GEM: Model of the Low-Latitude Boundary Layer

本文提出了一种低纬边界层（LLBL）的数值模型，该边界层位于地球磁层的赤道区域，靠近其外边界，磁层顶，并通过场向电流与日侧极光电离层耦合。该模型旨在为构建具有预测能力的全球地球空间环流模型（GGCM）提供基础。将开发两个版本的LLBL模型。首先，将建立一个相对复杂的稳定等离子体流、磁场和电流的自洽模型，并通过与观测结果的比较对其参数进行约束。在该模型中，偏微分方程在边界层近似下进行数值求解。根据从该模型中获得的结果和经验，将开发一个基于常微分方程的更简单版本，与现有模型或作为GEM计划一部分正在开发的模型并行使用，并与之耦合，该模型的一端是磁层内部对流，另一端是外部太阳风等离子体流。这个更简单的边界层模型的基本理论要素是：(i)在边界层厚度上积分的流动方向动量守恒方程和（ii）边界层变化的多项式（或其他参数）表示，其自由参数将允许模型适应内部循环模型和外部流动模型提供的边界条件，可能是迭代的。预期数值速度较快的较简单模型将通过与自洽边界层代码（计算速度太慢，无法用于GGCM）以及观测到的电离层和赤道层特征进行比较来作为基准。

It is proposed to develop a numerical model of the low- latitude boundary layer (LLBL), located in the equatorial region of the Earth's magnetoshpere adjacent to its outer boundary, the magnetopause, and the coupling of that layer to the dayside auroral ionosphere via field-aligned currents. The model is intended as a building block in the construction of a global geospace circulation model (GGCM) with predictive capability. Two versions of the LLBL model will be developed. First, a relatively sophisticated self-consistent model of steady plasma flow, magnetic fields and currents will be completed and its parameters constrained by comparison with observations. In this model, partial differential equations are solved numerically in the boundary layer approximation. On the basis of results and experience gained from this model a simpler version, based on ordinary differential equations, will be developed for use in parallel with, and coupled to, existing models, or models under development as part of the GEM program, of internal magnetospheric convection on one side of the layer and external solar-wind plasma flow on its other side. This simpler version of the boundary-layer model will have as its basic theoretical elements: (i) the equation of momentum conservation in the flow direction integrated over the thickness of the boundary layer and (ii) polynomial (or other parametric) representations of variations across the boundary layer, with free parameters that will allow the model to accommodate, perhaps iteratively, boundary conditions provided by an internal circulation model and an external flow model. The simpler model which is expected to be numerically fast will be benchmarked by comparison w ith the self-consistent boundary layer code (which is computationally too slow to be used in a GGCM), as well as with observed ionospheric and equatorial signatures of the layer.