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

NSF地球空间环境模型(GEM)计划的长期目标之一是建立地球空间通用环流模式(GGCM)，这是一个全面的、预测功能强大的地球空间数值模式。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“脊梁”，包括必要时的附加物理(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MHD脊椎方法取得了成功，但它也存在一些众所周知的缺陷，其中最严重的是MHD无法对被认为在磁重联中发挥关键作用的动力学过程进行建模。为了在全球MHD程序中实现快速重连，建模人员通常采用特别的局部、数值或电流相关的电阻率，但重连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减小时，结果是否会收敛，目前还不清楚。尽管重联在推动磁层动力学方面发挥了重要作用，但重联时间尺度问题并没有得到全球MHD模型人员的太多关注。这个问题常常被抛诸脑后，求助于“埃克斯福德猜想”，该猜想指出，重联率由外部边界条件决定，扩散区域进行调整以适应这些条件。现在有令人信服的证据表明，埃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联的MHD模拟中得到系统测试。这项研究计划将解决全球MHD风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)在全球MHD模拟中，磁层顶重联的物理过程如何依赖于在强的、持续的IMF太阳风向南驱动(即，典型地产生强风暴的条件)期间的电阻率模型？2)全球MHD模拟能否产生高Lundquist数限制的风暴和亚暴？3)风暴-亚暴关系如何依赖于电阻率模型？这项研究将结合直接的参数研究和事件研究模型/数据比较：我们将运行具有日冕物质抛射太阳风边界条件的OpenGGCM全球MHD程序，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的背景下对埃克斯福德猜想的最终检验。对风暴/亚暴关系的分析将需要开发Vlasov测试粒子代码(该代码将在OpenGGCM输出的指定电场和磁场中求解Vlasov方程)。该代码将主要由新汉普郡大学(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).