Role of Collisionless Reconnection Mechanisms in Solar Flares and Coronal Mass Ejections (CMEs)

人们普遍认为，磁场重联在太阳耀斑和日冕物质抛射(CME)的大多数理论模型中起着重要作用。这些模型一般基于阻性磁流体力学(MHD)方程，其中拓扑变化和能量耗散的动力学完全由等离子体电阻率控制。霍尔MHD模型可以描述一些最重要的无碰撞日冕效应。阻性MHD模型和霍尔MHD模型最重要的区别在于它们对欧姆定律的处理。阻性MHD模型采用标准的阻性欧姆定律，而霍尔MHD模型采用广义欧姆定律，它不仅包括等离子体的电阻率，还包括霍尔电流等附加项。有人提出，霍尔MHD效应有可能解决与太阳耀斑和日冕物质抛射有关的一些悬而未决的科学问题，这些太阳耀斑和日冕物质抛射的特征不仅是快速增长，而且还具有“冲动性”，即增长率的时间导数突然增加(所谓的“触发”问题)。提出了一种基于Hall MHD模型的无碰撞重连的理论研究方案，采用解析方法和采用自适应网格加密(AMR)的大规模并行代码。建议的任务包括彻底调查触发问题、重联点的结构和所涉及的流动(S)、足点切变对多个拱廊的影响以及磁通绳模型的完全自洽的Hall MHD模拟。这一研究计划的结果有望加强和补充日冕重联理论的现有框架，包括已被证明在实验室和其他空间等离子体研究中有价值的重要的无碰撞过程。拟议的努力整合了从高中到研究生的所有级别的教学和研究。它包括：(1)参加新汉普郡大学一个名为“通过研究培训实现科学和数学成就”(Project SMART)的暑期学院，为当地高中生提供令人兴奋的研究体验；(2)共同赞助“研究与发现”(RAD)计划中的一个试点暑期实习计划，供初三和高年级的本科生使用；以及(3)培训一名研究生并指导一名初级研究教员。这项工作还将促进与空间等离子体和实验室物理学家的广泛社区的跨学科互动。

Magnetic reconnection is widely believed to play a significant role in most theoretical models of solar flares and coronal mass ejections (CMEs). These models are generally based on resistive magnetohydrodynamic (MHD) equations, where the dynamics of topological change and energy dissipation are controlled entirely by the plasma resistivity. The Hall MHD model can describe some of the most significant collisionless effects for the solar corona. The most important difference between resistive MHD and Hall MHD models lies in their treatment of Ohm's law. Whereas the resistive MHD model employs the standard resistive Ohm's law, the Hall MHD model employs the generalized Ohm's law, which includes not only the plasma resistivity but also additional terms such as the Hall current. It is proposed that Hall MHD effects can potentially resolve some outstanding science issues pertaining to those solar flares and CMEs that are characterized not only by fast growth, but also by "impulsiveness," that is, a sudden increase in the time-derivative of the growth rate (the so-called "trigger" problem). A theoretical research program is proposed on collisionless reconnection based on the Hall MHD model, using analytical methods as well as massively parallel codes that employ adaptive mesh refinement (AMR). The tasks proposed include a thorough investigation of the trigger problem, the structure of and the flows involved in the reconnection site(s), the effects of foot point shear on multiple arcades, and fully self-consistent Hall MHD simulations of flux rope models. The results of this research program are expected to enhance and complement the existing framework of coronal reconnection theory by including important collisionless processes that have already proved to be valuable in studies of laboratory and other space plasmas.The proposed effort integrates teaching and research at all levels, from high school to graduate studies. It includes: (1) participation in a summer institute at the University of New Hampshire called "Project SMART" (Science and Mathematics Achievement through Research Training), which provides exciting research experiences for local high school students; (2) co-sponsorship of a pilot Summer Internship in the "Research And Discover" (RAD) program for undergraduates in their junior and senior years; and (3) training a graduate student and mentoring a junior research faculty member. This work will also promote interdisciplinary interactions with a broad community of space plasma and laboratory physicists.