Magnetosphere-Ionosphere Coupling in the Jupiter System

Magnetosphere-ionosphere coupling is a topic of central significance in understanding the properties of any planetary plasma environment. Here we briefly review the magnetosphere-ionosphere coupling mechanisms which play a role in the magnetospheres of the rapidly rotating gas giant magnetospheres. In general, we are interested in the extent to which the magnetosphere is driven by internal processes (e.g. sources of plasma, rapid planetary rotation) versus external mechanisms (e.g. upstream solar wind, interplanetary magnetic field). The dominance of one over the other dictates the nature of the large-scale plasma flows and dynamics within the magnetosphere. Large-scale field-aligned current systems are generated in regions of flow shear between the different regimes and flow between the ionosphere and magnetosphere. Thus field-aligned currents are fundamental to the processes that transfer stress between the magnetosphere and the ionosphere via the planetary magnetic field. At Earth the momentum is directed from the solar wind/magnetosphere toward the ionosphere. At Jupiter momentum is mostly transferred from the ionosphere to the magnetosphere. Theoretical modelling of this scenario produces broadly distributed downward field-aligned currents over the polar region (associated with upward electron acceleration to high energies) mapping to the tail and outer magnetosphere, closed principally through a ring of upward current mapping to the middle magnetosphere. The upward current requires downward acceleration of magnetospheric electrons, generating Jupiter's main auroral emission. The associated azimuthal magnetic perturbation, current intensity/density, electron energy, and acceleration region properties can be derived from theoretical modelling. The NASA Juno mission has now provided the first in situ evidence of these parameters in the polar magnetosphere, and this talk will focus on comparing the similarities and differences between observation and theory.