Corotation-driven magnetosphere-ionosphere coupling currents in Saturn’s magnetosphere and their relation to the auroras

Abstract

Abstract.We calculate the latitude profile of the equatorward-directed ionospheric Pedersen currents that are driven in Saturn's ionosphere by partial corotation of the magnetospheric plasma. The calculation incorporates the flattened figure of the planet, a model of Saturn's magnetic field derived from spacecraft flyby data, and angular velocity models derived from Voyager plasma data. We also employ an effective height-integrated ionospheric Pedersen conductivity of 1 mho, suggested by a related analysis of Voyager magnetic field data. The Voyager plasma data suggest that on the largest spatial scales, the plasma angular velocity declines from near-rigid corotation with the planet in the inner magnetosphere, to values of about half of rigid corotation at the outer boundary of the region considered. The latter extends to ∼15-20 Saturn radii (R S ) in the equatorial plane, mapping along magnetic field lines to ∼15 • co-latitude in the ionosphere. We find in this case that the ionospheric Pedersen current peaks near the poleward (outer) boundary of this region, and falls toward zero over ∼5 • -10 • equatorward of the boundary as the plasma approaches rigid corotation. The peak current near the poleward boundary, integrated in azimuth, is ∼ 6 MA. The field-aligned current required for continuity is directed out of the ionosphere into the magnetosphere essentially throughout the region, with the current density peaking at ∼10 nA m −2 at ∼20 • co-latitude. We estimate that such current densities are well below the limit requiring field-aligned acceleration of magnetospheric electrons in Saturn's environment (∼70 nA m −2 ), so that no significant auroral features associated with this ring of upward current is anticipated. The observed ultraviolet auroras at Saturn are also found to occur significantly closer to the pole (at ∼10 • -15 • co-latitude), and show considerable temporal and local time variability, contrary to expectations for corotation-related currents. We thus conclude that Saturn's 'main oval' auroras are not associated with corotationenforcing currents as they are at Jupiter, but instead are most probably associated with coupling to the solar wind as at Correspondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) Earth. At the same time, the Voyager flow observations also suggest the presence of radially localized 'dips' in the plasma angular velocity associated with the moons Dione and Rhea, which are ∼1-2 R S in radial extent in the equatorial plane. The presence of such small-scale flow features, assumed to be azimuthally extended, results in localized several-MA enhancements in the ionospheric Pedersen current, and narrow bi-polar signatures in the field-aligned currents which peak at values an order of magnitude larger than those associated with the large-scale currents. Narrow auroral rings (or partial rings) ∼0.25 • co-latitude wide with intensities ∼1 kiloRayleigh may be formed in the regions of upward field-aligned current under favourable circumstances, located at co-latitudes between ∼17 • and ∼20...

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

“…Given this angular velocity profile, the consequent red current profile in Figure 13a derived from equation (10) with fixed conductivity indicates distributed downward currents over the polar region, a layer of strong upward current at the OCB where the angular velocity of the plasma suddenly increases, followed by a more distributed region of weaker upward current at larger colatitudes, similar to the theoretical models of Cowley and Bunce [2003] and Cowley et al [2004aCowley et al [ , 2004b derived on an essentially equivalent basis. Comparison with the observed northern current profile in Figure 13a suggests, however, that variations of the effective Pedersen conductivity in the northern ionosphere must also play an important role, with conductivities much Figure 13.…”

“…Following the methodology of Cowley and Bunce [2003] which takes account of the north-south differences in the planetary ionospheric magnetic field and the polar flattening of the planetary atmosphere and ionosphere, we rearrange equation (10) into the form…”

“…The system is assumed axisymmetric to a first approximation. (c-e) The northern PPO-related system, where Figure 1c subcorotation at high latitudes first implies the presence of distributed downward currents in the polar region in both hemispheres, as in the models of Cowley and Bunce [2003] and Cowley et al [2004aCowley et al [ , 2004bCowley et al [ , 2008. These downward currents feed equatorward-directed Pedersen currents in the northern and southern ionospheres that increase with distance from the pole due to both the increasing neutral-plasma relative velocity with distance from the pole, and the expanding spherical geometry of the system.…”

“…Electric currents flowing transverse to the magnetic field in both the planetary ionosphere and the magnetosphere imply the presence of j × B forces on these plasma mediums, the transverse currents being linked by currents flowing along the planetary magnetic field lines between them to form large-scale closed-loop current systems. In the case of Saturn, the primary focus of the present paper, the principal current systems are those related to subcorotation of the magnetospheric plasma [e.g., Cowley and Bunce, 2003;Wilson et al, 2009;Pontius and Hill, 2009;Thomsen et al, 2014] and to the "planetary period oscillations" (PPOs) whose effects have been found to be ubiquitous in Saturn's magnetosphere [e.g., Carbary and Mitchell, 2013, and references therein]. In a recent paper Hunt et al [2014] examined the magnetic field perturbations observed on a sequence of~40 periapsis passes of the Cassini spacecraft over Saturn's nightside auroral regions in 2008 and separated the effects of these two current systems using their opposite symmetry properties.…”

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

“…Cowley and Bunce, 2001;Hill, 2001). 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.…”