Oscillation of Saturn's southern auroral oval

Abstract

Near-planetary-period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently highly axisymmetric. In addition, the period of the Saturn kilometric radiation has been shown to vary over time. In this paper we present results from the recent Hubble Space Telescope observations of Saturn's southern ultraviolet auroral emission. We show that the center of the auroral oval oscillates with period 10.76 h ± 0.15 h for both January 2007 and February 2008, i.e., close to the periods determined for oscillations in other magnetospheric phenomena. The motion of the oval center is described for 2007 by an ellipse with semimajor axis ∼1.4° ± 0.3° oriented toward ∼09-21 h LT, eccentricity ∼0.93, and center offset from the spin axis by ∼1.8° toward ∼04 h LT. For 2008 the oscillation is consistent with an ellipse with semimajor axis ∼2.2° ± 0.3° oriented toward ∼09-21 h LT, eccentricity ∼0.99, and a center offset from the spin axis by ∼2.2° toward ∼03 h LT. The motion of the auroral oval is thus highly elliptical in both cases, and the major oscillation axis is oriented toward prenoon/premidnight. This result places an independent constraint on the magnitude of the planet's dipole tilt and may also indicate the presence of an external current system that imposes an asymmetry in the ionospheric field modulated close to the planetary period.