This project will develop and employ a new class of current sheet equilibria in Earth's magnetosphere. These equilibria will be analytic solutions and can be used initiate dynamical studies of their time evolution using particle-in-cell (PIC) kinetic simulations of the plasma. Among the issues that will be addressed are the following: Can the many observed current sheet configurations in Earth's magnetosphere be modeled by the stable members of this new class of equilibria? How sensitive is undriven (spontaneous) reconnection to initial conditions? What are the underlying instabilities which destabilize current sheets? How is magnetic reconnection triggered in Earth's magnetotail? How well can the initiation of asymmetric reconnection at Earth's magnetopause be modeled by the new equilibria? How well can the new equilibria model transient reconnection onset in the magnetotail? The PIC simulations in two and three dimensions will determine the stability of these diverse kinetic equilibria against a variety of plasma processes including tearing, lower hybrid drift and other instabilities. In addition, preliminary results have demonstrated that non-equilibrium initial conditions can lead to self-generated current sheets. The investigation of how non-equilibrium conditions can evolve toward an equilibrium condition will improve our understanding of the dynamics of the magnetosphere.
The research has relevance to many different fields of plasma physics. Even within the main focus of magnetospheric physics the research will have diverse impacts. It will provide novel asymmetric kinetic current sheet equilibria for simulations of reconnection at the magnetopause as well as symmetric equilibria for a wider class of simulations of reconnection in the magnetotail and magnetosheath. Furthermore, it can contribute to our understanding of the many broad stable current sheets which have been observed in the magnetosphere and which cannot be modeled as Harris current sheet equilibria. Its potential impact extends to all plasma domains where collisionless magnetic reconnection occurs: to the magnetosphere, to the solar wind, to the solar corona, to laboratory reconnection experiments and to toroidal magnetic fusion devices in which "sawtooth" instabilities which inhibit confinement and heating are believed to involve reconnection. The project will also have an educational impact through the training of a graduate student in plasma theory, simulations, and data visualization.
Current sheets play a role in separating different regions of Earth’s space-plasma environment, such as the interface between the solar wind and the magnetosphere. Under suitable conditions, thin current sheets can initiate magnetic reconnection, a process capable of releasing large amounts of energy stored in magnetic fields. A simple current-sheet model know as a Harris sheet is a plasma equilibrium that has served as the starting point for many computational studies of magnetic reconnection. In this project we have developed generalizations of the Harris equilibrium that can better model complex current sheets separating plasmas with different densities, temperatures, and magnetic field strengths. A key feature of these generalized current sheets is that, unlike the Harris sheet, they are not strictly charge neutral, but instead they produce and are simultaneously influenced by spatially varying electrostatic fields and potentials. For example, these potentials can form barriers separating different populations of electrons and ions on opposite sides of the current sheet. The accompanying figure illustrates how these generalized equilibria can be used to model the interface between the thinner but hotter plasma of the magnetosphere from the colder but denser solar wind plasma. Because the density ratio across the interface is greater than the temperature ratio, the plasma pressure (the product of density and temperature) is higher on the solar-wind side. This imbalance in plasma pressure is compensated by a higher magnetic pressure (proportional to the square of the magnetic field strength) on the magnetosphere side – in accordance with satellite observations. These methods developed for generating new families of current-sheet configurations are easily implemented and can have application as an educational tool. Furthermore, these models can be useful for initializing computer simulations of magnetic reconnection in a number of space environments. Such simulations are critical to understanding the influence of solar activity on Earth’s space-plasma environment. These interactions, which comprise an aspect of "space weather" can have important societal impacts in areas such as the reliability of satellite communications and electrical power grids. To aid in the interpretation of the output of these simulations, new visualization methods have also been developed.
