Saturn's dayside ultraviolet auroras: Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field

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

Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

“…In a parallel study of these images, Meredith et al (2014) found no clear IMF response in the dawn emissions, while patchy auroras observed at higher latitudes in the noon to dusk sector were found to occur for northward but not for southward IMF, thus suggesting a connection with reconnection-related processes at the magnetopause as discussed previously by Radioti et al (2011 and Badman et al (2013). In addition, a prominent patch of polar emissions observed in one image for southward IMF was suggested to be due to high-latitude "lobe reconnection" between the IMF and open tail flux tubes poleward of the dayside cusp (e.g.…”

“…Nevertheless, this does not preclude the solar wind interaction from playing an important role in outer magnetosphere dynamics at the giant planets; furthermore, it is known, in particular, that bright auroral emissions extend to very high latitudes in the dawn sector at Saturn in response to strong magnetospheric compressions by the solar wind ). In addition, structured dayside auroral features observed in the post-noon sector have been interpreted as the signatures of time-dependent reconnection and openflux production at the dayside magnetopause, similar to "flux transfer events" at Earth (Radioti et al, 2011Badman et al, 2013;Meredith et al, 2014).…”

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

“…However, that paper examined only the data from the passes over the southern auroral region, such that here we complete the survey by also examining the northern data. We note that other variable current systems may also generally be present associated with the solar wind interaction [e.g., Cowley et al, 2004a;Southwood and Kivelson, 2009;Radioti et al, 2013;Meredith et al, 2013Meredith et al, , 2014, fixed in local time (LT) principally on the dayside, but since the observations to be discussed here are restricted to a narrow LT sector on the nightside, these will not be considered further here. symbols the current system, and the blue arrowed lines and symbols the perturbation field associated with that current system, while in the ionospheric diagram the red dashed lines show the associated atmospheric/ionospheric flow.…”

Open and partially closed models of the solar wind interaction with outer planet magnetospheres: the case of Saturn

“…It should be noted, however, that some observations of reconnection at the dayside magnetopause for northward IMF do nevertheless exist (e.g., McAndrews et al, 2008). Meredith et al (2014) studied the northern polar cap and found that post-noon auroral arcs exist for northward IMF, but not for southward IMF. They suggested that these were associated with low-latitude reconnection occurring for northward IMF.…”

“…Taking this argument into account, we conclude that the open model is probably appropriate for northward IMF also. Meredith et al (2014) explained that the absence of FTEs at the Kronian magnetopause was likely due to the different timescales of such phenomena relative to the case of Earth due to the much larger magnetospheric length scales involved. As noted above, the open-closed field line boundaries calculated for the two models using the paraboloid model do not show great difference for northward IMF.…”