SM33C-01 INVITED
Theoretical work on auroral current systems at the outer planets
This talk will review the current generation systems thought to be responsible for producing aurora in outer planet magnetospheres. Auroral emissions at Jupiter and Saturn exhibit some features that are qualitatively different from those observed at Earth. Along with solar wind-driven aurora apparent at the highest latitudes, these rapidly rotating magnetospheres exhibit the effects of purely internal current systems that have no ready terrestrial counterpart. For example, emissions at Jupiter are dominated by a steady, persistent main oval produced by currents that attempt to enforce corotation with the planet. In the absence of these currents, outward moving plasma from the Io torus would slow to a very low angular velocity, in conflict with spacecraft observations of substantially higher values. Saturn has a main auroral oval that intermittently assumes a distinct spiral pattern, but its cause is an ongoing subject of debate. Jupiter also exhibits sharply localized aurora at the magnetic footprints of the three inner Galilean satellites. This is a rare example of an auroral system where emissions can be associated unambiguously with specific drivers in precisely known remote locations.
SM33C-02
Generation of Parallel Electric Fields in the Jupiter-Io Torus Wake Region
Infrared and ultraviolet images have established that auroral emissions at Jupiter caused by the electromagnetic interaction with Io not only produce a bright spot, but an emission trail that extends in longitude from Io's magnetic footprint. Electron acceleration that produces the bright spot is believed to be dominated by Alfvén waves whereas we argue that the trail or wake aurora results from quasi-static parallel electric fields associated with large-scale, field-aligned currents between the Io torus and Jupiter's ionosphere. These currents ultimately transfer angular momentum from Jupiter to the Io torus. We examine the generation and the impact of the quasi-static parallel electric fields in the Io trail aurora. A critical component to our analysis is a current-voltage relation that accounts for the low-density plasma along the magnetic flux tubes that connect the Io torus and Jupiter. This low density region, ~ 2 RJ from Jupiter's center, can significantly limit the field-aligned current, essentially acting as a "high-latitude current choke". Once parallel electric fields are introduced, the governing equations that couple Jupiter's ionosphere to the Io torus become nonlinear and, while the large-scale behavior is similar to that expected with no parallel electric field, there are substantial deviations on smaller scales. The solutions, bound by properties of the Io torus and Jupiter's ionosphere, indicate that the parallel potentials are on the order of 1 kV when constrained by peak energy fluxes of ~1 miliWatt per meter squared. The parallel potentials that we predict are significantly lower than earlier reports.
SM33C-03
Reconnection and Flows in the Jovian Magnetotail as Inferred From Magnetometer Observations
Some of the auroral polar bright spots at Jupiter are thought to originate through reconnection in the Jovian magnetotail [Grodent et al., 2004; Radioti et al., 2008]. Understanding the statistics of magnetotail reconnection will, therefore, contribute to interpreting the polar aurora. Dynamics in the Jovian magnetotail have been identified from energetic particle data, but a thorough survey of the magnetic field data is yet to be completed. An initial study [Vogt et al., 2007] identified events whose magnetic signatures suggest that magnetic reconnection is taking place in the Jovian magnetotail at distances from 30 to 150 RJ. In the initial analysis, events were characterized by increases in |Bθ|, the north-south component of the magnetic field, over background levels, with less restrictive criteria placed on periods of negative Bθ. We have recently focused on improving our quantitative event selection criteria, with particular emphasis on establishing the required increase in |Bθ|. This has proved to be a delicate task. Although the number and duration of the events vary with the identification criteria, the strongest events are selected by all those applied. Here we analyze events that satisfy quite stringent identification criteria. We distinguish between locations inside or outside of a neutral line, which we infer through the sign of Bθ. The background Jovian field points southward (Bθ> 0) near the equator and we assume that a northward or negative Bθ occurs when the spacecraft is located tailward of a reconnection x-line. Most events with (Bθ < 0) are observed in the post-midnight sector and at radial distances larger than ~90 RJ. Using the sign of Bθ as a proxy for the flow direction, we compare with previous studies that identified intermittent flows from particle anisotropy [Kronberg et al. 2005, 2007]. In the specific cases illustrated there we find that our events occur in conjunction with increases in the particle anisotropies, though our strict selection criteria miss some intervals of increased anisotropy. However, all but one of the disturbed intervals reported by Kronberg et al. [2005, 2007] occur in the post- midnight sector, but we have identified scores of additional events pre-midnight. We have also examined changes to the bendback angle, which represents how swept back the field line is with respect to the radial direction. By assuming that in events lasting less than 5 hours the azimuthal component of the flow changes to conserve angular momentum and that the field is frozen into the flow, we can infer changes of the radial plasma flow from changes of the bendback angle. We find that large, fast changes of the bendback angle occur in the majority of our events. By conservation of angular momentum we expect that the bendback of the field line will increase as plasma flows outward; we also expect negative Bθ during such times. Therefore we examine the correlation between the sign of Bθ and the change of the bendback angle in our events. We will discuss the distribution of events and their inferred properties as functions of radial distance and local time.
SM33C-04
Response of Saturn's Magnetosphere and Ionosphere to Solar Wind Driving
The extent to which the solar wind drives Saturn's magnetosphere-ionosphere system is an ongoing topic of extensive study due to both the Cassini spacecraft presence in the Saturnian system as well as the recent (2007, 2008) Hubble Space Telescope campaign to image the UV aurora. Studies using these data have shown some evidence that solar wind dynamic pressure enhancements influence the aurora as well as the magnetosphere. In this study, we address the solar wind driving using our 3D global MHD model of Saturn's magnetosphere-ionosphere. In order to study the coupled solar wind-magnetosphere-ionosphere system it is necessary to know the upstream solar wind conditions. We have recently published a detailed validation of our MHD model which propagates the solar wind radially outward from the Earth (ACE) to Saturn [Zieger and Hansen, JGR, 2008]. These solar wind values can be used as input to our global, 3D, MHD model of the magnetosphere. From the global model we extract time series that can be compared directly to spacecraft data. These include the total auroral power as well as the locations of both the auroral oval and the open- closed field lines boundary. In addition, we are able to explore the global structure of the magnetosphere including auroral morphology and current structures that create the aurora as well as convection patterns in the magnetosphere. Results will be presented from the two time periods of extensive HST measurements