Discovery of a north‐south asymmetry in Saturn's radio rotation period

Abstract

[1] For many years it has been known that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies by up to one percent on a time scale of years. Recent radio observations from the Cassini spacecraft have revealed the appearance of a second component, with a rotation period of about 10.6 hours, significantly less than the period of the previously known component, which is currently about 10.8 hours. In this paper we show that the first component originates from the southern auroral region, and that the second component originates from the northern auroral region. This north-south asymmetry in the rotation period has potentially important implications on how angular momentum is transferred from the interior to the magnetosphere. Citation:

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

“…As indicated in Figures 1e and 1h, a colatitudinal (θ) component of the perturbation field is also present, consistent with the large-scale perturbation field loops shown in these diagrams. Each of these two systems then rotates at similar but separate periods close to the planetary rotation period, that have been found, initially through tracking related modulated radio wave emissions, to vary slowly with Saturn's seasons [Galopeau and Lecacheux, 2000;Kurth et al, 2008;Gurnett et al, 2009aGurnett et al, , 2009bGurnett et al, , 2011Lamy, 2011]. Essentially the same rotation periods have been derived by following the phase of the resulting magnetic field oscillations observed during periapsis passes of the Cassini spacecraft [Andrews et al, 2008[Andrews et al, , 2010bProvan et al, 2014].…”

Direct evidence of differences in magnetic rotation rate between Saturn's northern and southern polar regions

“…The presence of such currents is deduced on the basis of shear in the magnetic field, in particular in the B ' component of the planetary field. The discovery reported by Gurnett et al [2009a] of a difference in apparent rotation rate between Saturn kilometric radio emission from northern and southern hemispheres leads immediately to the question of whether there is a counterpart detectable in the Saturnian magnetic field. This paper reports such a direct detection.…”

“…[6] Gurnett et al [2009a] discovered that the Saturnian radio emission shows another surprising feature. Radio signals from the southern hemisphere have a period have a period slightly longer than that of signals from the north.…”

The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals

“…One of the most widely studied of these is Saturn Kilometric Radiation (SKR) which is an intense radio emission at kilometer-wavelengths discovered during the Voyager flybys of Saturn in 1980-1981[Kaiser et al, 1980. It is now known that the SKR rotational modulation period is not only variable, but has two periods [Kurth et al, 2008], one at about 10.6 hr that originates primarily from a source in the northern auroral region, and the other at about 10.8 hr that originates primarily from a source in the southern auroral region [Gurnett et al, 2009a]. Recent Cassini data show that these two periods reversed a few months after the August 2009 equinox [Gurnett et al, 2010a;Lamy, 2011].…”