The Need for Ultraviolet Remote Sensing in Tandem with In Situ Particle and Fields Measurements for a Complete Understanding of Planetary Systems

While observations from a single remote sensing or in situ instrument can check the boxes of unique science questions within a mission’s traceability matrix, it is common that combination of instrumental data from a host of instruments often leads to a more fundamental understanding of the physical processes occurring within the system. This is born out time and again in planetary missions of all sizes. In the Juno Mission an ultraviolet spectrograph (UVS) was included with a primary objective to measure and characterize the UV aurora to offer context and a connection to the instantaneous in situ measurements of the particles and fields at the Juno spacecraft. While the UV observations of Jupiter’s aurora have proved interesting in their own right (e.g., Bonfond et al., 2017; Bonfond et al., 2021; Greathouse et al., 2021; Hue et al., 2019), they have also been integral in relating the particle and fields results to a global view of Jupiter’s magnetosphere (e.g., Mauk et al., 2017; Sulaiman et al., 2022; Szalay et al., 2018). In addition to the planned science of a mission captured within a traceability matrix, there are often unplanned observational opportunities or surprising discoveries during a mission which can be more thoroughly understood/studied with a diverse dataset including remote and in situ observations. In this presentation, as an example, we will describe how the combination of ultraviolet spectral data along with in situ fields and particle measurements were leveraged by the Juno Mission to uncover new and exciting results about Ganymede (Waite et al., 2024) even though close flybys of Jupiter’s moons did not exist in the original Juno Mission design.
The UVS maps of Ganymede’s ultraviolet auroral emissions offer insights to how Ganymede’s magnetic field interacts with Jupiter’s magnetic field. The Juno Mission’s PJ34 UVS data provide a sparse, but high-resolution look at Ganymede’s aurora, and can be used to map the polar-most auroral emissions as a function of latitude to an accuracy of about one degree. This polar boundary may be the position of Ganymede’s last closed field line and if so, can be used as an instantaneous mapping of the Jupiter/Ganymede magnetic field geometry. The emission maps show surprising latitudinal structure of the auroral emissions on Ganymede’s leading hemisphere, where there is an intense and narrow auroral curtain exhibiting a sharp polar cut-off with a more slowly tapering of emission towards the equator. Observations of the auroral emissions on the limb of Ganymede are found to be vertically unresolved and located just above the surface. Detailed modelling of the JADE (the Jovian Auroral Distributions Experiment) electron and ionospheric measurements coupled with the UVS observations allow for a more detailed solution to the probable structure of Ganymede’s atmosphere and the precipitating electrons causing the auroral emissions.

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