Near-Earth solar-wind conditions, including disturbances generated by coronal mass ejections (CMEs), are routinely forecast using three-dimensional, numerical magnetohydrodynamic (MHD) models of the heliosphere. The resulting forecast errors are largely the result of uncertainty in the near-Sun boundary conditions, rather than heliospheric model physics or numerics. Thus ensembles of heliospheric model runs with perturbed initial conditions are used to estimate forecast uncertainty. MHD heliospheric models are relatively cheap in computational terms, requiring tens of minutes to an hour to simulate CME propagation from the Sun to Earth. Thus such ensembles can be run operationally. However, ensemble size is typically limited to 10 1 $10^{1}$ to 10 2 $10^{2}$ members, which may be inadequate to sample the relevant high-dimensional parameter space. Here, we describe a simplified solar-wind model that can estimate CME arrival time in approximately 0.01 seconds on a modest desktop computer and thus enables significantly larger ensembles. It is a one-dimensional, incompressible, hydrodynamic model, which has previously been used for the steady-state solar wind, but it is here used in time-dependent form. This approach is shown to adequately emulate the MHD solutions to the same boundary conditions for both steady-state solar wind and CME-like disturbances. We suggest it could serve as a “surrogate” model for the full three-dimensional MHD models. For example, ensembles of 10 5 $10^{5}$ to 10 6 $10^{6}$ members can be used to identify regions of parameter space for more detailed investigation by the MHD models. Similarly, the simplicity of the model means it can be rewritten as an adjoint model, enabling variational data assimilation with MHD models without the need to alter their code. The model code is available as an Open Source download in the Python language.

中文翻译：

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用于替代三维数值磁流体动力学模拟的计算效率高、时间相关的太阳风模型

近地太阳风条件，包括日冕物质抛射 (CME) 产生的扰动，通常使用日球层的三维磁流体动力学 (MHD) 数值模型进行预测。由此产生的预测误差主要是近太阳边界条件的不确定性造成的，而不是日光层模型物理或数值的结果。因此，在初始条件受到扰动的情况下运行的日光层模型集合用于估计预报的不确定性。MHD 日光层模型在计算方面相对便宜，需要几十分钟到一个小时来模拟从太阳到地球的 CME 传播。因此，这样的集合可以在操作上运行。但是，集成大小通常限制为 10 1 $10^{1}$ 到 10 2 $10^{2}$ 成员，这可能不足以对相关的高维参数空间进行采样。在这里，我们描述了一个简化的太阳风模型，它可以在一台普通的台式计算机上在大约 0.01 秒内估计 CME 的到达时间，从而实现更大的集合。它是一个一维、不可压缩的流体动力学模型，以前曾用于稳态太阳风，但这里以时间相关形式使用。这种方法被证明可以充分模拟稳态太阳风和类 CME 扰动的相同边界条件的 MHD 解。我们建议它可以作为全三维 MHD 模型的“替代”模型。例如，10 5 $10^{5}$ 到 10 6 $10^{6}$ 成员的集合可用于识别参数空间的区域，以便 MHD 模型进行更详细的调查。同样，模型的简单性意味着它可以重写为伴随模型，使用 MHD 模型实现变分数据同化，而无需更改其代码。模型代码以 Python 语言的开源下载形式提供。