This project will use a combination of global and local magnetohydrodynamic (MHD) simulations to examine three questions concerning magnetic reconnection on the dayside of Earth's magnetosphere. The three questions are: (1) Is the global reconnection rate controlled by local or external conditions? (2) Is the site of reconnection dependent on the physics that causes reconnection? (3) Is the reconnection process at the magnetopause continuous or intermittent? The simulations will couple local resistive MHD and Hall MHD codes to a global MHD code. The results of the simulatons will be compared to in situ observations from the Cluster and THEMIS spacecraft missions. The techniques that will be developed for coupling the local and the global codes will make it possible to study multi-scale coupling and is potentially transformative. Although the research is directed toward magnetic reconnection on the dayside of Earth's magnetosphere, the topic of magnetic reconnection and the techniques for coupling local to global simulations will have impacts on studies of reconnection in Earth's night-side magnetotail, reconnection in the solar wind and in the sun's corona and has the potential to impact studies of astrophysical and laboratory plasma processes.
The interaction of the solar wind with the Earth's magnetosphere is the most important driver ofSpace Weather, the phenomena thart can affect areas like power grids on the Earth as well assatellite-based infrastructure. Magnetic reconnection is the most important process by which thisinteraction sends energy from the solar wind to the Eart and the near-Earth space environment. Thisproject studied the effects of a number of physical phenomena on the reconnection occurrin at theEarth's dayside magnetopause - the boundary between the solar wind and the magnetosphere. One of the recent questions has been whether the reconnection rate is governed by local processes,such as the density on both sides of the magnetopause, or whether it is controlled by global aspectsof the flow. We studied the local rate of reconnection in a global MHD model of the magnetosphereand found that the rate agreed with a local theory put forward by Cassak and Shay. Borovsky used theCassak-Shay theory to come up with a total coupling function for the solar wind-Earth interactionwhich was independent of the flow conditions in the region downwind of the Earth's bowshock and ofionospheric coupling. Our results showed that the situation is more complicated that the system willattempt to compensate for reconnection shut off in one area by increased density ,say, by enhancingreconnection elsewhere. Another question has been whether reconnection theories which generally are concerned with verysimplified situations can be applied in isolation in a complicated system like the Earth'smagnetopause. We looked at the Kelvin-Helmholtz (K-H) instability at the magnetopause to see if it wasactive while reconnection was going on. It was and was stronger in periods where it combined withreconnection on the dayside. We studied the K-H in isolation by simulating periods of Northward IMFwhen reconnection on the dayside is mostly shut down. We found that the instability starts quiteearly on the dayside and also leads to waves that affect the internal dynamics of the magnetosphere. It has been suggested that reconnection can be stabilized by diamagnetic drifts. These drifts areproduced by the need for force balance when there is a gradient in the magnetic field and areassociated with the current carried by the plasma. The plasma ions move too fast, they cannotparticipate in the reconnection and, thus, stabilize the system. We performed gyro-kineticsimulations to test this idea. We found that, at least in the parameter range we studied, there wasalways some instability that lead to reconnection.
