Local time dependence of turbulent magnetic fields in Saturn's magnetodisc

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.

Section: Discussionmentioning

confidence: 94%

Radial Transport and Plasma Heating in Jupiter's Magnetodisc

Kaminker

et al. 2018

JGR Space Physics

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“…The red line has a spectral index of −5/3, and the blue line has a spectral index of −8/3 consistent with a turbulent cascade in the respective inertial and dissipative ranges (Galtier et al, ). Turbulent magnetic fluctuations are seen throughout Saturn's magnetosphere (Kaminker et al, ; von Papen et al, ), and KH‐related ion heating could be an important aspect of boundary layer processes.…”

Section: Resultsmentioning

confidence: 99%

Three‐Dimensional Hybrid Simulation of Viscous‐Like Processes at Saturn's Magnetopause Boundary

Geophysical Research Letters

Burkholder

2018

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Saturn's magnetosheath flows exhibit significant dawn/dusk asymmetry. The dawnside flows are reduced from expectation, suggesting significant momentum transport through the magnetopause boundary where the flow shear is maximized. It has been suggested that the solar wind interaction with the giant magnetospheres is, in fact, dominated by a viscous‐like interaction governed by the Kelvin‐Helmholtz instability. In three dimensions, the Kelvin‐Helmholtz instability can generate small‐scale and intermittent magnetic reconnection due, in part, to a twisted magnetic field topology. The net result is a field line threading of the magnetopause boundary and the generation of Maxwell shear stresses. Here we present three‐dimensional hybrid simulations (kinetic ions and massless fluid electrons) of conditions similar to Saturn's dawnside magnetopause boundary to quantify the viscous‐like, tangential drag. Using model‐determined momentum fluxes, we estimate the effect on dawnside sheath flows and find very good agreement with observations.

“…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

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.