Distortion of Saturn's Magnetopause Surface Observed by Cassini

ACHILLEOS, NICHOLAS A.; ARRIDGE, C. S.; BERTUCCI, C.; MCANDREWS, H. J.; DOUGHERTY, M. K.; COATES, A. J.; CASSINI MAGNETOMETER TEAM

Congreso; American Astronomical Society, DPS meeting #38; 2006

The Cassini spacecraft has now encountered Saturn's magnetopause boundary over 100 times, during more than 20 orbits of the planet. During the compilation of minimum variance analyses for the inbound `Rev 5' orbit (March 2005), we have detected what appears to be a significantly `distorted' orientation of the magnetopause for the final pair of crossings on the inbound leg. This pair of crossings encloses a brief ( 10 minute) excursion into the planetary magnetosheath (as revealed by the CAPS instrument), which occurred an unusually long time (>6 hours) and distance (3 RS = Saturn radii) after the earlier group of at least seven clear magnetopause encounters. Application of an equilibrium magnetopause model indicates that the standoff distance changed from 20 to 17 RS during this time interval and that the model solar wind pressure correspondingly changed by a factor of nearly two. Therefore, a strong magnetospheric compression is indicated. Minimum variance analysis (MVA) reveals that all the crossings correspond to a tangential discontinuity. Moreover, the angle between the MVA and model normals is in the range 10-35 deg for the early group of encounters, but increases to 50-60 deg for the pair associated with the compressed configuration. The magnetic data acquired by the MAG instrument show that the field during the brief ( 10 min) magnetosheath excursion associated with the compression reached orientations nearly orthogonal to a model of the unperturbed magnetopause surface. The observations are consistent with a sharp `depression' in the magnetopause surface with a sunward component of motion. If an anti-sunward moving disturbance is assumed, then we require an even more distorted, `folded' magnetopause in order to explain the observations.