Plasma transport driven by the Rayleigh‐Taylor instability

Abstract

Two important differences between the giant magnetospheres (i.e., Jupiter's and Saturn's magnetospheres) and the terrestrial magnetosphere are the internal plasma sources and the fast planetary rotation. Thus, there must be a radially outward flow to transport the plasma to avoid infinite accumulation of plasma. This radial outflow also carries the magnetic flux away from the inner magnetosphere due to the frozen‐in condition. As such, there also must be a radial inward flow to refill the magnetic flux in the inner magnetosphere. Due to the similarity between Rayleigh‐Taylor (RT) instability and the centrifugal instability, we use a three‐dimensional RT instability to demonstrate that an interchange instability can form a convection flow pattern, locally twisting the magnetic flux, consequently forming a pair of high‐latitude reconnection sites. This process exchanges a part of the flux tube, thereby transporting the plasma radially outward without requiring significant latitudinal convection of magnetic flux in the ionosphere.

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

“…The power spectrum (magnetic field) is turbulent. The apparent coalescence of the smaller‐scale waves into larger waves results in kinetic‐scale structure throughout the RT fingers. Diffusive transport is important during the inverse cascade, but eventually, strong guide field reconnection will contribute to transport. This type of double reconnection (Ma et al, ) appears as numerous patchy reconnection sites in the hybrid simulations. Magnetic field fluctuations are consistent with Cassini observations of disturbed magnetic field conditions in Saturn's middle and outer magnetosphere. The fluctuations are characterized by multiple current sheet crossings with substantial fluctuations in all three components (Delamere et al, ). The perpendicular scale of the RT fingers (several ion inertial lengths) can be limited by the parallel wavelengths (i.e., fundamental mode) permitted by the magnetodisc resonant cavity.…”

“…The dynamics of these giant planet magnetospheres are strongly influenced by the internal plasma sources and on average the input plasma source rate must balance the loss rate to the solar wind (Achilleos et al, 2015;Delamere et al, 2015a;Kivelson, 2015). The specific radial transport mechanism is thought to be a centrifugally driven flux tube interchange instability akin to the gravitationally driven Rayleigh-Taylor (RT) instability (Gold, 1959;Ma et al, 2016). Observational evidence for radial transport includes energy dispersed signatures of ions and electrons due to energy-dependent gradient/curvature drift (Hill et al, 2005;Mauk et al, 1999;Paranicas et al, 2016), anomalous magnetic field signatures (André et al, 2005), and disturbed magnetic field conditions exhibiting multiple current sheet crossings in Saturn's middle and outer magnetosphere (Delamere et al, 2015b).…”

“…At L ∼ 16.5 and local time ∼19.3, the separation of the current sheets (disturbed case) is of the order of several ion inertial lengths. Ma et al (2016) examined a local region of the magnetodisc extending to higher latitudes using a 3-D MHD model, finding evidence that RT instabilities in the magnetodisc cause a twist in the magnetic field lines at high latitude. Antiparallel in-plane components of the magnetic field are brought together and reconnect, decoupling plasma from the ionosphere, illustrating the importance of a parallel electric field (purple isosurface in Figure 2 for one hemisphere).…”

Plasma Transport Driven by the Three‐Dimensional Kelvin‐Helmholtz Instability

“…The boundary conditions along the x direction are given by v x =0, and ∂ x =0 for other quantities. For the boundary conditions along the z direction, we add an artificial friction term, − ν ( z ) ρ ( V − V 0 ), on the right‐hand side of the momentum equation, localized at the top and bottom boundaries to mimic the magnetic flux tube being carried by the fast tailward moving solar wind and to limit the KH unstable region along the z direction (Ma et al, ; Ma, Otto, & Delamere, ). Here V 0 is the initial sheared flow profile, and the friction coefficient is given by , (see Figure ).…”

Response of Jupiter's Aurora to Plasma Mass Loading Rate Monitored by the Hisaki Satellite During Volcanic Eruptions at Io

“…Equatorial cross sections of the inward/outward moving flux tubes have finger‐ or bubble‐like shapes, which are expected from the in situ magnetic field measurements (Kivelson et al, ; Thorne et al, ). The finger‐shape was often set for initial conditions in the MHD simulations (Hiraki et al, ; Ma et al, ; Wu et al, ; Yang et al, ). The finger‐like shape is displayed in Figure .…”