We use global and local hybrid (kinetic ions and fluid electrons) simulations to investigate the conditions under which foreshock bubbles (FBs) form and how their topology changes with solar wind conditions. FBs form as a result of the interaction between solar wind discontinuities and backstreaming ion beams in the foreshock. They consist of an outer shock and its associated sheath plasma and a low density high temperature core with low magnetic field strength. The structure of FBs is determined by the angle between the interplanetary magnetic field and the normal to the solar wind discontinuity. We show that interaction of rotational discontinuities with the foreshock during small angles between the interplanetary magnetic field and discontinuity normal results in the formation of a nearly spherical bubble with a radius that scales with the width of the foreshock. As this angle increases, FBs become more elongated and eventually become nearly planar structures with dimensions that scale with the length of the foreshock. Despite this transformation, the signatures of FBs in spacecraft time series data remain the same in agreement with the observations. Global simulation results show that FBs form when the solar wind flow speed corresponds to high or intermediate Alfvén Mach numbers (approximately >7 M_A). In general, this is tied to the relative speed between the solar wind and ion beams and drop in density of the backstreaming ions.

中文翻译：

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前震气泡的形成和拓扑

我们使用全局和局部混合（运动离子和流体电子）模拟研究前兆气泡（FBs）形成的条件以及它们的拓扑如何随太阳风条件而变化。由于前风中太阳风的不连续性和回流离子束之间的相互作用而形成FB。它们由外激波及其相关的鞘等离子体和磁场强度低的低密度高温铁心组成。FBs的结构由行星际磁场与太阳风不连续法线之间的夹角确定。我们表明，在行星际磁场与不连续法线之间的小角度期间，旋转不连续与前震的相互作用会导致形成一个近似球形的气泡，其半径与前震的宽度成比例。随着该角度的增加，FB变得更加细长，并最终变成几乎平面的结构，其尺寸随前躯的长度成比例。尽管进行了这种转换，但航天器时间序列数据中的FB签名与观察一致。整体模拟结果表明，当太阳风的流速对应于高或中等的Alfvén马赫数（大约> 7 M）时，便形成了FB。FB变得更加细长，并最终变成近乎平面的结构，其尺寸随前震的长度成比例。尽管进行了这种转换，但航天器时间序列数据中的FB签名与观察一致。整体模拟结果表明，当太阳风的流速对应于高或中等的Alfvén马赫数（大约> 7 M）时，便形成了FB。FB变得更细长，最终变成几乎平面的结构，其尺寸随前震的长度成比例。尽管进行了这种转换，但航天器时间序列数据中的FB签名仍然与观测一致。整体模拟结果表明，当太阳风的流速对应于高或中等的Alfvén马赫数（大约> 7 M）时，便形成了FB。_A）。通常，这与太阳风和离子束之间的相对速度以及回流离子的密度下降有关。