Longitudinal and local time asymmetries of magnetospheric turbulence in Saturn's plasma sheet

Papen, Michael von; Saur, Joachim

doi:10.1002/2016ja022427

Cited by 12 publications

(14 citation statements)

References 78 publications

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“…Delamere et al () analyzed current sheet crossings using Cassini magnetometer data and found a greatly increased number of possible magnetodisc reconnection sites near the dusk flank of Saturn. They conclude that a continuous “drizzle” of small and patchy reconnection events in this region is likely to contribute significantly to the continuous magnetic flux circulation in the magnetosphere—in line with earlier theoretical results (e.g., Bagenal, ; Bagenal & Delamere, ; Delamere & Bagenal, , and references therein) and more recent investigations of magnetic turbulence in Saturn's plasma sheet (Kaminker et al, ; von Papen & Saur, ). This process is similar to small plasma bubbles breaking off the outer edge of Jupiter's magnetodisc and moving down the dusk flank as proposed by Kivelson and Southwood ().…”

Section: Resultssupporting

confidence: 78%

Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time‐Resolution UV Auroral Imagery

et al. 2019

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Saturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short-lived ∼1-hr quasiperiodic high-energy electron injections, frequently observed in in situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time-resolution ultraviolet (UV) auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low-frequency waves and instead directly caused by recurrent small-scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site toward the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent-back to a more dipolar configuration. This manifests as a sawtooth-shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release.Recent surveys have statistically investigated the occurrence of such short periodicities throughout the Kronian magnetosphere. Roussos et al. (2016) and analyzed quasiperiodic injections of relativistic electrons and found that most events occurred at ∼1-hr periodicities and outside of Titan's orbit (∼20 R S ), spread through almost all the outer magnetosphere-although with a significant location bias toward dusk local times (LTs). further observed strong radio bursts in the auroral hiss collocated with the electron injections and higher growth rates of the pulses at high latitudes, suggesting a high-latitude acceleration region. The observed location at which these injections take place points to magnetopause or Vasyliunas-cycle reconnection as possible trigger mechanisms . Kelvin-Helmholtz waves are deemed unlikely to effectuate the observed LT disparity.Based on radio measurements from the entire Cassini mission, Carbary et al. (2016) observed similarly increased occurrence rates of periodicities in plasma wave intensity near dusk and at high latitudes, although noting that this bias might be explained with higher auroral hiss observation rates in these regions. They

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Section: Resultssupporting

confidence: 78%

Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time‐Resolution UV Auroral Imagery

et al. 2019

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“…Turbulent heating can account for magnetodisc heating at Saturn too. Kaminker et al (), von Papen et al (), and von Papen and Saur () showed that magnetic field fluctuations measured by the Cassini magnetometer (MAG) instrument are consistent with the requisite turbulent heating rate density (Bagenal & Delamere, ). Using the 1‐s‐averaged MAG data, Kaminker et al () compared the heating rate density in both the inertial subrange (MHD scale) and the dissipation scale (kinetic scale) and found that the kinetic scale heating was typically larger.…”

Section: Discussionmentioning

confidence: 94%

Radial Transport and Plasma Heating in Jupiter's Magnetodisc

Delamere

Kaminker

et al. 2018

JGR Space Physics

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The ion temperature of the magnetosphere of Jupiter derived from Galileo PLS data was observed to increase by about an order of magnitude from 10 to 40 Jupiter radii. This suggests the presence of heating sources that counteract the adiabatic cooling effect of expanding plasma. There have been different attempts of explaining this phenomena, including a magnetohydrodynamic (MHD) turbulent heating model which is based on flux tube diffusion (Saur, Astrophys. J. Lett., 602, L137, 2004). We explore an alternate turbulent heating model based on advection, similar to models commonly used in solar wind heating. Based on spectral analysis of Galileo magnetometer (MAG) data, we find that observed MHD turbulence could potentially provide the required heating to explain some of the increase in plasma temperature. This indicates that advection is a more appropriate way to describe radial transport of plasma in the Jovian magnetosphere beyond 10 Jupiter radii.

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“…The turbulence observed is a result of counter propagating waves along the magnetic field (Kaminker et al, ; Saur et al, ; von Papen & Saur, ). The formation of surface waves in the unseeded simulation has an initial phase that varies along y .…”

Section: Discussionmentioning

confidence: 99%

Hybrid Simulations of Magnetodisc Transport Driven by the Rayleigh‐Taylor Instability

Stauffer

Delamere

et al. 2019

JGR Space Physics

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Plasma transport in the rapidly rotating giant magnetospheres is thought to involve a centrifugally driven flux tube interchange instability, similar to the Rayleigh‐Taylor (RT) instability. In three dimensions, the convective flow patterns associated with the RT instability can produce strong guide field reconnection, allowing plasma mass to move radially outward while conserving magnetic flux (Ma et al., 2016, https://doi.org/10.1002/2015JA022122). We present a set of hybrid (kinetic ion/fluid electron) plasma simulations of the RT instability using high plasma beta conditions appropriate for the inner and middle magnetosphere at Jupiter and Saturn. A density gradient, combined with a centrifugal force, provide appropriate RT onset conditions. Pressure balance requires only a temperature gradient as the magnetic pressure is constant. Pressure balance is achieved with a temperature gradient in a fixed magnetic field. The three‐dimensional simulation domain represents a local volume of the magnetodisc resonant cavity. Simulated RT growth rates compare favorably with linear theory, where the fundamental mode of the resonant cavity determines the largest (stabilizing) parallel wavelength. We suggest that the perpendicular scale of RT structures is determined by the fundamental mode, which limits growth due to magnetic tension. Finally, we investigated strong guide field magnetic reconnection and diffusive processes as plausible mechanisms to facilitate kinetic‐scale radial transport.

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Longitudinal and local time asymmetries of magnetospheric turbulence in Saturn's plasma sheet

Cited by 12 publications

References 78 publications

Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time‐Resolution UV Auroral Imagery

Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time‐Resolution UV Auroral Imagery

Radial Transport and Plasma Heating in Jupiter's Magnetodisc

Hybrid Simulations of Magnetodisc Transport Driven by the Rayleigh‐Taylor Instability

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