Local Time Variation in the Large‐Scale Structure of Saturn's Magnetosphere

Sorba, Arianna; Achilleos, N.; Sergis, N.; Guio, P.; Arridge, C. S.; Dougherty, M. K.

doi:10.1029/2018ja026363

Cited by 12 publications

(13 citation statements)

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“…This may be due to the presence of a plasmapause‐like boundary. Further evidence of radial flow boundaries extrapolated from remotely sensed auroral imagery might be useful but relies on improved magnetic mapping models (for example, Sorba et al () have recently computed local‐time‐dependent ionosphere‐magnetodisk mapping profiles at Saturn).…”

Section: Discussionmentioning

confidence: 99%

“…All PPO phase data (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) are available in the University of Leicester Research Archive (http://hdl.handle.net/2381/ 42436). We thank Nick Achilleos and Arianna Sorba at UCL for their provision of ionosphere-magnetosphere field mapping profiles, which have since been computed for various local time sectors, and are available in the supporting information of Sorba et al (2019). J.K., S.V.B., and C.S.A.…”

Section: Acknowledgmentsmentioning

confidence: 99%

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Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

et al. 2020

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Saturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning.

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Section: Discussionmentioning

confidence: 99%

Section: Acknowledgmentsmentioning

confidence: 99%

Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

et al. 2020

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“…In the simplified model of a dipole plus magnetodisc examined by Achilleos et al (2010), dipolar field lines that cross 10-12 Saturn radii at the equator are displaced to equatorial distances 16-25 Rs by the external currents. A more advanced version of this UCL/AGA model (Sorba et al, 2019) shows that the mapping from the ionosphere varies with local time: In the midnight region, an ionospheric latitude of 71.6°North maps to downtail distances of 16.5 and 31.4 Rs under "compressed" and "expanded" conditions of the model, respectively. Mapping from the same latitude in the southern hemisphere produces even larger equatorial crossing distances (36 and 54 Rs, respectively).…”

Section: 1029/2019ja027075mentioning

confidence: 99%

Saturn's Plasmapause: Signature of Magnetospheric Dynamics

Thomsen

Coates

2019

JGR Space Physics

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We explore the paradigm that Saturn's plasmapause marks the boundary between the magnetic flux tubes that have been circulating around the planet for some time, accumulating a dense load of Enceladus-sourced material, and those that have recently undergone tail reconnection, shedding the bulk of the cold plasma and retaining a more tenuous, heated population. A centrifugally driven interchange instability should develop at this boundary, producing fingers of outward propagating dense plasma and of inward propagating hot, tenuous plasma. The plasmapause should thus be identifiable as a transition from mostly-dense-with-some-tenuous to mostly-tenuous-with-some-dense plasma populations. Electron densities from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) instrument are used to identify the location of this transition for all of the low-latitude (<5°from the magnetic equator) passes through Saturn's inner/middle magnetosphere. The boundary is typically found near and somewhat beyond L=10 (i.e., at~10 Rs from the planet), with a local time asymmetry such that it is closer to the planet on the night side than on the day side.

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“…Staniland et al () tracked the strength of the current sheet throughout the Cassini mission and identified a local time asymmetry, revealing the field to be more compressed at dusk than dawn. Empirical models and simulations have also identified this asymmetry, showing a 5–10% difference in the location of the magnetopause at dawn and dusk (Jia & Kivelson, ; Pilkington et al, ; Sorba et al, ). On average, for radial distances greater than >15 R S , a region defined as the magnetodisc‐proper (Arridge et al, ), the magnetic field becomes predominantly radial and the current sheet determines the field geometry.…”

Section: Introductionmentioning

confidence: 96%

Determining the Nominal Thickness and Variability of the Magnetodisc Current Sheet at Saturn

et al. 2020

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The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half-thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5-3 R S . The current sheet is on average 50% thicker in the nightside quasi-dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon-midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non-PPO drivers of variability. Plain Language SummaryA key discovery of the Cassini orbital mission at Saturn was that the small moon Enceladus was geologically active. Water from a subsurface ocean escapes as vapor through geysers in the moon's southern hemisphere. A significant portion becomes ionized and interacts with the magnetic environment that surrounds Saturn, called its magnetosphere. Due to the rapid rotation rate of the planet, this layer of charged particles sourced from Enceladus is stretched into a thin disc around Saturn, known as the magnetodisc current sheet. In this study, we explore the thickness of the current sheet, identifying its average structure and potential sources of variability. We find that the current sheet thickness depends on both the distance from Saturn and position in local time with respect to the Sun. We also find that some of the variability about its average state is somewhat described by the presence of a magnetic perturbation system that occurs close to the rotation period of Saturn. These results are essential for accurately modeling the magnetosphere of Saturn and understanding how the system responds to different drivers of variability.

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Local Time Variation in the Large‐Scale Structure of Saturn's Magnetosphere

Cited by 12 publications

References 62 publications

Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

Saturn's Plasmapause: Signature of Magnetospheric Dynamics

Determining the Nominal Thickness and Variability of the Magnetodisc Current Sheet at Saturn

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