First Evidence of a Cushion Region at Saturn and a Reconsideration of Why It Forms

The presence of an internal plasma source within the magnetosphere of Saturn distorts the magnetic field away from a dipole to magnetodisc configuration. The magnetic field lines are radially stretched by the equatorially confined plasma and supported by an azimuthal current. It has been suggested that as the field lines stretch down the magnetotail and reconnection occurs, mass-depleted flux tubes will convect towards the dayside through dawn, producing a dipolar region between the outer edge of the current sheet and the magnetopause, known as the cushion region. Whilst this structure has been observed at Jupiter, it has yet to be at Saturn.

We present the first evidence of a cushion region at Saturn at dusk rather than dawn, as would be expected. We suggest this region is not a return flow channel following reconnection, but the result of greater turbulent heating and a reduced magnetic field strength at dusk compared to dawn. This increases the gyroradius implying a magnetodisc instability is more likely in this local time sector, as this length-scale approaches the radius of curvature and plasma can escape the equatorial current sheet. We further find that this phenomenon is more likely to occur at Jupiter compared to Saturn, possibly explaining why this has been more commonly observed at Jupiter.