Particle acceleration in Jupiter’s polar region

Observations from Juno have indicated that Jupiter’s polar cap is a unique plasma environment, with plasma density as low as 10⁻³ cm⁻³, precipitating heavy ions at megavolt energies, broadband upgoing energetic electrons and strong wave emissions.  Low plasma densities suggest that the ionospheric plasma in this region is held down by ambipolar potentials due to the large gravitational potentials that limit the ability of thermal plasma to escape the ionosphere.  Despite this barrier, Juno observes upward, energetic electron beams, requiring a low-altitude acceleration of these electrons.  The low plasma density favors the formation of parallel electric fields that could accelerate these electrons, even in the presence of weak downward field-aligned currents associated with co-rotation.  However, observations of precipitating heavy ions (Oxygen, Sulfur) suggest that these field lines are closed in the Jovian plasma sheet.  These ions may be scattered into the loss cone by electromagnetic cyclotron waves that propagate as kinetic Alfvén waves toward the ionosphere, enhancing the field-aligned current.  These processes will be investigated using a combination of fluid and kinetic modeling of polar field lines at Jupiter.