Recent observations [McComas et al., 2012] indicate that the flow of the LISM onto the heliopause is sub-magnetosonic, a situation resembling the Ganymede situation more closely than the situation at planets embedded in the solar wind. Not only does this imply that there is no bow shock standing upstream of the heliopause [McComas et al., 2012], but it also implies a configuration of the magnetosphere different from that previously assumed. A magnetohydrodynamic simulation shows that the region in which magnetic reconnection occurs in a Ganymede-like magnetosphere forms a belt whose shape and spatial extent is close to that observed for the source of ENAs observed by IBEX and Cassini. The regions of high pressure on Ganymede's upstream magnetopause for an appropriately chosen upstream field follow contours that are similar to those defining the regions of maximum ENA emissions on the heliopause. The shift off the equatorial plane requires a finite component of the upstream field along the flow direction (as inferred by McComas et al. for the interaction with the LISM).Above, schematic heliospheric configuration [McComas et al., 2009]. Below, cross-sections of Ganymede's magnetosphere [Jia et al., 2009]