GEM: What Determines the Magnetosphere Plasma Entropy Distribution

University Of New Hampshire, Durham NH

Investigators

Abstract

This project will address compelling questions about the processes that control the energetics and dynamics of Earth's magnetosphere, a region carved out by the extension of Earth's magnetic field into space. The matter in the magnetosphere is in the form of plasma, a fully ionized gas containing approximately equal numbers of ions and electrons, the motion of which is strongly affected by magnetic fields. A fundamental quantity that describes the energy state(s) of this system is called entropy. When a physical process in this system is adiabatic, meaning no energy or mass is transferred to the surroundings from the system, then the specific entropy is conserved. For example, when a parcel of plasma travels through the magnetosphere adiabatically, it's volume will increase or decrease but it will contain the same number of particles. It's density and temperature will change in order to keep constant the entropy it had when it started it's journey. If the entropy of the parcel changes, then the parcel is being heated or particles are being lost. The methodology for this investigation is to produce maps of specific entropy throughout the Earth's magnetosphere using both global magnetohydrodynamic (MHD) simulations and observations. These maps will then be used to identify the locations where plasma heating is taking place and to investigate the characteristics of the processes responsible. The location of these regions and the time history of the entropy changes provide important clues to the processes involved, and fundamental insights into the energetics and dynamics of the magnetosphere. The project has significant broader impacts. The questions addressed are key to understanding how space weather disturbances originate and evolve in the Earth's vicinity. These disturbances are capable of damaging or destroying satellites that underlie critical societal infrastructures, of creating hazardous radiation for human explorers, and of inducing strong currents in the solid Earth problematic for power grid operation. In the longer term, advances in understanding space weather will feed directly into improved space weather forecast models of value to society. In addition, the project provides training for a graduate student and early career scientist at the University of New Hampshire contributing to the future scientific workforce. Questions addressed in this project focus on the processes that raise the entropy of the plasma sheet in the magnetosphere compared to the entropy of the plasma sources that feed it. In the magnetosphere, entropy can be used to identify the origins of the plasma in a given region. There are only two sources of plasma supplying the magnetosphere - the solar wind and the ionosphere. In the source regions, the entropy values of these populations are very different. If the processes that move these parcels of plasma into the magnetosphere are adiabatic, then the entropy retains the value of the sourch region and is like a dye that marks the origin and entry pathway of the plasma parcel. However, if along these pathways there are regions where the entropy is changing, then these mark the locations where the plasma in the parcel is being lost, mixed, heated or cooled. The processes that are believed to raise the entropy of the plasma sheet include: magnetic reconnection, Bursty Bulk Flows (BFFs), Dipolarization Fronts (DFs), dual lobe reconnection, Kelvin-Hemholtz waves, and turbulent heating. Each of these will be examined to understand their contributions to the increases in entropy in the plasma sheet under different conditions.