Figure 6. The SWMF is being used to study how plasma production and loss are balanced in Saturn's magnetosphere. One mechanism for loss is the formation of plasmoids in the magnetotail.

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Magnetospheric configuration and dynamics of Saturn’s magnetosphere: A global MHD simulation

Xianzhe Jia1, Kenneth C. Hansen1, Tamas I. Gombosi1, Margaret G. Kivelson1,2, Gabor Tóth1, Darren L. DeZeeuw1, and Aaron J. Ridley1

1. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA.
2. Department of Earth and Space Sciences, University of California, Los Angeles, California, USA.

We investigate the solar wind interaction with Saturn’s magnetosphere by using a global magnetohydrodynamic simulation driven by an idealized time-varying solar wind input that includes features of Corotating Interaction Regions typically seen at Saturn.

Our model results indicate that the compressibility of Saturn’s magnetosphere is intermediate between the Earth’s and Jupiter’s, and the magnetopause location appears insensitive to the orientation of the interplanetary magnetic field. The modeled dependences of both the magnetopause and bow shock locations on the solar wind dynamic pressure agree reasonably well with those of data-based empirical models.

Our model shows that the centrifugal acceleration of mass-loaded flux tubes leads to reconnection on closed field lines forming plasmoids, an intrinsic process (“Vasyliūnas-cycle”) in Saturn’s magnetosphere taking place independent of the external conditions. In addition, another type of reconnection process involving open flux tubes (“Dungey-cycle”) is also seen in our simulation when the external condition is favorable for dayside reconnection. Under such circumstances, plasmoid formation in the tail involves reconnection b etween open field lines in the lobes, producing stronger global impacts on the magnetosphere and ionosphere compared to that imposed by the Vasyliūnas-cycle directly.

Our model also shows that large-scale tail reconnection may be induced by compressions driven by interplanetary shocks. In our simulation, large-scale tail reconnection and plasmoid formation take place in a quasi-periodic manner but the recurrence rate tends to be higher as the dynamic pressure becomes higher. While large-scale plasmoid release clearly is an important process in controlling the magnetospheric dynamics, it appears insufficient to account for all the losses of plasma added by the magnetospheric sources. We find that a large fraction of the planetary plasma is lost through the magnetotail near the flanks probably through relatively small-scale plasmoids, a situation that may also exist at Jupiter.

Figure 5a

3D perspective (as viewed from the north near the dawnside flank) of the structure of a plasmoid formed under southward IMF condition (at T = 90 h). Color traces are sampled field lines extracted from the simulation with green showing field lines that thread the plasmoid and magenta showing field lines that surround the plasmoid. The background colors represent contours of Bz. The two dashed lines show the bow shock and magnetopause boundaries identified by tracing flow streamlines. The orange balls mark every 10 RS along the axes.

Figure 8

A 3D perspective from a viewpoint above the equator in the noon meridian plane of the flux tubes returning from tail reconnection site to the magnetosphere as seen in the simulation at T = 225 h. Plotted in the equatorial plane are color contours of Vphi/Vcor overlaid with line contours of plasma density. The pattern of field-aligned currents along with unit flow vectors color coded with Vphi/Vcor in the northern ionosphere are shown in the inset as well as in the magnetospheric plot (mapped to a sphere of radius 4 RS for clarity). Green traces show some sampled field lines traced through the region of rapidly moving flows in the magnetosphere.

Figure 13

A snapshot of global convection and the distribution of flux tube content extracted from the simulation at a time (T = 482 h) when the IMF is northward. The background color contours represent the horizontal flow velocity (Vx) according to the bottom-right color bar and the color contours on a circular disk surrounding Saturn represent FACs intensity in the northern ionosphere (mapped to 4 RS) according to the top-right color bar. The intersections of sampled closed field lines with the equatorial plane are plotted as balls color coded with their corresponding flux tube content (according to the bottom-left color bar). Also plotted are unit flow vectors of the closed field lines showing the direction of their motion. The orange traces show some representative field lines that form the axes.

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