GEM: Can Global Magnetohydrodynamics (MHD) Codes Model Magnetic Reconnection in the High Lundquist Number Limit?

GEM：全球磁流体动力学 (MHD) 代码能否模拟高伦德奎斯特数限制下的磁重联？

• 批准号: 0802727
• 负责人: Amitava Bhattacharjee
• 金额: --
• 依托单位: University of New Hampshire
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-06-01 至 2011-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0802727&HistoricalAwards=false
• 关键词: GEM; Global; Magnetohydrodynamics; MHD; Codes

One of the long term goals of NSF's Geospace Environment Modeling (GEM) program is the construction of a Geospace General Circulation Model (GGCM), a comprehensive and predictively powerful numerical model of geospace. One popular approach to GGCM design consists of using a global magnetohydrodynamics (MHD) code as the computational "spine" of the model, including additional physics where needed (e.g., by coupling the global MHD code to a kinetic model of the ring current). Despite its successes, however, the MHD spine approach suffers from a number of well known deficiencies, the most serious of which is the inability of MHD to model the kinetic processes which are thought to play an essential role in magnetic reconnection. In order to achieve fast reconnection in global MHD codes, modelers usually resort to ad hoc localized, numerical or current dependent resistivities, and it is not clear how the reconnection dynamics depends on the resistivity or whether the results even converge as the numerical resistivity decreases with increasing grid resolution. Despite the fundamental role reconnection plays in driving magnetospheric dynamics, the reconnection time scale problem hasn't received much attention by global MHD modelers. The issue is often dismissed with appeals to the "Axford Conjecture" which states that the reconnection rate is determined by external boundary conditions, with the diffusion region adjusting to accommodate these conditions. There is now compelling evidence that the Axford Conjecture does not apply at the dayside magnetopause and it has never been systematically tested in global MHD simulations of magnetotail reconnection. This research program will attack the problem of how the generation and dynamics of global MHD storms and substorms depend on the resistivity. It will address the following issues: 1) How does the physics of magnetopause reconnection in global MHD simulations depend on the resistivity model during strong, sustained southward IMF solar wind driving (i.e., conditions which typically produce strong storms)? 2) Can global MHD simulations produce storms and substorms in the high Lundquist number limit? 3) How does the storm-substorm relationship depend on the resistivity model? The research will combine a straightforward parameter study with event study model/data comparisons: we will run the OpenGGCM global MHD code with coronal mass ejection solar wind boundary conditions, varying both the solar wind parameters as well as the resistivity model. The primary new result will be a definitive test of the Axford Conjecture in the context of magnetic storm and substorm dynamics.The analysis of the storm/substorm relationship will require the development of a Vlasov test-particle code (which will solve the Vlasov equation in prescribed electric and magnetic fields output from OpenGGCM). This code will be developed primarily by a graduate student at the University of New Hampshire (UNH).

NSF 地球空间环境建模 (GEM) 计划的长期目标之一是构建地球空间大气环流模型 (GGCM)，这是一个全面且具有强大预测能力的地球空间数值模型。 GGCM 设计的一种流行方法包括使用全局磁流体动力学 (MHD) 代码作为模型的计算“脊柱”，包括需要的附加物理场（例如，通过将全局 MHD 代码耦合到环电流的动力学模型）。然而，尽管 MHD 脊柱方法取得了成功，但它仍然存在许多众所周知的缺陷，其中最严重的是 MHD 无法模拟动力学过程，而动力学过程被认为在磁重联中发挥着重要作用。为了在全球 MHD 代码中实现快速重联，建模者通常采用临时的局部、数值或电流相关电阻率，并且不清楚重联动力学如何取决于电阻率，也不清楚结果是否会随着数值电阻率的减小而收敛。增加网格分辨率。尽管重联在驱动磁层动力学方面发挥着重要作用，但重联时间尺度问题并没有受到全球 MHD 建模者的太多关注。这个问题经常被诉诸“阿克斯福德猜想”而被驳回，该猜想指出，重连率是由外部边界条件决定的，扩散区域会进行调整以适应这些条件。现在有令人信服的证据表明，阿克斯福德猜想不适用于白天磁层顶，并且从未在磁尾重联的全球 MHD 模拟中进行过系统测试。该研究计划将解决全球 MHD 风暴和亚暴的产生和动态如何取决于电阻率的问题。它将解决以下问题：1）全球 MHD 模拟中磁层顶重联的物理原理如何依赖于强烈、持续南向 IMF 太阳风驱动（即通常产生强风暴的条件）期间的电阻率模型？ 2) 全球 MHD 模拟能否产生高伦德奎斯特数限制的风暴和亚暴？ 3）风暴-亚暴关系如何取决于电阻率模型？该研究将把简单的参数研究与事件研究模型/数据比较结合起来：我们将在日冕物质抛射太阳风边界条件下运行 OpenGGCM 全球 MHD 代码，同时改变太阳风参数和电阻率模型。主要的新结果将是在磁暴和亚暴动力学背景下对阿克斯福德猜想的明确测试。风暴/亚暴关系的分析将需要开发弗拉索夫测试粒子代码（它将求解OpenGGCM 输出的指定电场和磁场）。该代码将主要由新罕布什尔大学 (UNH) 的一名研究生开发。