Generation of a global longitudinal asymmetry in Saturn’s magnetosphere

Abstract:

Numerous empirical studies have indicated that a global longitudinal asymmetry of Saturn’s magnetospheric plasma population (the m=1 term in a Fourier expansion, where m is the azimuthal wavenumber) is required to explain observed spin-periodic modulations (~ 10.7 hr) of magnetospheric behavior (e.g., Carbary and Mitchell [2013], Rev. Geophys., 51, doi:10.1029/2012RG000416, and references therein). This is true in spite of the fact that Saturn’s intrinsic magnetic field is well described by a perfectly spin-aligned dipole. The Rice Convection Model (RCM) has been used previously to simulate plasma transport in Saturn’s magnetosphere with a spin-aligned dipole field and a longitudinally symmetric plasma source that is largely confined to the radial range L ~ 5 - 8 (e.g., Liu et al. [2010], J. Geophys. Res., 115, doi:10.1029/2010JA015859; Liu and Hill [2012], J. Geophys. Res., 117, doi:10.1029/2012JA017827). We have begun to explore the consequences of a global-scale longitudinal asymmetry of the plasma source. Preliminary results indicate that a small (~1%) asymmetry of the imposed plasma source produces a magnetospherically significant (factor ~2) m=1 asymmetry of the resultant plasma distribution in the middle magnetosphere (L ~ 8-15). Our objective is not to explain the (still mysterious) underlying cause of the source asymmetry, but rather to investigate the effects of a given source asymmetry on the plasma distribution in the more distant magnetosphere.