title: Internally driven large-scale changes in the size of Saturn's magnetosphere.
abstract: Saturn's magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system's dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10-15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively "plasma-loaded" or "plasma-depleted" states, defined in terms of the internal suprathermal plasma pressure.
authors.affiliation: Atmospheric Physics Laboratory, Department of Physics and Astronomy University College London London UK; The Centre for Planetary Sciences UCL/Birkbeck London UK.
authors.forename: N M
authors.initials: NM
authors.lastname: Pilkington
authors.affiliation: Atmospheric Physics Laboratory, Department of Physics and Astronomy University College London London UK; The Centre for Planetary Sciences UCL/Birkbeck London UK.
authors.forename: N
authors.initials: N
authors.lastname: Achilleos
authors.affiliation: Physics Department Lancaster University Lancaster UK.
authors.forename: C S
authors.initials: CS
authors.lastname: Arridge
authors.affiliation: Atmospheric Physics Laboratory, Department of Physics and Astronomy University College London London UK; The Centre for Planetary Sciences UCL/Birkbeck London UK.
authors.forename: P
authors.identifier: 0000-0002-1607-5862
authors.initials: P
authors.lastname: Guio
authors.affiliation: Blackett Laboratory Imperial College London London UK.
authors.forename: A
authors.initials: A
authors.lastname: Masters
authors.affiliation: Atmospheric Physics Laboratory, Department of Physics and Astronomy University College London London UK; The Centre for Planetary Sciences UCL/Birkbeck London UK.
authors.forename: L C
authors.initials: LC
authors.lastname: Ray
authors.affiliation: Academy of Athens Office of Space Research and Technology Athens Greece.
authors.forename: N
authors.initials: N
authors.lastname: Sergis
authors.affiliation: Planetary Science Institute Tucson Arizona USA.
authors.forename: M F
authors.initials: MF
authors.lastname: Thomsen
authors.affiliation: The Centre for Planetary Sciences UCL/Birkbeck London UK; Mullard Space Science Laboratory, Department of Space and Climate Physics University College London Dorking UK.
authors.forename: A J
authors.identifier: 0000-0002-6185-3125
authors.initials: AJ
authors.lastname: Coates
authors.affiliation: Blackett Laboratory Imperial College London London UK.
authors.forename: M K
authors.initials: MK
authors.lastname: Dougherty
keywords: Saturn; magnetopause; magnetosphere; plasma; solar wind