Deep Auroral Ionization and High-Energy Electron Precipitation in Jupiter’s North Polar Region from Juno MWR

During Juno’s extended mission, the spacecraft’s periapsis migrated toward Jupiter’s north pole, enabling high-resolution observations of auroral regions. At microwave wavelengths, these auroral features appear as localized reductions in brightness temperature (“cold spots”). Here we present observations from Juno Perijoves 56–72 using the Microwave Radiometer (MWR). MWR measures thermal emission from Jupiter’s deep atmosphere, which is partially absorbed by ionization produced by precipitating energetic electrons in the ionosphere. By exploiting MWR’s multi-frequency observations, we use the frequency dependence of electron–neutral collisional absorption to probe the vertical extent of auroral ionization at depths well below those accessible to ultraviolet measurements.

We find systematic differences among auroral regions. The deepest ionization occurs at the Io footprint, is moderate along the main auroral oval, and is shallowest in the polar cap. Modeling of the multi-frequency absorption indicates that the Io footprint requires a substantial population of precipitating electrons with energies in the tens-of-MeV range, whereas the main oval can be explained without invoking such high-energy electrons. These results place new constraints on the energy and depth of electron precipitation in Jupiter’s aurora and demonstrate the unique capability of MWR multi-frequency measurements to diagnose deep ionospheric structure, complementing ultraviolet, infrared, and radio occultation observations.