Magnetospheric period oscillations of Saturn's bow shock

Using magnetic field and plasma electron data from 35 Cassini orbits on which the spacecraft crossed Saturn's magnetopause and bow shock during 2004–2007, we provide first evidence for bow shock oscillations related to oscillations in the magnetic field and plasma inside the magnetosphere near the ∼11 h planetary rotation period, termed here the “magnetospheric period.” Two case studies are presented showing both bow shock and magnetopause oscillations on given spacecraft passes, together with a statistical study showing that bow shock oscillations observed within a timing window between 0.6 and 1.6 of the magnetospheric period are significantly organized by the phase of the interior field oscillations. Both case and statistical studies indicate that the bow shock and magnetopause oscillate approximately in phase, within a phase uncertainty of about ±25°. The phasing of the oscillations is such that the interior oscillatory field within the magnetospheric “core” region (magnetic shells with equatorial crossing distances less than ∼15 Saturn radii) is rotated ∼130°–160° counterclockwise (as viewed from the north) of the instantaneous location of outward boundary excursion maxima. The overall data set suggests that bow shock oscillations are commonly observed over the whole local time range that the spacecraft crossed the boundary (∼5–17 h LT) with typical amplitudes of ∼1–2 R S , comparable to results found previously for the corresponding oscillations of the magnetopause. These results thus reveal a new aspect of Saturn's bow shock dynamics, providing first evidence for modulation of its position by the periodic processes observed throughout Saturn's magnetosphere.