[1] The shape and location of a planetary magnetopause can be determined by balancing the solar wind dynamic pressure with the magnetic and thermal pressures found inside the boundary. Previous studies have found the kronian magnetosphere to show rigidity (like that of Earth) as well as compressibility (like that of Jupiter) in terms of its dynamics. In this paper we expand on previous work and present a new model of Saturn's magnetopause. Using a Newtonian form of the pressure balance equation, we estimate the solar wind dynamic pressure at each magnetopause crossing by the Cassini spacecraft between Saturn Orbit Insertion in June 2004 and January 2006. We build on previous findings by including an improved estimate for the solar wind thermal pressure and include low-energy particle pressures from the Cassini plasma spectrometer's electron spectrometer and high-energy particle pressures from the Cassini magnetospheric imaging instrument. Our improved model has a size-pressure dependence described by a power law

Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma βconditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti‐parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheathβconditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind‐magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth‐like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.

[1] We present Cassini observations of a plasma vortex in Saturn's dayside outer magnetosphere. The vortex encounter took place on 13 December 2004 as Cassini was travelling toward the planet. The spacecraft crossed the magnetopause 3 times, before being immersed in the low-latitude boundary layer. During the transition between the boundary layer and the magnetosphere proper, the spacecraft observed deflected boundary layer plasma, a twisted magnetic field topology, and high-energy (>20 keV) directional electron fluxes. These observations are consistent with an encounter with a vortex on the inner edge of the boundary layer, an interface that is expected to be susceptible to the growth of the Kelvin-Helmholtz (K-H) instability due to its low magnetic shear. The size of the vortex is determined to be at least 0.55 R S , and a simple model of the current system resulting from the formation of the vortex is proposed. The possible acceleration mechanisms responsible for the high-energy electrons are discussed. The identification of the structure provides compelling evidence of the operation of the nonlinear K-H instability at Saturn's morning magnetospheric boundaries and has implications for our understanding of the transfer of energy and momentum between the solar wind and Saturn's magnetosphere.

[1] The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft provides measurements of the energetic ion population within the magnetosphere of Saturn. Energetic ion directional intensities, energy spectra and ion composition, are measured by the Charge Energy Mass Spectrometer (CHEMS) over the range $3 to 236 keV per charge and by the Low Energy Magnetospheric Measurements System (LEMMS) for ions in the range 0.024 < E < 18 MeV. This work reports preliminary results of partial particle pressure distributions throughout the equatorial magnetosphere and comparison with in situ measurements of the magnetic pressure provided by Cassini's magnetometer. The results cover 11 passes from late 2005 to early 2006, when the spacecraft was particularly close to the nominal magnetic equator in the range 5 < R < 20 R S and can be summarized as follows:(1) the plasma b (particle pressure/magnetic pressure) profile increases radially outward to maximum values of ! 1 at L >10 R S ; (2) most particle pressure is contained in the range of 10 < E < 150 keV; and (3) in the high beta region 10 < L < 19, where the apparent ring current resides, oxygen generally contributes more than 50% of the total particle pressure. The results demonstrate that typical assumptions of MHD models, whereby particle pressure is presumed to reside with the cold plasma, are not supported by the data. Citation: Sergis, N., S. M. Krimigis, D. G.

[1] The Magnetospheric Imaging Instrument on board Cassini has been providing measurements of energetic ion intensities, energy spectra, and ion composition, combining the Charge Energy Mass Spectrometer over the range 3 to 236 keV/e, the Low Energy Magnetospheric Measurements System for ions in the range 0.024 to 18 MeV, and the Ion and Neutral Camera for ions and energetic neutral atoms in the range 3 to > 200 keV. Results of the energetic (E > 3 keV) particle pressure distribution throughout the Saturnian magnetosphere and comparison with in situ measurements of the magnetic pressure are presented. The study offers a comprehensive depiction of the average, steady state hot plasma environment of Saturn over the 3 years since orbit insertion on 1 July 2004, with emphasis on ring current characteristics. The results may be summarized as follows:(1) The Saturnian magnetosphere possesses a dynamic, high-beta ring current located approximately between 8 and $15 R S , primarily composed of O + ions, and characterized by suprathermal (E > 3 keV) particle pressure, with typical values of 10 À9 dyne/cm 2 . (2) The planetary plasma sheet shows significant asymmetries, with the dayside region being broadened in latitude (±50°) and extending to the magnetopause, and the nightside appearing well confined, with a thickness of $10 R S and a northward tilt of some 10°with respect to the equatorial plane beyond $20 R S . (3) The average radial suprathermal pressure gradient appears sufficient to modify the radial force balance and subsequently the azimuthal currents. (4) The magnetic perturbation due to the trapped energetic particle population is $7 nT, similar to values from magnetic field-based studies (9 to 13 nT).