[1] The magnetotails of Jupiter and Earth are known to be hinged so that their orientation is controlled by the magnetic field of the planet at small distances and asymptotically approach the direction of the flow of the solar wind at large distances. In this paper we present Cassini observations showing that Saturn's magnetosphere is also similarly hinged. Furthermore, we find that Saturn's magnetosphere is not only hinged in the tail but also on the dayside, in contrast to the Jovian and terrestrial magnetospheres. Over the midnight, dawn, and noon local time sectors we find that the current sheet is displaced above Saturn's rotational equator, and thus the current sheet adopts the shape of a bowl or basin. We present a model to describe the warped current sheet geometry and show that in order to properly describe the magnetic field in the magnetosphere, this hinging must be incorporated. We discuss the impact on plasma observations made in Saturn's equatorial plane, the influence on Titan's magnetospheric interaction, and the effect of periodicities on the mean current sheet structure.

[1] The location and shape of a planetary magnetopause is principally determined by the dynamic pressure, D p , of the solar wind, the orientation of the planet's magnetic dipole with respect to the solar wind flow, and by the distribution of stresses inside the magnetosphere. The magnetospheres of Saturn and Jupiter have strong internal plasma sources compared to the solar wind source and also rotate rapidly, causing an equatorial inflation of the magnetosphere and consequently the magnetopause. Empirical studies using Voyager and Pioneer data concluded that the kronian magnetopause was Earth-like in terms of its dynamics (Slavin et al., 1985) as revealed by how the position of the magnetopause varies with dynamic pressure. In this paper we present a new pressuredependent model of Saturn's magnetopause, using the functional form proposed by Shue et al. (1997). To establish the pressure-dependence, we also use a new technique for fitting a pressure-dependent model in the absence of simultaneous upstream pressure measurements. Using a Newtonian form of the pressure balance across the magnetopause boundary and using model rather than minimum variance normals, we estimate the solar wind dynamic pressure at each crossing. By iteratively fitting our model to magnetopause crossings observed by the Cassini and Voyager spacecraft, in parallel with the pressure balance, we obtain a model which is self-consistent with the dynamic pressure estimates obtained. We find a model whose size varies as $D p À1/4.3 and whose flaring decreases with increasing dynamic pressure. This is interpreted in terms of a different distribution of fields and particles stresses which has more in common with the jovian magnetosphere compared with the terrestrial situation. We compare our model with the existing models of the magnetopause and highlight the very different geometries. We find our results are consistent with recent MHD modeling of Saturn's magnetosphere (Hansen et al., 2005).Citation: Arridge, C. S., N. Achilleos, M. K. Dougherty, K. K. Khurana, and C. T. Russell (2006), Modeling the size and shape of Saturn's magnetopause with variable dynamic pressure,

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Cassini's successful orbit insertion has provided the first examination of Saturn's magnetosphere in 23 years, revealing a dynamic plasma and magnetic environment on short and long time scales. There has been no noticeable change in the internal magnetic field, either in its strength or its near-alignment with the rotation axis. However, the external magnetic field is different compared with past spacecraft observations. The current sheet within the magnetosphere is thinner and more extended, and we observed small diamagnetic cavities and ion cyclotron waves of types that were not reported before.