Identification of Jupiter's magnetic equator through H3+ ionospheric emission

Our understanding of Jupiter's magnetic field has been developed through a combination of spacecraft measurements at distances >1.8R_J and images of the aurora^1-7. These models all agree on the strength and direction of the Jovian dipole magnetic moments, but because higher-order magnetic moments decay more strongly with distance from the planet, past spacecraft measurements could not easily resolve them. In the past 2 years, the Juno mission has measured very close to the planet (>1.05R_J), observing a strongly enhanced localized magnetic field in some orbits^8,9, and resulting in models that identify strong hemispheric asymmetries at mid-to-high latitudes^10,11. These features could be better resolved by identifying changes in the ionospheric density caused by interactions with the magnetic field, but past observations have been unable to spatially resolve such features^12-14. In this study, we identify a dark sinusoidal ribbon of weakened H₃⁺ emission near the jovigraphic equator, which we show to be an ionospheric signature of Jupiter's magnetic equator. We also observe complex structures in Jupiter's mid-latitude ionosphere, including one dark spot that is coincident with a localized enhancement in Jupiter's radial magnetic field observed recently by Juno¹⁰. These features reveal evidence of complex localized interactions between Jupiter's ionosphere and its magnetic field. Our results provide ground-truth for Juno spacecraft observations and future ionospheric and magnetic field models.