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We investigate magnetic data showing the presence of field-aligned magnetosphere-ionosphere coupling currents on 31 Cassini passes across Saturn's southern postmidnight auroral region. The currents are strongly modulated in magnitude, form, and position by the phase of the southern planetary period oscillations (PPOs). PPO-independent currents are separated from PPO-related currents using the antisymmetry of the latter with respect to PPO phase. PPO-independent downward currents~1.1 MA per radian of azimuth flow over the polar open field region indicative of significant plasma subcorotation are enhanced in an outer plasma sheet layer of elevated ionospheric conductivity carrying~0.8 MA rad À1and close principally in an upward directed current sheet at~17°-19°ionospheric colatitude carrying 2.3 MA rad À1 that maps to the outer hot plasma region in Saturn's magnetosphere (equatorial rangẽ 11-16 Saturn radii (R S )) colocated with the UV oval. Subsidiary downward and upward currents~0.5 MA rad À1 lie at~19°-20.5°colatitude mapping to the inner hot plasma region, but no comparable currents are detected at larger colatitudes mapping to the cool plasma regime inside~8 R S . PPO-related currents at~17.5°-20°colatitude overlap the main upward and subsidiary downward currents and carry comparable rotating upward and downward currents peaking at~1.7 MA rad À1 . The overall current layer colatitude is also modulated with 1°amplitude in the PPO cycle, maximum equatorward adjacent to the peak upward PPO current and maximum poleward adjacent to peak downward PPO current. This phasing requires the current system to be driven from the planetary atmosphere rather than directly from the magnetosphere.

We investigate the magnetic perturbations associated with field‐aligned currents observed on 34 Cassini passes over the premidnight northern auroral region during 2008. These are found to be significantly modulated not only by the northern planetary‐period oscillation (PPO) system, similar to the southern currents by the southern PPO system found previously, but also by the southern PPO system as well, thus providing the first clear evidence of PPO‐related interhemispheric current flow. The principal field‐aligned currents of the two PPO systems are found to be co‐located in northern ionospheric colatitude, together with the currents of the PPO‐independent (subcorotation) system, located between the vicinity of the open‐closed field boundary and field lines mapping to ~9 Saturn radius (Rs) in the equatorial plane. All three systems are of comparable magnitude, ~3 MA in each PPO half‐cycle. Smaller PPO‐related field‐aligned currents of opposite polarity also flow in the interior region, mapping between ~6 and ~9 Rs in the equatorial plane, carrying a current of ~ ±2 MA per half‐cycle, which significantly reduce the oscillation amplitudes in the interior region. Within this interior region the amplitudes of the northern and southern oscillations are found to fall continuously with distance along the field lines from the corresponding hemisphere, thus showing the presence of cross‐field currents, with the southern oscillations being dominant in the south, and modestly lower in amplitude than the northern oscillations in the north. As in previous studies, no oscillations related to the opposite hemisphere are found on open field lines in either hemisphere.

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[1] We examine the planetary-period oscillations in Saturn's magnetic field observed by the Cassini spacecraft on 23 near-equatorial periapsis passes in the inner magnetosphere spanning October 2004 to July 2006. Overall, we find that the phase of the magnetic oscillations is well organized by the long-timescale modulation phase of Saturn kilometric radiation (SKR) determined over the same interval by Kurth et al. (2007), suggesting that the slow period variation of the latter relates to inner magnetosphere processes. The relative phases of the oscillations in the spherical polar r and 8 magnetic field components imply the presence of a quasi-uniform equatorial field rotating near the SKR period, while the sense of the q component indicates that the perturbation field lines form loops with apices in the Northern Hemisphere. No consistent evidence is found for a sign reversal in any field component across the equatorial plane, within ±20°in latitude. The relative SKR phasing is such that the peak radio power occurs when the r and q component maxima lie at $0200 LT ± 2 hours. However, a slow drift of the magnetic phase relative to the SKR phase is also discerned, amounting to $75°over the study interval. This drift lies within the envelope of scatter in the SKR phase determinations, suggesting that it represents the refinement of a common periodicity. A revised magnetic phase or longitude model is derived that should form an improved organizational system for oscillatory phenomena observed during this interval of the Cassini mission. The magnetic oscillations are also found to exhibit pass-to-pass phase ''jitter'' about the long-term variation, of RMS amplitude $20°, with r and 8 strongly correlated, but not q. The relation with the solar wind-modulated short-timescale phase variations reported in SKR data by Zarka et al. (2007) remains to be investigated, though the latter are 5 times larger in magnitude.

Abstract. We consider the flows and currents in Saturn's polar ionosphere which are implied by a three-component picture of large-scale magnetospheric flow driven both by planetary rotation and the solar wind interaction. With increasing radial distance in the equatorial plane, these components consist of a region dominated by planetary rotation where planetary plasma sub-corotates on closed field lines, a surrounding region where planetary plasma is lost down the dusk tail by the stretching out of closed field lines followed by plasmoid formation and pinch-off, as first described for Jupiter by Vasyliunas, and an outer region driven by the interaction with the solar wind, specifically by reconnection at the dayside magnetopause and in the dawn tail, first discussed for Earth by Dungey. The sub-corotating flow on closed field lines in the dayside magnetosphere is constrained by Voyager plasma observations, showing that the plasma angular velocity falls to around half of rigid corotation in the outer magnetosphere, possibly increasing somewhat near the dayside magnetopause, while here we provide theoretical arguments which indicate that the flow should drop to considerably smaller values on open field lines in the polar cap. The implied ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed downward currents on open field lines in the polar cap, a narrow ring of upward current near the boundary of open and closed field lines, and regions of distributed downward and upward current on closed field lines at lower latitudes associated with the transfer of angular momentum from the planetary atmosphere to the sub-corotating planetary magnetospheric plasma. Recent work has shown that the upward current associated with sub-corotation is not sufficiently intense to produce significant auroral acceleration and emission. Here we suggest that the observed auroral oval at Saturn instead corresponds to the ring of upward current bounding the region of open and closed field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be produced byCorrespondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV energy, if the current flows in a region which is sufficiently narrow, of the order of or less than ∼1000 km (∼1 • latitude) wide. Arguments are also given which indicate that the auroras should typically be significantly brighter on the dawn side of the oval than at dusk, by roughly an order of magnitude, and should be displaced somewhat towards dawn by the down-tail outflow at dusk associated with the Vasyliunas cycle. Model estimates are found to be in good agreement with data derived from high quality images newly obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, both in regard to physical parameters, as well as local time effects. The implication of this picture is that the form, position, and brightness of Saturn's main auroral o...

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter and Saturn are driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter's aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn.

[1] We propose a simple illustrative axisymmetric model of the plasma flows and currents that occur in Saturn's polar ionosphere which are due to both internal magnetospheric plasma processes and the solar wind interaction. The features of the model are based on previous physical discussion, guided quantitatively by both Voyager plasma observations on closed field lines and remote-sensing IR Doppler observations on open field lines. With increasing latitude the flow features represented include a region poleward of $25°colatitude where the angular velocities decrease continuously from rigid corotation to $60% of rigid corotation due to plasma production from internal sources in the central magnetosphere, a narrow band of higher but still subcorotating angular velocities mapping to Dungey cycle return flow and Vasyliunas cycle flow regions in the outer closed field magnetosphere, and, finally, a region of low angular velocities, $30% of rigid corotation, on open field lines in the polar cap. We show that these flows require a four-region pattern of field-aligned currents. With increasing latitude, these consist of regions of upward and downward current on closed field lines peaking at a few tens of nanoamperes per square meter (for an effective ionospheric Pedersen conductivity of 1 mho), a narrow ring of upward field-aligned current across the open-closed field line boundary of order 100 nA m À2 , and distributed downward currents on open field lines of order 10 nA m À2