Internally Driven, Dynamical Behaviour of Saturn's Magnetosphere

Tuesday, 16 December 2014: 2:40 PM
Nathan Michael Pilkington1,2, Nicholas A Achilleos1,2, Christopher Stephen Arridge2,3, Patrick Guio1,2, Adam Masters4, Nick Sergis5, Andrew J Coates2,3 and Michele Karen Dougherty6, (1)University College London, London, United Kingdom, (2)The Centre for Planetary Sciences at UCL/Birkbeck, Gower St., London, WC1E 6BT, London, United Kingdom, (3)University College London, Mullard Space Science Laboratory, London, United Kingdom, (4)Imperial College London, Blackett Laboratory, London, SW7, United Kingdom, (5)Academy of Athens, Athens, Greece, (6)Imperial College London, Blackett Laboratory, London, United Kingdom
We have used 7 years of in-situ magnetic and and particle data from the CAPS and MIMI instruments onboard the Cassini spacecraft to study Saturn’s magnetopause boundary throughout the mission. In addition to the solar wind dynamic pressure, we find that magnetopause size is also strongly modulated by changing conditions inside the magnetosphere for which the usual scaling law (stand-off distance versus dynamic pressure) cannot account. At a fixed dynamic pressure, the stand-off distance can vary by 10-15 Saturn radii (Rs) depending on the plasma pressure inside the magnetosphere. We have quantified the variability in stand-off distance as a function of both dynamic pressure and interior plasma beta, both of which show considerable variability at Saturn. We modify the power law that is usually used to specify the size of a magnetosphere as a function of dynamic pressure by adding an additional dependency on plasma beta. We have also fitted empirical surfaces, using both 'old' and 'new' power laws, to observed magnetopause crossings. To describe the magnetopause shape and scale, we have used the original analytical form of Shue et al. (1997), as modified by Pilkington et al. (2014) to incorporate polar flattening. Using the new power law reduces the discrepancy between where the boundary is observed and where the model predicts it should be by ~1Rs on average, which is ~20% of the typical r.m.s. deviation between observed and modelled location. Hence, the internal variation in plasma beta strongly influences the magnetopause location at Saturn and, presumably, must also be taken into account for Jupiter and other magnetised planets with strong internal plasma sources.