We examine a unique data set from seven Hubble Space Telescope (HST) “visits” that imaged
Saturn’s northern dayside ultraviolet emissions exhibiting usual circumpolar “auroral oval” morphologies,
during which Cassini measured the interplanetary magnetic field (IMF) upstream of Saturn’s bow shock over
intervals of several hours. The auroras generally consist of a dawn arc extending toward noon centered near
~15° colatitude, together with intermittent patchy forms at ~10° colatitude and poleward thereof, located
between noon and dusk. The dawn arc is a persistent feature, but exhibits variations in position, width, and
intensity, which have no clear relationship with the concurrent IMF. However, the patchy postnoon auroras
are found to relate to the (suitably lagged and averaged) IMF Bz, being present during all four visits with
positive Bz and absent during all three visits with negative Bz. The most continuous such forms occur in the
case of strongest positive Bz. These results suggest that the postnoon forms are associated with reconnection
and open flux production at Saturn’s magnetopause, related to the similarly interpreted bifurcated auroral arc
structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data,
whose details remain unresolved in these HST images. One of the intervals with negative IMF Bz however
exhibits a prenoon patch of very high latitude emission extending poleward of the dawn arc to the magnetic/
spin pole, suggestive of the occurrence of lobe reconnection. Overall, these data provide evidence of
significant IMF dependence in the morphology of Saturn’s dayside auroras.