Model‐Based Assessments of Magnetic Reconnection and Kelvin‐Helmholtz Instability at Jupiter's Magnetopause

Abstract

The interaction between the solar wind and Jupiter's magnetic field confines the planetary field to the largest magnetosphere in the Solar System. However, the full picture of when and where key processes operate at the magnetopause boundary of the system remains unclear. This is essential for testing understanding with observations and for determining the relative importance of different drivers of Jovian magnetospheric dynamics. Here we present a global analytical model of Jovian magnetopause conditions under steady state, which forms the basis of boundary process assessments. Sites of magnetic reconnection at Jupiter's magnetopause are expected to be in regions of sufficiently high magnetic shear across the boundary, controlled by the orientation of the interplanetary magnetic field. Reconnection rates are also most sensitive to changes in the highly variable IMF, followed by changes in the solar wind plasma mass density. The largest plasma flow shear across the boundary is in the equatorial dawn region, producing a region that is typically unstable to growth of the Kelvin‐Helmholtz (K‐H) instability. Compared to magnetopause reconnection site locations, this K‐H‐unstable region at dawn is less sensitive to changing conditions. Motion of K‐H boundary perturbations typically includes dawn‐to‐dusk motion across the subsolar region. Model‐predicted reconnection voltages are typically hundreds of kV but rely on steady solar wind conditions on a time scale that is longer than typical at Jupiter's orbit. How the reconnection voltage compares to the voltage applied due to the “viscous‐like” interaction involving K‐H instability remains unclear.

A More Viscous‐Like Solar Wind Interaction With All the Giant Planets

“…Although this therefore appears to have the potential to exaggerate our conclusion (see Figure 2), spacecraft observations of PDLs at different planetary magnetopauses suggest that the related increase in solar wind Alfvén speeds is similar (Anderson et al, 1997;Gershman et al, 2013;Masters et al, 2014), implying that this effect is instead negligible. Note that the modeling results summarized in Table 1 include a treatment of PDL formation that is consistent with observations (e.g., Masters, 2017).…”

“…The decrease in solar wind Alfvén speed with distance from the Sun should be reflected in a similarly monotonic decrease in the typical Alfvén speed in the shocked solar wind adjacent to the magnetopauses of the magnetized planets (equivalent to increasing typical nearmagnetopause plasma β, ratio of plasma to magnetic pressure), which is consistent with spacecraft observations (e.g., Gershman et al, 2013;Masters et al, 2012;Trenchi et al, 2008). Table 1 explores this known variation in near-magnetopause solar wind Alfvén speeds by employing an established analytical modeling approach (see Masters, 2017, and references therein). For each planet the magnetopause is described as a parabolic conic section and the upstream IMF is perpendicular to the solar wind flow vector.…”

“…Table explores this known variation in near‐magnetopause solar wind Alfvén speeds by employing an established analytical modeling approach (see Masters, , and references therein). For each planet the magnetopause is described as a parabolic conic section and the upstream IMF is perpendicular to the solar wind flow vector.…”

“…A larger obstacle to a solar wind of constant speed promotes both external drivers discussed here and also increases the duration of steady solar wind required for approximately steady state external conditions across the global dayside magnetopause. Conditions at all the giant planet magnetopauses are therefore expected to typically deviate from steady state to a greater extent, likely affecting global magnetic reconnection more than the ongoing viscous‐like interaction (e.g., Masters, ; McComas & Bagenal, ).…”

“…Debate on this topic has often been in the context of Jupiter's giant magnetosphere. While the potential applied by global magnetic reconnection can be estimated (Badman & Cowley, ; Masters, ), it has been proposed and contested that the cycling of magnetic flux through Jupiter's magnetosphere differs from the terrestrial case (Cowley et al, ; McComas & Bagenal, , ). The question of external driving of the Jovian system was addressed by Delamere and Bagenal (, ).…”