Abstract. Bounded by the bow shock and the magnetopause, the magnetosheath forms the interface between solar wind and magnetospheric plasmas and regulates solar wind-magnetosphere coupling. Previous works have revealed pronounced dawn-dusk asymmetries in the magnetosheath properties. The dependence of these asymmetries on the upstream parameters remains however largely unknown. One of the main sources of these asymmetries is the bow shock configuration, which is typically quasi-parallel on the dawn side and quasi-perpendicular on the dusk side of the terrestrial magnetosheath because of the Parker spiral orientation of the interplanetary magnetic field (IMF) at Earth. Most of these previous studies rely on collections of spacecraft measurements associated with a wide range of upstream conditions which are processed in order to obtain average values of the magnetosheath parameters. In this work, we use a different approach and quantify the magnetosheath asymmetries in global hybrid-Vlasov simulations performed with the Vlasiator model. We concentrate on three parameters: the magnetic field strength, the plasma density and the flow velocity. We find that the Vlasiator model reproduces accurately the polarity of the asymmetries, but that their level tends to be higher than in spacecraft measurements, probably because the magnetosheath parameters are obtained from a single set of upstream conditions in the simulation, making the asymmetries more prominent. We investigate how the asymmetries change when the angle between the IMF and the Sun-Earth line is reduced and when the Alfven Mach number decreases. We find that a more radial IMF results in a stronger magnetic field asymmetry and a larger variability of the magnetosheath density. In contrast, a lower Alfven Mach number leads to a reduced magnetic field asymmetry and a decrease in the variability of the magnetosheath density and velocity, the latter likely due to weaker foreshock processes. Our results highlight the strong impact of the foreshock on global magnetosheath properties, in particular on the magnetosheath density, which is extremely sensitive to transient foreshock processes.

中文翻译：

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地球日侧磁鞘的不对称性：全球混合 Vlasov 模拟的结果

摘要。受弓形激波和磁层顶的约束，磁鞘形成太阳风和磁层等离子体之间的界面，并调节太阳风-磁层耦合。以前的工作揭示了磁鞘特性中明显的黎明-黄昏不对称性。然而，这些不对称性对上游参数的依赖性在很大程度上仍然未知。这些不对称性的主要来源之一是弓形激波配置，由于行星际磁场 (IMF) 的帕克螺旋方向，它通常在地球磁鞘的黎明侧准平行而在黄昏侧准垂直在地球。大多数这些先前的研究依赖于与各种上游条件相关的航天器测量的集合，这些测量被处理以获得磁鞘参数的平均值。在这项工作中，我们使用不同的方法并量化使用 Vlasiator 模型执行的全局混合 Vlasov 模拟中的磁鞘不对称性。我们专注于三个参数：磁场强度、等离子体密度和流速。我们发现 Vlasiator 模型准确地再现了不对称性的极性，但它们的水平往往高于航天器测量中的水平，这可能是因为磁鞘参数是从模拟中的一组上游条件中获得的，使得不对称性更加突出. 我们研究了当 IMF 和太阳-地球线之间的角度减小以及阿尔文马赫数减小时不对称性如何变化。我们发现更径向的 IMF 会导致更强的磁场不对称性和更大的磁鞘密度可变性。相比之下，较低的阿尔文马赫数会导致磁场不对称性降低以及磁鞘密度和速度的可变性降低，后者可能是由于较弱的前震过程。我们的结果强调了前震对全球磁鞘特性的强烈影响，特别是对磁鞘密度的强烈影响，磁鞘密度对瞬态前震过程极为敏感。我们发现更径向的 IMF 会导致更强的磁场不对称性和更大的磁鞘密度可变性。相比之下，较低的阿尔文马赫数会导致磁场不对称性降低以及磁鞘密度和速度的可变性降低，后者可能是由于较弱的前震过程。我们的结果强调了前震对全球磁鞘特性的强烈影响，特别是对磁鞘密度的强烈影响，磁鞘密度对瞬态前震过程极为敏感。我们发现更径向的 IMF 会导致更强的磁场不对称性和更大的磁鞘密度可变性。相比之下，较低的阿尔文马赫数会导致磁场不对称性降低以及磁鞘密度和速度的可变性降低，后者可能是由于较弱的前震过程。我们的结果强调了前震对全球磁鞘特性的强烈影响，特别是对磁鞘密度的强烈影响，磁鞘密度对瞬态前震过程极为敏感。