Periodicities in Saturn's magnetosphere
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Abstract
Abstract Although the exact rotation period of Saturn is unknown, Saturn's magnetosphere displays an abundance of periodicities near ~10.7 h. Such modulations appear in charged particles, magnetic fields, energetic neutral atoms, radio emissions, motions of the plasma sheet and magnetopause, and even in Saturn's rings themselves. Known to an accuracy of four significant figures, these periodicities do not remain constant but vary by ~1% over time scales of a year or longer. Magnetospheric periodicities also display slightly different periods in the northern and southern hemispheres: ~10.6 h and ~10.8 h, respectively. The magnetic and spin axes of Saturn are aligned to within ~1°, so that Saturn's magnetospheric periodicities cannot be explained as “wobble” caused by a geometric tilt, unlike those of the Earth and Jupiter. Furthermore, the variations in periodicity argue against a cause related to changes interior to an object as large as Saturn. Several models have been proposed for the periodicities, including rotating planetary vortices, periodic plasma releases, and a flapping magnetodisk, but none can satisfactorily explain all of Saturn's periodicities. This review discusses the observations of these periodicities from their initial discovery during the Pioneer flyby to their long‐term surveillance by Cassini and examines the various struggles to explain and model them. Understanding Saturn's periodicity may elucidate periodic phenomena in other magnetospheric environments.