Abstract
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.Key PointsWe examine seven cases of joint HST Saturn auroral images and Cassini IMF dataThe persistent but variable dawn arc shows no obvious IMF dependencePatchy postnoon auroras are present for northward IMF but not for southward IMF
Highlights
It has long been supposed that the dynamics of the magnetospheres of the gas giant planets are dominated by the rotational flows imposed by ion-neutral collisions in the planetary ionosphere [e.g., Brice and Ioannidis, 1970; Kennel and Coroniti, 1975]
The patchy postnoon auroras are found to relate to the interplanetary magnetic field (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 interpreted bifurcated auroral arc structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data, whose details remain unresolved in these Hubble Space Telescope (HST) images
In this paper we have examined a unique set of HST observations of Saturn’s dayside auroras during which the Cassini spacecraft was located in the solar wind just upstream from Saturn’s bow shock, measuring the IMF over concurrent intervals of at least several hours
Summary
It has long been supposed that the dynamics of the magnetospheres of the gas giant planets are dominated by the rotational flows imposed by ion-neutral collisions in the planetary ionosphere [e.g., Brice and Ioannidis, 1970; Kennel and Coroniti, 1975]. In a related auroral case study when Cassini was located nearer to the magnetopause, Badman et al [2013] have shown that the simultaneous near-noon magnetosheath field was directed northward, favoring lower latitude reconnection and open flux production at Saturn, with signatures present of escaping magnetospheric electrons. We note in this regard that Saturn’s magnetic dipole is directed parallel to the planet’s spin axis, opposite to the case of the Earth, such that open flux production is favored for northward directed interplanetary magnetic field (IMF) at Saturn rather than for southward IMF at Earth. Belenkaya et al (Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns - implications for auroral source regions, submitted to Annales Geophysicae, 2014), where the bright auroral features have been mapped along magnetic field lines using the “paraboloid” model of Saturn’s magnetospheric magnetic field, which employs the concurrent Cassini IMF values as input
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
