Characterization of auroral current systems in Saturn's magnetosphere: High‐latitude Cassini observations

Abstract

We have identified seven periapsis passes during the first high‐latitude phase of the Cassini mission, from mid‐2006 to mid‐2007, in which the spacecraft traversed at intermediate altitudes the region between open field lines at highest latitudes and the region inside the inner edge of the ring current. Varying azimuthal magnetic fields indicative of the presence of field‐aligned currents were observed in both hemispheres on all these passes, corresponding to the dawn and prenoon sector in the summer southern hemisphere and the dusk to premidnight sector in the winter northern hemisphere. In the southern hemisphere, strongly “lagging” fields observed on open field lines are observed to decline rapidly across the open‐closed field line boundary, usually then reversing in sense to a “leading” configuration in a narrow layer of closed field lines, before declining to smaller values determined by the phase of the planetary period oscillation in the inner region. These observations suggest that the plasma flow in this sector increases sharply across the boundary from subcorotation on open field lines to a layer of supercorotation on closed field lines, accompanied by a major layer of upward‐directed field‐aligned current that is shown to be colocated with the statistical location of the southern auroral oval. Downward current then flows in the inner region as the leading field declines. In the northern hemisphere, however, only weak azimuthal fields are observed on open field lines, suggesting weak conductivity in the winter ionosphere if the plasma similarly subcorotates, while a layer of stronger lagging field indicative of subcorotation is observed immediately equatorward in the closed field region. The field‐aligned currents in this case are thus directed downward just inside the boundary and upward in the interior region, opposite to the southern hemisphere, but are again modulated by the field of the planetary period oscillations on either side.

Magnetospheric mapping of the dayside UV auroral oval at Saturn using simultaneous HST images, Cassini IMF data, and a global magnetic field model

“…Corresponding results have also been found for selected images within the January 2004 data set obtained under related interplanetary conditions, thus supporting the generality of these findings. The results are in excellent accord with the findings of Talboys et al (2009), who studied Cassini observations of field-aligned current signatures in high-latitude dayside magnetic field data, and found that the region of upward currents spanned the outer magnetosphere from near the outer boundary of the ring current to near the boundary of open field lines. Upward currents are potentially associated with downward accelerated electrons and bright aurora.…”

“…A modest exception occurs in our results in the dusk sector, where the equatorward boundary of the auroras moves inward toward ∼9 R S corresponding to the central ring current, as the latitude of the equatorward boundary in image C moves to ∼ −70 • in this sector. We note that these results are in excellent accord with the findings of Talboys et al (2009) concerning the location of the dayside upward-directed magnetosphereionosphere coupling field-aligned currents at Saturn, specifically in the southern pre-noon sector in that case. They found that the region of upward current, which may be associated with downward electron acceleration and UV auroras, maps to the closed field region between the boundary of open field lines and the outer edge of the ring current.…”

Signatures of field‐aligned currents in Saturn's nightside magnetosphere

“…We have projected the spacecraft footprint along model magnetic field lines into the northern and southern ionospheres for the intervals when Cassini was located north and south of the equatorial plane, respectively. The projection employed a magnetic model consisting of the 'Cassini Saturn Orbit Insertion (SOI)' internal field model of Dougherty et al [2005] and a typical ring current model derived by Bunce et al [2008a], as used for similar purposes by Bunce et al [2008b] and Talboys et al [2009]. The mapped trajectories are shown by solid lines with dots and labels at 3 hourly intervals as in Figures 1a and 1b, which are projected onto a polar grid with dotted circles shown every 5°of colatitude to 25°from the pole, viewed 'onto' the northern hemisphere in Figure 1c and 'through' the planet in the southern hemisphere in Figure 1d.…”

“…Given the sense of Saturn's planetary field, a 'lagging' field means negative azimuthal field in the northern hemisphere and positive azimuthal field in the southern hemisphere, and vice-versa for 'leading' fields.) Talboys et al [2009] also examined field-aligned current signatures in the northern hemisphere pre-dusk sector. It was shown in these cases that the azimuthal field signatures on open field lines were much reduced in magnitude, possibly due to northern winter conditions and reduced conductivities, while a layer of strongly 'lagging' fields then appears on outer closed field lines, associated with downward-directed currents at highest latitudes together with an adjacent layer of upward currents at lower latitudes.…”

“…We note, however, that Stallard et al [2008] have also reported the presence of weaker IR auroral emissions at lower latitudes that may be related to plasma corotation breakdown deeper inside the magnetosphere. Study of the data from all the Cassini passes during the first high-latitude mission phase by Talboys et al [2009] confirmed the findings of Bunce et al [2008b], but also showed on pre-noon orbits that penetrated deeper inside the magnetosphere in the southern hemisphere that the upward field-aligned current layer often extends into a region of 'leading' fields in the outer magnetosphere, suggestive of plasma super-corotation, which then relaxes in the inner ring current region via a layer of downwarddirected field-aligned currents. ('Lagging' and 'leading' fields refer to field lines that are either swept back or swept forward out of meridian planes relative to the sense of planetary rotation.…”