Turbulent Ion Heating in the Magnetosheath

地球的磁场把它包裹在一个被称为磁层的保护性茧中，使它周围的大部分太阳风发生偏转。太阳风是一股带电粒子流，大约有等量的离子和电子，以及从太阳向外吹出的磁场，其强度随太阳活动而变化。太阳风移动得如此之快，以至于当它遇到磁层时，会在其上游形成激波。当太阳风等离子体穿过激波时，它的速度会减慢，然后在一个叫做磁鞘的磁层周围流动。在这个区域，磁场是不稳定的或湍流的，有证据表明，当这种湍流消散时，磁鞘离子被加热。这个话题是引人注目的，因为湍流和相关的离子加热的潜在重要性直到最近才被认识到。磁鞘中的离子最终穿过磁层顶，成为磁层等离子体的主要来源，因此它们的温度和其他特性的变化会对磁层本身的稳定性产生深远的影响，这可能导致地磁暴和亚暴期间能量的爆炸性释放。这项研究的方法是创造性的，是以往研究的重要进步，因为它是第一个对这种湍流及其耗散的自洽综合处理。因此，新发现的可能性很大。理解的进步将有益于社会，因为从长远来看，它们将有助于提高预测能力的发展，从而将此类事件的影响降到最低。这项研究通过培养一名博士后和一名研究生，促进了科研队伍的发展。研究成果将通过出版物、会议和社区外展来传播。为了实现其目标，本研究探讨了Alfvenic湍流如何加热磁鞘离子，加热过程如何饱和，能量转移到离子的速率，电磁能量如何传输以支持加热过程，加热过程中二次波辐射如何发展，以及加热过程如何影响磁鞘等离子体的体积特性。主要工具是三维混合粒子细胞模型。该建议使用THEMIS卫星观测到的波/离子加热事件来指定初始条件并验证模拟结果。一个创新的方面是耦合处理与等离子体湍流相关的级联过程和离子分布中发展的各向异性引起的额外不稳定性。

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.