The giant‐planet magnetodiscs are shaped by the radial transport of plasma originating in the inner magnetosphere. Magnetic flux transport is a key aspect of the stretched magnetic field configuration of the magnetodisc. While net mass transport is outward (ultimately lost to the solar wind), magnetic flux conservation requires a balanced two‐way transport process. Magnetic reconnection is a critical aspect of the balanced flux transport. We present a comprehensive analysis of current sheet crossings in Saturn's magnetosphere using Cassini magnetometer data from 2004 to 2012 in an attempt to quantify the circulation of magnetic flux, emphasizing local time dependence. A key property of flux transport is the azimuthal bend forward or bend back of the magnetic field. The bend back configuration is an expected property of the magnetodisc with net mass outflow, but the bend forward configuration can be achieved with the rapid inward motion of mostly empty flux tubes following reconnection. We find a strong local time dependence for the bend forward cases, localized mostly in the postnoon sector, indicating that much of the flux‐conserving reconnection occurs in the subsolar and dusk sector. We suggest that the reconnection occur in a complex and patchy network of reconnection sites, supporting the idea that plasma can be lost on small scales through a “drizzle”‐like process. Auroral implications for the observed flux circulation will also be presented.

It has been well demonstrated that the nonlinear Kelvin‐Helmholtz (KH) instability plays a critical role for the solar wind interaction with the Earth's magnetosphere. Although the two‐dimensional KH instability has been fully explored during the past decades, more and more studies show the fundamental difference between the two‐ and three‐dimensional KH instability. For northward interplanetary magnetic field (IMF) conditions, the nonlinear KH wave that is localized in the vicinity of the equatorial plane can dramatically bend the magnetic field line, generating strong antiparallel magnetic field components at high latitudes in both North and South Hemispheres, which satisfy the onset condition for magnetic reconnection. This high‐latitude double reconnection process can exchange the portion of magnetosheath and magnetospheric flux tubes between those two reconnection sites. This study used a high‐resolution 3‐D magnetohydrodynamic simulation to demonstrate that nonlinear KH waves can generate a large amount of double‐reconnected flux during the northward IMF condition, which can efficiently transport the plasma with a high diffusion coefficient of 1 × 1010 m2 s−1 for typical magnetopause conditions at the Earth. The presence of the magnetic field component along the shear flow direction not only decreases the KH growth rate but also causes north‐south asymmetry, which generates more open flux and reduces the efficiency of the plasma transport process.

At the Earth's magnetopause, both magnetic reconnection and the Kelvin-Helmholtz (KH) instability can operate simultaneously for southward interplanetary magnetic field conditions. The dynamic evolution of such a system can be expected to depend on the importance of KH wave evolution versus reconnection and therefore on the respective initial perturbations. In this study, a series of local three-dimensional MHD and Hall MHD simulations are carried out to investigate the situation where the Kelvin-Helmholtz instability is initially the primary process. It is demonstrated that magnetic reconnection is driven and strongly modified by nonlinear KH waves. The highest reconnection rate is close to the Petschek rate, but the total open flux is limited by the size of the nonlinear KH wave. Most of the total open magnetic flux has no flux rope structure and originates from reconnection at thin current layers which connect adjacent vortices. In contrast, complex flux ropes generated by patchy reconnection within the KH vortices dominate the vicinity of the equatorial plane; however, the associated open flux with flux ropes is a minor contribution to the total open flux. Although the presence of Hall physics leads to a fast early increase of the reconnection rate, the maximum reconnection rate and the total amount of open magnetic flux at saturation are the same as in the MHD case.

Citation:Ma, X., A. Otto, and P. A. Delamere (2014) Abstract A typical property of magnetopause reconnection is a significant perpendicular shear flow due to the fast streaming magnetosheath plasma. Therefore, the magnetopause represents a large magnetic and flow shear boundary during periods of southward interplanetary magnetic field, which can be unstable to Kelvin-Helmholtz (KH) modes and to magnetic reconnection. A series of local three-dimensional MHD and Hall MHD simulations is carried out to investigate the interaction of reconnection and nonlinear KH waves considering magnetic reconnection as the primary process. It is demonstrated that the onset reconnection causes a thinning of the shear flow layer, thereby generating small wavelength KH modes. In turn, the growing KH modes modify the current layer width, which modulate the diffusion regions, increase the local reconnection rates, and generate field-aligned currents. The simulation results imply a limitation of total amount of open flux likely caused by nonlinear saturation of KH growth and the associated diffusion. It is also demonstrated that the reconnection rate maximizes for conditions that allow a strong nonlinear evolution of KH waves, i.e., fast shear flow and limited guide magnetic field. The presence of Hall physics increases the reconnection rate in the early stage; however, the maximum reconnection rate and the total amount of open flux at saturation are the same as in the MHD case.