Numerical investigation of the impact of seasonal variability on Saturn's magnetosphere

Abstract:

Saturn's significant axial tilt (approx. 26°) combines with the close alignment of its magnetic dipole and rotational axes to produce significant seasonal variability in the overall morphology of its magnetospheric structure. This is most apparent in the change in curvature of the current sheet as the planet moves from equinox to solstice and back: the angle of attack (AOA - the effective angle between the dipole axis and the oncoming solar wind flow) varies from 0° at equinox, resulting in little to no curvature, to 26° at Northern and Southern solstice, when the magnetosphere is deformed into a "bowl" or "basin" shape.

We use the Saturn multifluid MHD model to investigate the changes in magnetospheric morphology and dynamics that result from seasonal variation. We have incorporated a static representation of Saturn's neutral cloud into the multifluid simulation, using scale height and equatorial density profiles for water-group neutrals, developed by H. T. Smith. This enables the simulation to calculate energy-dependent rate coefficients for electron impact ionization, and including the ability to specify a radially-dependent energetic/suprathermal electron distribution. Model outputs for a range of AOA values are used to study the specific response of current sheet curvature to the solar wind angle as a function of local time, as well as the implications of this for production and transport of plasma in the inner and middle magnetosphere.