Cassini's View of Stress Balance in Saturn's Magnetosphere

Details Cassini's View of Stress Balance in Saturn's Magnetosphere Cassini's View of Stress Balance in Saturn's Magnetosphere

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p51e..05a&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P51E-05

Other

2721 Field-Aligned Currents And Current Systems (2409), 2740 Magnetospheric Configuration And Dynamics, 2756 Planetary Magnetospheres (5443, 5737, 6033), 2764 Plasma Sheet, 6275 Saturn

Scientific paper

Observations by Pioneer 11 and the two Voyager spacecraft indicated that the dominant source of mechanical stress in Saturn's magnetosphere is provided by the centrifugal force of cold ions [McNutt 1983]. In order to explain a significant localised peak in the field stress from Voyager observations, the role of hot ion pressure gradients were suggested to balance the centrifugal and magnetic stresses [Mauk et al. 1985]. Using observations from the Cassini spacecraft in orbit around Saturn we are revisiting these issues with a greatly expanded data set. The magnetic force has been calculated directly from the magnetometer data in a limited spatial region [Arridge et al. 2005], and complemented by calculations of the magnetic force implicit in new empirical magnetic field models [Khurana et al. 2005; Arridge et al. 2005]. Using these calculations of the radial component of the magnetic stress, and assuming a particular balance of pressure between fields and particles, the mass content in the plasma sheet can be calculated. By assuming a steady state we can also calculate the radial plasma outflow velocity for various values of the mass-loading rate in the inner magnetosphere. In this paper we extend this work and explore the relative contributions of particle stresses in balancing the magnetic stresses, as calculated from the magnetometer data and the field models. Particular attention is given to the spatial variation of hot particle pressures and the effect of neutrals on these populations in the inner magnetosphere, and to the radial variation in the cold ion mass at the centrifugal equator and hence the implied radial outflow rates. These rates are compared with observed radial velocities [Sittler et al. 2005]. The possible role of neutrals in transporting plasma out of Saturn's magnetosphere [Saur et al. 2004] is discussed in reference to the observed outflow velocities. The results of our study are compared with stress balance investigations in the terrestrial magnetosphere [e.g. Zaharia and Cheng 2005] and at the outer planets [e.g. McNutt 1983; Paranicas et al. 1991; Russell et al. 1999]. We comment on the implications of our study on the global configuration of the magnetosphere, and other macroscopic observable consequences, for example the size and shape of the magnetopause boundary [Arridge et al. 2005; Hansen et al. 2005].

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