| Home > External Publications > Vita Publications > Energetic electron observations of Rhea’s magnetospheric interaction |
| Journal Article | FZJ-2016-06660 |
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2012
Academ. Press
Orlando, Fla.
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Please use a persistent id in citations: doi:10.1016/j.icarus.2012.07.006
Abstract: Saturn’s moon Rhea is thought to be a simple plasma absorber, however,energetic particle observations in its vicinity show a variety ofunexpected and complex interaction features that do not conform withour current understanding about plasma absorbinginteractions. Energetic electron data are especially interesting, asthey contain a series of broad and narrow flux depletions on eitherside of the moon’s wake. The association of these dropouts withabsorption by dust and boulders orbiting within Rhea’s Hill sphere wassuggested but subsequently not confirmed, so in this study we reviewdata from all four Cassini flybys of Rhea to date seeking evidence foralternative processes operating within the moon’s interactionregion. We focus on energetic electron observations, which we put incontext with magnetometer, cold plasma density and energetic iondata. All flybys have unique features, but here we only focus onseveral structures that are consistently observed. The mostinteresting common feature is that of narrow dropouts in energeticelectron fluxes, visible near the wake flanks. These are typicallyseen together with narrow flux enhancements inside the wake. Aphase-space-density analysis for these structures from the first Rheaflyby (R1) shows that Liouville’s theorem holds, suggesting that theymay be forming due to rapid transport of energetic electrons from themagnetosphere to the wake, through narrow channels. A series ofpossibilities are considered to explain this transport process. Weexamined whether complex energetic electron drifts in the interactionregion of a plasma absorbing moon (modeled through a hybrid simulationcode) may allow such a transport. With the exception of severalfeatures (e.g. broadening of the central wake with increasing electronenergy), most of the commonly observed interaction signatures inenergetic electrons (including the narrow structures) were notreproduced. Additional dynamical processes, not simulated by thehybrid code, should be considered in order to explain the data. Forthe small scale features, the possibility that a flute (interchange)instability acts on the electrons is discussed. This instability isprobably driven by strong gradients in the plasma pressure and themagnetic field magnitude: magnetometer observations show clearlysignatures consistent with the (expected) plasma pressure loss due toion absorption at Rhea. Another potential driver of the instabilitycould have been gradients in the cold plasma density, which are,however, surprisingly absent from most crossings of Rhea’s plasmawake. The lack of a density depletion in Rhea’s wake suggests thepresence of a local cold plasma source region. Hybrid plasmasimulations show that this source cannot be the ionized component ofRhea’s weak exosphere. It is probably related to acceleratedphotoelectrons from the moon’s negatively charged surface, indicatingthat surface charging may play a very important role in shaping Rhea’smagnetospheric interaction region.
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