Turbulent Ion Heating in the Magnetosheath

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Earth's magnetic field encloses it in a protective cocoon, called the magnetosphere, deflecting most of the solar wind around it. The solar wind is a stream of charged particles, approximately equal numbers of ions and electrons, and magnetic fields blowing outward from the Sun, which varies in intensity with solar activity. The solar wind moves so fast that when it encounters the magnetosphere a shock forms upstream from it. As solar wind plasma passes through the shock it is slowed down and then flows around the magnetosphere in a sheath region, called the magnetosheath. In this region the magnetic field is erratic or turbulent and evidence suggests that magnetosheath ions are heated as this turbulence is dissipated. The topic is compelling because the potential importance of turbulence and associated ion heating has only recently been recognized. Ions in the magnetosheath eventually cross the magnetopause to provide the dominant source of magnetospheric plasmas, thus changes in their temperature and other properties can have a profound effect on the stability of the magnetosphere itself, which can lead to explosive releases of energy during geomagnetic storms and substorms. The methodology for this study is creative and an important advance over previous studies because it is the first self-consistent comprehensive treatment of this turbulence and its dissipation. As such, the potential for new discoveries is high. Advances in understanding will have benefit to society because, in the longer term, they will contribute to the development of an improved predictive capability that can be used to minimize the impacts of such events. This research contributes to the development of a scientific workforce by training a postdoctoral student and a graduate student. Research results will be disseminated through publications, conferences, and community outreach. In order to fulfill its objectives, this study investigate how Alfvenic turbulence heats magnetosheath ions, how the heating process saturates, the rate at which energy is transferred to the ions, how electromagnetic energy is transported to support the heating process, how secondary wave emissions develop during the heating, and how the heating process influences the bulk characteristics of the magnetosheath plasma. The primary tool is a 3D hybrid particle in cell model. The proposal uses wave/ion heating events observed by the THEMIS satellites to both specify initial conditions and to validate simulation results. An innovative aspect is the coupled treatment of both the cascade processes associated with plasma turbulence and the additional instabilities that arise from the anisotropies that develop in the ion distributions.