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

美国国家科学基金会的地球空间环境建模（GEM）计划的长期目标之一是建立一个地球空间环流模型（GGCM），一个全面的和预测功能强大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（例如，通过将全局MHD代码耦合到环电流的动力学模型）。尽管它的成功，但是，MHD脊柱的方法遭受了一些众所周知的缺陷，其中最严重的是MHD的动力学过程被认为是在磁重联中发挥重要作用的模型的能力。为了实现快速重联在全球MHD代码，建模者通常求助于特设本地化，数值或电流依赖的电阻率，它是不清楚的重联动力学如何取决于电阻率或结果是否收敛，甚至作为数值电阻率随着网格分辨率的增加而降低。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还没有得到全球MHD建模者的重视。这个问题往往被驳回上诉的“阿克斯福德猜想”，其中指出，重联率是由外部边界条件，与扩散区调整，以适应这些条件。现在有令人信服的证据表明，阿克斯福德猜想并不适用于昼侧磁层顶，它从来没有在全球磁流体动力学模拟磁尾重联系统测试。该研究计划将解决全球MHD风暴和亚暴的生成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，磁层顶重连的物理学如何依赖于强的、持续的向南IMF太阳风驱动期间的电阻率模型（即，通常会产生强烈风暴的条件）。2)全球MHD模拟能产生高Lundquist数极限的风暴和亚暴吗？3)风暴-亚暴关系如何依赖于电阻率模型？该研究将结合联合收割机一个简单的参数研究与事件研究模型/数据比较：我们将运行OpenGGCM全球MHD代码与日冕物质抛射太阳风边界条件，改变太阳风参数以及电阻率模型。主要的新结果将是在磁暴和亚暴动力学的背景下对阿克斯福德猜想的确定性测试。风暴/亚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输出的指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州（UNH）大学的研究生开发。

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).