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 work on this project has been a collaborative effort aimed at investigation of magnetic reconnection at the dayside magnetosphere. The Earth's magnetosphere is a magnetic field cocoon that largely shields the planet from the harmful action of solar disturbances. Magnetic reconnection can partly open this protective shield making humans and technological systems vulnerable to penetrating energetic particles and electromagnetic fields. One of the ways of studying the magnetosphere and its interaction with the solar wind is via numerical simulations employing the method called magnetohydrodynamics (MHD). This approach can be very successful in describing magnetospheric physical processes on a global and meso scale, but fails to reproduce micro-physics, which requires more detailed approximations. Magnetic reconnection is just such a micro-physical process, and arguably the most critical one. The goal of this project has therefore been to explore ways of including descriptions of micro-physics of magnetic reconnection in the global MHD context. The role of the JHU/APL team in the project has been to help develop simulation techniques toward that goal. Most of the work has been devoted to the development and testing of a new version of our primary simulation tool, the Lyon-Fedder-Mobarry (LFM) global MHD model, suitable for numerical investigations of local and micro-physical reconnection processes in limited spatial domains. The original LFM model has been developed for global simulations of the magnetosphere. It uses a highly conservative numerical scheme meaning that it very effectively captures sharp features in the solutions. The accuracy of the solutions comes at the price of relatively complex algorithms. Although the code is written to work on arbitrary numerical grids as long as the cells are hexahedra (distorted cubes), adapting it to a new grid and new initial and boundary conditions has taken a significant effort. Despite the difficulty, the task has now been accomplished, the new version of the code is in use and is currently being applied to reconnection studies, which will continue far beyond the lifetime of this project. As our work on dayside reconnection progressed, our simulations revealed an important process in the dayside magnetosphere, which can interact with reconnection, but is poorly understood in the global magnetospheric context. The phenomenon is called the Kelvin-Helmholtz instability (KHI), which formes when two plasmas (fluids made up of charged particles, e.g., electrons and protons) stream past each other. This is exactly the kind of situation that occurs when solar wind flows around the magnetosphere. Dayside reconnection and KHI occur in overlapping regions of the magnetosphere and can interfere, but their interaction has not been studied before in global MHD models because until recently they lacked sufficient resolution. In this study we found that the magnetospheric boundary is globally unstable to KHI waves, meaning that it is not a smooth surface but rather a perturbed surface with undulations of multiple frequencies that grow and decay as the waves propagate along the boundary away from the sun. This is an important finding, since aside from their poorly understood interaction with reconnection in the global magnetosphere setting, the KHI is thought to excite various wave populations inside the magnetosphere, which can, for instance, affect transport and energization of particles in the radiation belts. The results of this study have now been published in the Journal of Geophysical Research (Merkin, et al. (2013), J. Geophys. Res. Space Physics, 118, doi:10.1002/jgra.50520). The problem of magnetic reconnection in the dayside magnetosphere is vastly complex and is far from being solved. Reconnection is still poorly understood on the kinetic micro-physics level; understanding it in a complex geometry and asymmetric conditions of the real dayside magnetosphere is exceedingly difficult. This project has helped us to make a significant step forward in two aspects: i) we advanced our technical ability to include local reconnection physics in global magnetosphere simulations; ii) we investigated an important process in the dayside magnetosphere, the Kelvin-Helmholtz instability, which can interact with reconnection and is only starting to be investigated in the global magnetosphere environment. We will continue pursuing these directions in our future studies.
