The plasma depletion layer in Saturn's magnetosheath

Abstract

A plasma depletion layer (PDL) of reduced plasma density and enhanced magnetic field strength can form in the magnetosheath (shocked solar wind) adjacent to the magnetopause boundary of a planetary magnetosphere. The dominant factor controlling the level of plasma depletion and field enhancement in Earth's PDL is the magnetic shear across the magnetopause, due to the influence of this parameter on magnetic reconnection at the boundary. Here we examine the PDL in Saturn's magnetosheath using Cassini spacecraft observations. For most Cassini magnetosheath intervals analyzed, there is some level of both plasma depletion and magnetic field enhancement approaching Saturn's magnetopause, consistent with the presence of a PDL. We find some evidence that the Saturnian PDL responds to the solar wind dynamic pressure, in the same sense as Earth's PDL. However, we find no evidence for a response of Saturn's PDL to the magnetic shear across the magnetopause, in contrast with the terrestrial case. Our results thus suggest that the rate of magnetic flux transport due to reconnection at Saturn's magnetopause is generally small in comparison to the rate of magnetic flux advection by the solar wind and are consistent with the expectation that conditions at Saturn's magnetopause are less favorable for magnetopause reconnection than those at Earth's magnetopause.

Can magnetopause reconnection drive Saturn's magnetosphere?

“…This leaves 262 crossings where we have the magnetic field vector and electron number density in both the adjacent magnetosheath and magnetosphere. Note that the captured conditions in the magnetosheath are affected by the presence of a plasma depletion layer, and those in the magnetosphere are affected by the presence of a low‐latitude boundary layer (e.g., Figure ) [ Masters et al ., , ]. Conditions on both sides of Saturn's magnetopause are highly variable, and our statistics capture the level of this variability.…”

“…This is most likely because the local magnetosheath magnetic field strength (which primarily controls the electric field strength) is similarly not guaranteed to be relatively high when the system is compressed (see supporting information). We suggest that this is due to magnetosheath variability, particularly the presence of mirror mode structures that can significantly reduce the local magnetic field strength [ Violante et al ., ; Masters et al ., ].…”

“…Spacecraft observations have confirmed that reconnection takes place at Saturn's magnetopause [ Huddleston et al ., ; McAndrews et al ., ; Lai et al ., ], transient auroral features have been attributed to this phenomenon [ Radioti et al ., , ; Badman et al ., , ], and it is thought that Saturn's main auroral emission is associated with the boundary between open and closed magnetic field lines [ Cowley et al ., ; Badman et al ., ]. However, Saturn's low‐latitude boundary layer, plasma depletion layer, and magnetopause position do not show a clear, Earth‐like, reconnection‐related response to the Interplanetary Magnetic Field (IMF) orientation [ Masters et al ., , ; Lai et al ., ]. Although the full picture remains unclear, reported observations are currently consistent with our expectations about magnetopause reconnection at Saturn.…”

The Near-Saturn Magnetic Field Environment

“…By testing its validity, this will also provide additional means in approximating the magnetic field in a region where Cassini is 128 not present to make observations. The magnetosheath during these four traversals was measured to have relatively steady IMF orientations throughout the traversal, also used in the work by Masters et al [2014]. This one-to-one approach is not suitable with the MHD simulation since the grid size near the magnetopause and bow shock is ~0.5 R s .…”

“…Gershman et al (2013) also reported a large-scale PDL often forming just exterior of Mercury's subsolar magnetopause as a result of piled-up draped magnetic flux around the magnetosphere. This has been attributed to the low av- Masters et al (2014), on the other hand, show no PDL response to cross-magnetopause magnetic shear because the magnetic flux transport rates associated with reconnection are too low to have any effect. Nonetheless, it is expected that the IMF orientation strongly controls where reconnection is occurring because of the β-imposed constraint of close to anti-parallel fields required for reconnection onset [Masters et al, 2012].…”