Abstract. The foreshock is a region of space upstream of the Earth's bow shock extending along the interplanetary magnetic field (IMF). It is permeated by shock-reflected ions and electrons, low-frequency waves, and various plasma transients. We investigate the extent of the He2+ foreshock using Vlasiator, a global hybrid-Vlasov simulation. We perform the first numerical global survey of the helium foreshock and interpret some historical foreshock observations in a global context. The foreshock edge is populated by both proton and helium field-aligned beams, with the proton foreshock extending slightly further into the solar wind than the helium foreshock and both extending well beyond the ultra-low frequency (ULF) wave foreshock. We compare our simulation results with Magnetosphere Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA) measurements, showing how the gradient of suprathermal ion densities at the foreshock crossing can vary between events. Our analysis suggests that the IMF cone angle and the associated shock obliquity gradient can play a role in explaining this differing behaviour. We also investigate wave–ion interactions with wavelet analysis and show that the dynamics and heating of He2+ must result from proton-driven ULF waves. Enhancements in ion agyrotropy are found in relation to, for example, the ion foreshock boundary, the ULF foreshock boundary, and specular reflection of ions at the bow shock. We show that specular reflection can describe many of the foreshock ion velocity distribution function (VDF) enhancements. Wave–wave interactions deep in the foreshock cause de-coherence of wavefronts, allowing He2+ to be scattered less than protons.

中文翻译：

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地球前震中的氦气：全球 Vlasiator 调查

摘要。前震是沿行星际磁场 (IMF) 延伸的地球弓形激波上游的空间区域。它被冲击反射的离子和电子、低频波和各种等离子体瞬变所渗透。我们使用 Vlasiator（一种全局混合 Vlasov 模拟）研究 He2+ 前震的范围。我们对氦前震进行了第一次全球数值调查，并在全球范围内解释了一些历史前震观测。前震边缘由质子和氦场对齐的光束填充，质子前震比氦前震稍微更深入地延伸到太阳风中，并且都远远超出了超低频 (ULF) 波前震。我们将模拟结果与磁层多尺度 (MMS) 热等离子体成分分析仪 (HPCA) 测量结果进行比较，显示前震交叉处的超热离子密度梯度如何在不同事件之间变化。我们的分析表明，IMF 锥角和相关的激波倾斜梯度可以在解释这种不同的行为方面发挥作用。我们还使用小波分析研究了波-离子相互作用，并表明 He2+ 的动力学和加热必须由质子驱动的 ULF 波引起。发现离子同向性的增强与例如离子前激波边界、ULF 前激波边界和弓形激波处离子的镜面反射有关。我们表明镜面反射可以描述许多前震离子速度分布函数 (VDF) 增强。