Cassini observations of the Interplanetary Medium Upstream of Saturn and their relation to the Hubble Space Telescope aurora data☆
Introduction
During January 2004, a unique ‘solar wind campaign’ took place whereby magnetic field, plasma and radio wave instruments onboard the Cassini–Huygens spacecraft measured the in-situ solar wind and embedded interplanetary magnetic field (IMF) whilst the Hubble Space Telescope (HST) simultaneously observed the far ultraviolet (FUV) aurora in Saturn’s southern hemisphere. Previous fly-bys of Saturn by Pioneer-11, Voyager-1 and -2 during the interval 1979–1981 supplied detailed information on the morphology of Saturn’s magnetosphere (e.g. Smith et al., 1980, Ness et al., 1981, Ness et al., 1982, Sittler et al., 1983, Richardson and Sittler, 1990), however, minimal information on the interaction between the solar wind and IMF with the magnetosphere has been available until now. Voyager measurements showed that the Saturn kilometric radiation (SKR) could be positively correlated with the solar wind dynamic pressure (Desch, 1982, Desch and Rucker, 1983), but the details of the interaction have remained unclear. Since the new era of high resolution FUV imaging using the HST, we have learnt that Saturn’s auroras are variable in both intensity and morphology (Gérard et al., 1995, Gérard et al., 2004, Trauger et al., 1998, Cowley et al., 2004a), but their dependence on the upstream solar wind conditions is only now beginning to be understood (Clarke et al., 2005, Crary et al., 2005, Kurth et al., 2005, Cowley et al., 2005). In this paper, we will describe the Cassini magnetic field and plasma measurements and their relation to the HST images of Saturn’s aurora. We will then discuss the effects of shocks associated with compression regions on Saturn’s magnetosphere and aurora, in relation to the specific theoretical ideas of Cowley et al. (2005).
We will discuss the Cassini and HST data in the theoretical framework presented by Cowley et al., 2004a, Cowley et al., 2004b. They have suggested that Saturn’s auroras are associated with a ring of upward current along the open-closed field line boundary generated by the difference in angular velocity between open and outer magnetosphere closed field lines. Previous work by Cowley and Bunce (2003) and Cowley et al. (2004b) indicates that the main auroral oval at Saturn is not produced by the effects of sub-corotation of equatorial plasma as is found to be the case at Jupiter. They find that the magnetosphere–ionosphere coupling currents are of insufficient magnitude, and flow at the wrong co-latitude to account for the auroral emissions at Saturn. In Fig. 1 we show a view of the flows and currents which are present in Saturn’s ionosphere, due to three main flow regimes discussed previously by Hill, 1979, Vasyliunas, 1983, and Dungey (1961). These regimes are shown in Fig. 1 in a view which is looking down onto the northern ionosphere, with the pole at the centre, in a frame which is fixed relative to the Sun. The solid lines indicate plasma streamlines, whilst the circled dots and crosses indicate regions of field-aligned current directed upward and downward respectively. The three flow regions are as follows (1) a lower-latitude region which maps to the sub-corotating plasma in the equatorial plane, associated with the field-aligned current system which transfers momentum from the ionosphere to the magnetosphere, which according to the modelling work discussed above is unlikely to account for the main oval auroras at Saturn, (2) a higher-latitude region of sub-corotating flows where field lines are stretched out downtail and eventually pinch off, forming a plasmoid, which is subsequently released downtail (the ‘Vasyliunas cycle’), and (3) a region of flow which is driven by reconnection at the dayside magnetopause in which ‘open’ field lines mapping to the tail lobes flow anti-sunward over the poles, and following reconnection in the tail, return to the dayside, drawn here principally via dawn, in a single cell convection pattern (the ‘Dungey cycle’). The newly closed flux tubes return to the dayside via dawn due to the presence of the Vasyliunas cycle on the duskside, and also due to the effect of planetary rotation on the open field lines. A slow rotation of the open field region was suggested by Cowley et al., 2004a, Cowley et al., 2004b and measured by Stallard et al. (2004) using infrared aurora data. It is the upward-directed field-aligned current along the boundary between open and closed field lines (shown by the inner dashed circle) which is thought to be associated with the main auroral oval at Saturn. Of course, if this is the case, the main oval aurora will be strongly modulated by the upstream solar wind conditions, which we will now discuss with respect to the joint Cassini-HST observations.
Section snippets
Simultaneous measurements of the upstream solar wind conditions at ∼9 AU and Saturn’s southern aurora
Recently, Jackman et al. (2004) have investigated the reconnection-driven interaction of the solar wind with Saturn’s magnetosphere with particular focus on the consequences for magnetospheric dynamics. In this study, IMF data obtained by the Cassini spacecraft en route to Saturn were collected for 8 complete solar rotations which allow the variation of the field structure to be investigated. They find that the solar wind magnetic field structure is consistent with that expected to be produced
Theoretical interpretation
An interpretation of the auroral response to the onset of a CIR compression region is shown in Fig. 4, taken from Cowley et al. (2005). It is supposed that the forward shock of a CIR compression region will trigger an interval of rapid Dungey-cycle reconnection in Saturn’s magnetic tail in which a significant fraction of the open flux in the tail lobe will be closed over an interval of several hours (less than one complete rotation of Saturn). In Fig. 4(a) and (b) we see the equatorial and
Summary
In this paper, we have for the first time described the Cassini magnetic field and plasma data along with the HST images of Saturn’s aurora which were taken in January 2004. We have discussed these ideas within the framework of the Cowley et al., 2004a, Cowley et al., 2004b model for Saturn’s main auroral oval. They suggest that the main aurora at Saturn is associated with upward-directed field-aligned currents flowing along the open-closed field line boundary, and will be strongly modulated by
Acknowledgements
This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the AURA, Inc. for NASA. EJB was supported during the course of this study by PPARC Postdoctoral Fellowship PPA/P/S/2002/00168. SWHC was supported by PPARC Senior Fellowship PPA/N/S/2000/00197. JTC was supported by grants from NASA and STScI. FJC was supported by NASA/JPL contract 1243218 with SwRI.
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Submitted to Advances in Space Research, as part of the 35th COSPAR proceedings, October 2004.