  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 magnetospheric states. Novel arc-detection techniques are used
to determine new reference ovals for the ME from perijoves 1 through 54, in
both hemispheres, and analyse 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 and the dusk-side ME typically
contracted compared to the reference oval, and the dusk-side ME being typically
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.
