Effect of magnetospheric conditions on the morphology of Jupiter’s ultraviolet main auroral emission as observed by Juno-UVS

Abstract

Auroral emissions are a reflection of magnetospheric processes, and at Jupiter, it is not entirely certain how the morphology of the UV main emission (ME) varies with magnetospheric compression or the strength of the central current sheet. This work leverages the observations from Juno-UVS to link ME variability with particular magnetospheric states. We employed novel arc-detection techniques to determine new reference ovals for the ME from perijoves 1 through 54, in both hemispheres, and analysed how the size and shape of the ME vary compared to this reference oval. The morphology and brightness of the ME vary in local time: the dawn-side ME is typically expanded, while the dusk-side ME is contracted, compared to the reference oval, and the dusk-side ME is twice as bright as the dawn-side ME. Both the northern and southern ME and the day-side and night-side ME expand and contract from their reference ovals synchronously, which indicates that the variable size of the ME is caused by a process occurring throughout the Jovian magnetosphere. The poleward latitudinal shift of the auroral footprint of Ganymede correlates with the poleward motion of the ME, whereas a similar relation is not present for the footprint of Io. Additionally, the expansion of the ME correlates well with an increase in magnetodisc current. These two results suggest that a changing current-sheet magnetic field is partially responsible for the variable size of the ME. Finally, magnetospheric compression is linked to a global ME contraction and brightening, though this brightening occurs predominantly in the day-side ME. This observation, and the observation that the dusk-side ME is typically brighter than the dawn-side ME, stands in contrast to the modelled and observed behaviour of field-aligned currents and thus weakens the theoretical link between field-aligned currents and the generation of the auroral ME.