Spontaneous axisymmetry breaking of the external magnetic field at Saturn
Abstract
Saturn's magnetic field is remarkably axisymmetric. Early evidence for nonaxisymmetry came from the periodicity of Saturn's kilometric radio bursts (SKR). Subsequently, percent-level variations of the SKR period were found to occur on timescales of years. A recent breakthrough has been the direct detection of a nonaxisymmetric component of the field that rotates with a period close to that of the SKR and whose magnitude varies only weakly with distance from Saturn. The latter implies that it must be supported by currents external to the planet. We explore the hypothesis that centrifugally driven convection spontaneously breaks the axisymmetry of the external magnetic field at Saturn. The density of the outflowing plasma close to its source is assumed to contain a substantial part that varies as cos? and rotates uniformly. We demonstrate that the plasma stream must narrow with distance from the planet, while the field-aligned currents joining stream to ionosphere increase rapidly. These currents produce a nonaxisymmetric component of magnetic field whose magnitude varies inversely with radial distance in the planet's equatorial plane. For a rate of plasma outflow 104 ≲ ? ≲ 105g s-1, this component's strength is compatible with that observed. Additionally, we postulate that the SKR is associated with the narrow range of longitudes over which large currents flow along magnetic field lines connecting the tip of the outflow to the auroral ionosphere.
- Publication:
-
Journal of Geophysical Research (Space Physics)
- Pub Date:
- May 2007
- DOI:
- 10.1029/2006JA012163
- arXiv:
- arXiv:astro-ph/0608296
- Bibcode:
- 2007JGRA..112.5225G
- Keywords:
-
- Magnetospheric Physics: Planetary magnetospheres (5443;
- 5737;
- 6033);
- Magnetospheric Physics: Magnetosphere/ionosphere interactions (2431);
- Magnetospheric Physics: Plasma convection (2463);
- Planetary Sciences: Fluid Planets: Magnetic fields and magnetism;
- Saturn;
- magnetic field;
- plasma convection;
- Astrophysics
- E-Print:
- 24 pages, 2 figures, submitted to JGRA