A statistical analysis of the location and width of Saturn's southern auroras

Abstract

Abstract.A selection of twenty-two Hubble Space Telescope images of Saturn's ultraviolet auroras obtained during [1997][1998][1999][2000][2001][2002][2003][2004] has been analysed to determine the median location and width of the auroral oval, and their variability. Limitations of coverage restrict the analysis to the southern hemisphere, and to local times from the post-midnight sector to just past dusk, via dawn and noon. It is found that the overall median location of the poleward and equatorward boundaries of the oval with respect to the southern pole are at ∼14 • and ∼16 • co-latitude, respectively, with a median latitudinal width of ∼2 • . These median values vary only modestly with local time around the oval, though the poleward boundary moves closer to the pole near noon (∼12.5 • ) such that the oval is wider in that sector (median width ∼3.5 • ) than it is at both dawn and dusk (∼1.5 • ). It is also shown that the position of the auroral boundaries at Saturn are extremely variable, the poleward boundary being located between 2 • and 20 • co-latitude, and the equatorward boundary between 6 • and 23 • , this variability contrasting sharply with the essentially fixed location of the main oval at Jupiter. Comparison with Voyager plasma angular velocity data mapped magnetically from the equatorial magnetosphere into the southern ionosphere indicates that the dayside aurora lie poleward of the main upward-directed field-aligned current region associated with corotation enforcement, which maps to ∼20 • -24 • co-latitude, while agreeing reasonably with the position of the open-closed field line boundary based on estimates of the open flux in Saturn's tail, located between ∼11 • and ∼15 • . In this case, the variability in location can be understood in terms of changes in the open flux present in the system, the changes implied by the Saturn data then matching those observed at Earth as fractions of the total planetary flux. We infer that the broad (few degrees) diffuse auroral emissions and sub-corotating auroral patches observed in the dayside Correspondence to: S. V. Badman (svb4@ion.le.ac.uk) sector at Saturn result from precipitation from hot plasma sub-corotating in the outer magnetosphere in a layer a few Saturn radii wide adjacent to the magnetopause, probably having been injected either by Dungey-cycle or Vasyliunascycle dynamics on the nightside.

IMF dependence of the open-closed field line boundary in Saturn's ionosphere, and its relation to the UV auroral oval observed by the Hubble Space Telescope

“…From an analysis of the latter current system, Cowley and Bunce (2003) concluded that the corotation-enforcement currents at Saturn are too weak and occur at too low a latitude to explain the observed UV auroras, and thus suggested that a solar windrelated origin was the more likely. These results have subsequently been amplified by further modelling work presented by Cowley et al (2004b), and also by the statistical analysis of the location of the UV auroras presented by Badman et al (2006), who showed that they occur well poleward of the latitude where the magnetospheric plasma departs from near-rigid corotation according to Voyager data.…”

“…We can also compare these calculated open field boundaries with the typical position of the UV oval determined by Badman et al (2006). These authors presented an analysis of a selection of twenty-two HST images of Saturn's auroras obtained during [1997][1998][1999][2000][2001][2002][2003][2004], including the images shown here in Fig.…”

“…More recent HST observations of higher spatial resolution have Correspondence to: E. S. Belenkaya (elena@dec1.sinp.msu.ru) shown that they generally form a ring around each pole ∼1 • -3 • thick, located between ∼10 • -20 • co-latitude (e.g. Cowley et al, 2004a;Gérard et al, 2004;Badman et al, 2006). Such auroral rings can generally be formed either by the solar wind interaction, leading to emissions in the vicinity of the boundary between open and closed field lines, or by the current system associated with the maintenance of magnetospheric plasma corotation deeper within the magnetosphere (e.g.…”

Pitch angle distributions of energetic electrons at Saturn

“…Mapping the distances of 8, 10, and 12 R S from the equatorial crossing point to the Saturn's southern ionosphere using the Khurana model and assuming an ionosphere 1000 km above Saturn's ellipsoid. Also shown are locations of the southern auroral boundaries as found from HST observations [Badman et al, 2006] and the 20 R S orbit of Titan. The grid shows planetocentric colatitude as the radial coordinate and local time as the azimuthal coordinate (sun toward right).…”

“…The Khurana field model was traced from the equator to the ionosphere using the same techniques used to map the pitch angle distributions. Figure 7 exhibits mapping [Badman et al, 2006].…”