Abstract
AbstractDuring certain portions of the Cassini mission to Saturn, Cassini made repeated and periodic crossings of the magnetospheric current sheet that lies near the magnetic equator and extends well down the magnetospheric tail. These repeated crossings are part of the puzzling set of planetary period variations in numerous magnetospheric properties that have been discovered at Saturn. During 2010 these periodic crossings often display asymmetries such that the northbound crossing occurs faster than the southbound crossing or vice versa, while at other times the crossings are more symmetric. The character of the crossings is well organized by the relative phase of the northern versus southern perturbation currents inferred in earlier analyses of the magnetic field observations. Further, the dependence of the character of the crossings on the relative phase is consistent with similar asymmetries predicted both by the dual rotating current systems inferred from magnetic field observations and by global MHD models that incorporate the effects of hypothesized atmospheric vortices. The two models are themselves in generally good agreement on those predictions. In both models the asymmetries are attributable to a periodic thickening and thinning of the magnetospheric current sheet, combined with a periodic vertical flapping of the sheet. The Cassini observations thus provide additional observational support to such current systems as a likely explanation for many of the known magnetospheric planetary period variations.
Highlights
Saturn’s magnetosphere has been visited by four spacecraft (Pioneer 11, Voyagers 1 and 2, and Cassini) and remotely observed by the Ulysses radio wave experiment
The dependence of the character of the crossings on the relative phase is consistent with similar asymmetries predicted both by the dual rotating current systems inferred from magnetic field observations and by global MHD models that incorporate the effects of hypothesized atmospheric vortices
One of the most puzzling discoveries from these missions has been the occurrence of periodic variations in a wide range of magnetospheric observables, from bursts of Saturn Kilometric Radiation (SKR) [e.g., Warwick et al, 1981; Desch and Kaiser, 1981; Galopeau and Lecacheux, 2000; Gurnett et al, 2009; Lamy, 2011] to in situ magnetic field [e.g., Espinosa and Dougherty, 2000; Cowley et al, 2006; Southwood and Kivelson, 2007; Andrews et al, 2008], energetic particle [e.g., Carbary and Krimigis, 1982; Carbary et al, 2007, 2008], and plasma [e.g., Burch et al, 2009; Arridge et al, 2011; Nemeth et al, 2016] properties
Summary
Saturn’s magnetosphere has been visited by four spacecraft (Pioneer 11, Voyagers 1 and 2, and Cassini) and remotely observed by the Ulysses radio wave experiment. One of the most puzzling discoveries from these missions has been the occurrence of periodic variations in a wide range of magnetospheric observables, from bursts of Saturn Kilometric Radiation (SKR) [e.g., Warwick et al, 1981; Desch and Kaiser, 1981; Galopeau and Lecacheux, 2000; Gurnett et al, 2009; Lamy, 2011] to in situ magnetic field [e.g., Espinosa and Dougherty, 2000; Cowley et al, 2006; Southwood and Kivelson, 2007; Andrews et al, 2008], energetic particle [e.g., Carbary and Krimigis, 1982; Carbary et al, 2007, 2008], and plasma [e.g., Burch et al, 2009; Arridge et al, 2011; Nemeth et al, 2016] properties These periodicities occur at approximately the planetary rotation rate, which is inferred from tracking identifiable cloud features in the atmosphere since at Saturn it is not possible to observe a solid planetary body. These findings were reviewed by Carbary and Mitchell [2013] and have subsequently been supplemented by a number of studies, many of which are summarized by Cowley et al [2016a]
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