Abstract
Using magnetic field and plasma electron data from ∼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 ∼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 ∼60% of passes through the boundary region show one or more such oscillations. Of those that do, ∼65% show one oscillation, ∼10% show two, and ∼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 ∼15 Saturn radii (RS) 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 ∼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 ∼1.2 RS but sometimes reaches ∼2–3 RS and is occasionally as great as ∼4–5 RS.
Highlights
[2] Saturn’s internally generated magnetic field is found to be remarkably symmetric about the planet’s spin axis, the dipole axis being tilted by less than ∼0.5° [e.g., Connerney et al, 1982; Davis and Smith, 1990; Dougherty et al, 2005; Giampieri et al, 2006]
We have used magnetic field and plasma electron data from ∼40 Cassini orbits that crossed the magnetopause, which occurred during the interval from Saturn orbit insertion (SOI) in July 2004 to Rev 55 inbound at the end of 2007
While occasional intervals of multiple boundary crossings separated near the magnetospheric period provide convincing evidence for such boundary oscillations, more frequently only one or two reentries occur, with a broad range of intervals between like crossings of the boundary that can be due both to the finite speed of the spacecraft through the boundary region and the effect of other physical processes that modulate the boundary position
Summary
[2] Saturn’s internally generated magnetic field is found to be remarkably symmetric about the planet’s spin axis, the dipole axis being tilted by less than ∼0.5° [e.g., Connerney et al, 1982; Davis and Smith, 1990; Dougherty et al, 2005; Giampieri et al, 2006]. Carbary and Krimigis [1982] reported periodic variations in Voyager energetic ion and electron spectra, which they linked to the SKR modulations, while Espinosa and Dougherty [2000, 2001] and Espinosa et al [2003a, 2003b] identified magnetic oscillations in both Pioneer 11 and Voyager data and showed via their polarization that the perturbations were not due to a rotating tilted planetary dipole. The SKR power is found to peak when this field points radially outward in the postmidnight sector, at a local time (LT) of ∼0200 h [Andrews et al, 2008; Provan et al, 2009a] This field perturbs the background planetary field, tilting the magnetic equator toward the direction in which the transverse field points, leading to the oscillatory tilting of the equatorial ring current/plasma sheet reported by Carbary et al [2008b], which tilts maximally away from the Sun at SKR maxima. We attempt to isolate and study the oscillations near the planetary period observed by Cassini over the interval from orbit insertion in July 2004 to the end of 2007 and to consider their relation to the magnetospheric period phenomena discussed above
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