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

Geospace General Circulation Model （GGCM）是美国国家科学基金会地球空间环境模拟（GEM）项目的长期目标之一，是建立一个全面的、具有强大预测能力的地球空间数值模型。一种流行的GGCM设计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”，包括需要的额外物理（例如，通过将全局MHD代码耦合到环电流的动力学模型）。然而，尽管取得了成功，MHD脊柱入路仍存在许多众所周知的缺陷，其中最严重的是MHD无法模拟被认为在磁重联中起重要作用的动力学过程。为了在全局MHD代码中实现快速重联，建模者通常采用特别的局部、数值或电流相关的电阻率，并且不清楚重联动力学如何依赖于电阻率，或者当数值电阻率随着网格分辨率的增加而降低时，结果是否会收敛。尽管重联在驱动磁层动力学中起着重要的作用，但重联的时间尺度问题并没有受到全球MHD建模者的重视。这个问题经常被“阿克斯福德猜想”所驳回，该猜想指出，重联率是由外部边界条件决定的，扩散区域根据这些条件进行调整。现在有令人信服的证据表明，阿克斯福德猜想不适用于日侧磁层顶，而且它从未在全球磁尾重连的MHD模拟中得到系统测试。本研究计划将研究全球磁暴和亚暴的产生和动力学如何依赖于电阻率的问题。它将解决以下问题：1)全球MHD模拟中磁层顶重联的物理特性如何依赖于强的、持续的南向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模拟能否在高伦德奎斯特数极限下产生风暴和亚风暴？3)暴雨-次暴雨关系如何依赖于电阻率模型？该研究将直接的参数研究与事件研究模型/数据比较相结合：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最终检验。风暴/亚风暴关系的分析将需要开发一个弗拉索夫测试粒子代码（它将在OpenGGCM输出的规定电场和磁场中求解弗拉索夫方程）。该代码将主要由新罕布什尔大学（UNH）的一名研究生开发。

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).