Magnetopause oscillations near the planetary period at Saturn: Occurrence, phase, and amplitude

Using magnetic field and plasma electron data from similar to 40 Cassini orbits that crossed Saturn's magnetopause during 2004-2007, we make a first systematic study of the magnetopause oscillations that are related to the oscillations in the magnetic field and plasma inside the magnetosphere near the similar to 11 h planetary rotation period, here termed the magnetospheric period. Because the motion of the spacecraft through the boundary region, combined with other boundary effects, produces a broad spread in timings between successive like crossings of the boundary, boundary oscillation events were selected for analysis using a broad timing window, 0.4-1.6 of the magnetospheric period. We find these events to be highly organized by the phase of the interior field oscillations, showing that they relate to boundary oscillations that have a closely common period. We find that similar to 60% of passes through the boundary region show one or more such oscillations. Of those that do, similar to 65% show one oscillation, similar to 10% show two, and similar to 25% show three or more. The oscillations are observed at all local times at which Cassini crossed the boundary and over a range of latitudes and are thus a global phenomenon. The phasing of the boundary oscillations is such that the rotating quasi-uniform equatorial field within the quasi-dipolar core magnetosphere (within similar to 15 Saturn radii (R-S) of the planet) points approximately away from the maximum outward boundary displacement. However, the boundary oscillation phase is found to depend somewhat on radial distance to the boundary, consistent with outward radial propagation in the magnetosphere at phase speeds of similar to 250 km s(-1). Taking account of the radial propagation, analysis shows that the phase of maximum outward boundary displacement is directly related to the phase of the density maximum in the Enceladus torus. The oscillation amplitude is estimated typically to be similar to 1.2 R-S but sometimes reaches similar to 2-3 R-S and is occasionally as great as similar to 4-5 R-S.