Magnetohydrodynamic Simulations Of Plasma Dynamics In The Magnetospheric Cusp Region

The Earth's magnetospheric cusp regions are rich in interesting plasma physics. The geo-magnetic cusps offer solar wind plasma a relatively easy entry point into the magnetosphere through magnetic reconnection with the interplanetary magnetic field. The cusp regions are characterized by various interesting and important observations such as low energy particle precipitation, significant outflow of ionospheric material, and the frequent presence of energetic particles in regions of depressed magnetic field strength. The physical mechanisms which lead to these observations are often unresolved. For instance, the acceleration mechanism for energetic cusp populations is not understood, nor is it known what implications they may have on magnetospheric dynamics It is however, well accepted that magnetic reconnection plays a critical role in the vicinity of the cusps and is likely responsible for much of the dynamics in the region. Modeling of the geomagnetic cusps is notoriously challenging. Global magnetospheric models have proven indispensable in the study of the interaction of the solar wind plasma with the Earth's magnetosphere, however, the exterior cusp region poses a significant challenge for these models due to their relatively small scale. I have developed a mesoscale cusp-like magnetic field model in order to provide a better resolution (up to 300 km) of the entire cusp region than is possible in these global models. Typical observational features of the high-altitude cusps are well reproduced by the simulation. Results for both strongly northward and strongly southward interplanetary magnetic field indicate extended regions of depressed magnetic field and strongly enhanced plasma ß (cusp diamagnetic cavities). The Alfvénic nature of the outer boundary between the cusp and magnetosheath, in addition to the flow characteristics in the region, indicate that magnetic reconnection plays an important role in structuring the high-altitude cusp region. The inner boundaries with magnetosphere are gradual transitions forming a clear funnel. These cavities further present a unique configuration in which reconnecting magnetic flux tubes may gain a significant amount of flux tube entropy (H = p1/V) through topological changes due to magnetic reconnection.

This collection of papers will address the question "What is the Magnetospheric Cusp?" and what is its role in the coupling of the solar wind to the magnetosphere as well as its role in the processes of particle transport and energization within the magnetosphere. The cusps have traditionally been described as narrow funnel-shaped regions that provide a focus of the Chapman-Ferraro currents that flow on the magnetopause, a boundary between the cavity dominated by the geomagnetic field (i.e., the magnetosphere) and the external region of the interplanetary medium. Measurements from a number of recent satellite programs have shown that the cusp is not confined to a narrow region near local noon but appears to encompass a large portion of the dayside high-latitude magnetosphere and it appears that the cusp is a major source region for the production of energetic charged particles for the magnetosphere. Audience: This book will be of interest to space science research organizations in governments and industries, the community of Space Physics scientists and university departments of physics, astronomy, space physics, and geophysics.

An overview of current knowledge and future research directions in magnetospheric physics In the six decades since the term 'magnetosphere' was first introduced, much has been theorized and discovered about the magnetized space surrounding each of the bodies in our solar system. Each magnetosphere is unique yet behaves according to universal physical processes. Magnetospheres in the Solar System brings together contributions from experimentalists, theoreticians, and numerical modelers to present an overview of diverse magnetospheres, from the mini-magnetospheres of Mercury to the giant planetary magnetospheres of Jupiter and Saturn. Volume highlights include: Concise history of magnetospheres, basic principles, and equations Overview of the fundamental processes that govern magnetospheric physics Tools and techniques used to investigate magnetospheric processes Special focus on Earth’s magnetosphere and its dynamics Coverage of planetary magnetic fields and magnetospheres throughout the solar system Identification of future research directions in magnetospheric physics The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals. Find out more about the Space Physics and Aeronomy collection in this Q&A with the Editors in Chief

Abstract: Mass, momentum and energy are transferred from the solar wind into the magnetosphere via their interface, the magnetospheric boundaries. High latitude boundaries including the high latitude magnetopause, cusp, entry layer and mantle have been rarely studied since only a few spacecraft have visited there. There are many long standing open questions about high latitude boundaries, e.g., what is the magnetic structure of high latitude boundaries during various interplanetary magnetic field (IMF) conditions? Do the boundaries lose their distinct well-defined edge during southward IMF conditions? How do they respond to outside (solar wind) and inside (magnetic storm and substorm) conditions? What is the behavior of energetic particles in these regions? This dissertation addresses these questions via extensive Cluster data analysis and comparison with global MHD simulations. First, this dissertation presents a statistical study of energetic particles in the cusp region. It presents the first observation that energetic ions exist in the high latitude magnetospheric boundary regions for 80% of the cusp crossings. The spectra of energetic particles with energies greater than 30 keV become flatter for higher solar wind speeds. Second, the high latitude magnetopause has also been studied. When the IMF is northward, the magnetopause adjacent to the cusp is associated with sharp changes in plasma density, velocity, temperature and magnetic field. However, this interface becomes uncertain when the IMF turns southward. A superposed epoch analysis was applied to study the average variations of key plasma parameters across the magnetopause under different conditions for the first time. This dissertation reports the first in-situ observation of collisionless Hall reconnection at the high latitude magnetopause when the IMF B y dominates. Finally, this dissertation compares observations to MHD simulations for a real cusp event. Although the simulated magnetospheres are smaller than the real magnetosphere, the simulated magnetic fields and the amplitude of the model-derived plasma parameters of density, velocity and temperature in the cusp region agree reasonably well with observations. The MHD code qualitatively simulated the responses of the cusp position to the solar wind azimuthal flow for the first time and the formation of the cold dense plasma sheet.

Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 44. Existing models of the plasma distribution and dynamics in magnetosphere / ionosphere systems form a patchwork quilt of different techniques and boundaries chosen to define tractable problems. With increasing sophistication in both observational and modeling techniques has come the desire to overcome these limitations and strive for a more unified description of these systems. On the observational side, we have recently acquired routine access to diagnostic information on the lowest energy bulk plasma, completing our view of the plasma and making possible comparisons with magnetohydrodynamic calculations of plasma moments. On the theoretical side, rising computational capabilities and shrewdly designed computational techniques have permitted the first attacks on the global structure of the magnetosphere. Similar advances in the modeling of neutral atmospheric circulation suggest an emergent capability to globally treat the coupling between plasma and neutral gases. Simultaneously, computer simulation has proven to be a very useful tool for understanding magnetospheric behaviors on smaller space and time scales.

The reconnection of magnetic fields is one of the most fascinating processes in plasma physics, responsible for phenomena such as solar flares and magnetospheric substorms. The concept of reconnection has developed through recent advances in exploring the magnetospheres of the Sun and Earth through theory, computer simulations and spacecraft observations. The great challenge in understanding it stems from balancing the large volumes of plasma and magnetic fields involved with the energy release with the physical mechanism which relies on the strongly localized behavior of charged particles. This book, edited by and with contributions from leading scientists in the field, provides a comprehensive overview of recent theoretical and observational findings concerning the physics of reconnection and the complex structures that may give rise to, or develop from, reconnection. It is intended for researchers and graduate students interested in the dynamics of plasmas.

Following on from the companion volume Principles of Magnetohydrodynamics, this textbook analyzes the applications of plasma physics to thermonuclear fusion and plasma astrophysics from the single viewpoint of MHD. This approach turns out to be ever more powerful when applied to streaming plasmas (the vast majority of visible matter in the Universe), toroidal plasmas (the most promising approach to fusion energy), and nonlinear dynamics (where it all comes together with modern computational techniques and extreme transonic and relativistic plasma flows). The textbook interweaves theory and explicit calculations of waves and instabilities of streaming plasmas in complex magnetic geometries. It is ideally suited to advanced undergraduate and graduate courses in plasma physics and astrophysics.

The aim of this book is twofold: to provide an introduction for newcomers to state of the art computer simulation techniques in space plasma physics and an overview of current developments. Computer simulation has reached a stage where it can be a highly useful tool for guiding theory and for making predictions of space plasma phenomena, ranging from microscopic to global scales. The various articles are arranged, as much as possible, according to the - derlying simulation technique, starting with the technique that makes the least number of assumptions: a fully kinetic approach which solves the coupled set of Maxwell’s equations for the electromagnetic ?eld and the equations of motion for a very large number of charged particles (electrons and ions) in this ?eld. Clearly, this is also the computationally most demanding model. Therefore, even with present day high performance computers, it is the most restrictive in terms of the space and time domain and the range of particle parameters that can be covered by the simulation experiments. It still makes sense, therefore, to also use models, which due to their simp- fying assumptions, seem less realistic, although the e?ect of these assumptions on the outcome of the simulation experiments needs to be carefully assessed.

Magnetohydrodynamics, or MHD, is a theoretical way of describing the statics and dynamics of electrically conducting uids. The most important of these uids occurring in both nature and the laboratory are ionized gases, called plasmas. These have the simultaneous properties of conducting electricity and being electrically charge neutral on almost all length scales. The study of these gases is called plasma physics. MHD is the poor cousin of plasma physics. It is the simplest theory of plasma dynamics. In most introductory courses, it is usually afforded a short chapter or lecture at most: Alfven ́ waves, the kink mode, and that is it. (Now, on to Landau damping!) In advanced plasma courses, such as those dealing with waves or kinetic theory, it is given an even more cursory treatment, a brief mention on the way to things more profound and interesting. (It is just MHD! Besides, real plasma phy- cists do kinetic theory!) Nonetheless, MHD is an indispensable tool in all applications of plasma physics.