An MHD model of Ganymede's mini‐magnetosphere suggests that the heliosphere forms in a sub‐Alfvénic flow

Abstract

AbstractDirectional fluxes of energetic neutral atoms (ENAs) measured by the Interstellar Boundary Explorer spacecraft reveal a Ribbon of strong emission whose source lies beyond the termination shock. Intense emissions of ENAs (energies 0.1 to 6 keV) appear along an extended arc significantly displaced from the nose of the heliosphere, the point on the boundary defined by the Sun's motion relative to the local interstellar medium (LISM). The locus of the Ribbon differs from expectations based on early models of the interaction of the solar wind with the LISM, assumed to flow at a super‐Alfvénic speed. Here we argue that the distribution of the ENA source can be understood if the flow is sub‐Alfvénic. We use a magnetohydrodynamic model of the mini‐magnetosphere of Ganymede, embedded in the sub‐Alfvénic flow of Jupiter's magnetospheric plasma, to establish where heated ions are distributed on the magnetopause. If the flow of the LISM is sub‐Alfvénic, reconnection would occur along an arc centered away from the nose for an appropriately chosen field orientation. Charge exchange with ions heated by reconnection would produce an ENA source distributed in a manner close to that observed. Heating of ions by reconnection can account also for the way ENA images vary with energy. Sub‐Alfvénic flow implies not only that reconnection on the heliopause can be centered well away from the nose, but also that no bow shock forms upstream of the heliopause. It also seems probable that the configuration of the heliosphere differs from the bullet shape frequently illustrated.