The Early-Phase Growth of ULF Waves in the Ion Foreshock Observed in a Hybrid-Vlasov Simulation

IF 2.6 2区 地球科学 Q2 ASTRONOMY & ASTROPHYSICS
Kun Zhang, Seth Dorfman, Lucile Turc, Urs Ganse, Chen Shi, Hongyang Zhou, Minna Palmroth
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Abstract

Large-amplitude ultra-low frequency (ULF) waves in Earth's ion foreshock play a crucial role in the dayside dynamics and solar wind-magnetosphere coupling. This study uses global hybrid-Vlasov simulation results from Vlasiator to investigate the detailed physical processes in the early growth phase of the foreshock ULF waves. Using both spatial and temporal information, the wave phase speed is determined and used to track a specific phase front as the wave evolves. The space-time evolution of the foreshock waves and the backstreaming ions responsible for the wave growth is analyzed and presented in the wave frame for the first time. We employ a state-of-the-art linear dispersion solver, LEOPARD, to solve the wave dispersion relations using the ion distributions and compare the theoretical predictions with the measured wave phase speed and growth rate. The measured phase speed in the spacecraft (or stationary) frame is unexpectedly high at the initial growth stage but later decreases to the predicted level and exhibits an increasing trend over time that aligns with theoretical expectations. The measured and predicted growth rates share the same decreasing trend over time, but the predicted values are consistently lower than the measured growth rate by ${\sim} $ 25%. The comparison suggests that the foreshock waves in the Vlasiator simulation are likely generated through the ion-ion right-hand resonant instability, but there are discrepancies with linear theory that are not explained yet and require further investigation.

混合vlasov模拟中观察到的离子前震中ULF波的早期生长
地球离子前震中的大振幅超低频波在日侧动力学和太阳风-磁层耦合中起着至关重要的作用。本研究利用Vlasiator的全球混合vlasov模拟结果,研究了前震ULF波早期生长阶段的详细物理过程。利用空间和时间信息,确定波的相位速度并用于跟踪波演变时的特定相位前。本文首次在波框中分析了前激波的时空演化和引起波增长的回流离子。我们使用最先进的线性色散求解器LEOPARD来求解离子分布的色散关系,并将理论预测与测量的波相速度和增长率进行比较。航天器(或静止)框架中测量到的相速度在初始增长阶段出乎意料地高,但随后下降到预测水平,并随着时间的推移呈现出与理论预期一致的增加趋势。测量和预测的增长率随时间的推移呈相同的下降趋势,但预测值始终比测量的增长率低~ ${\sim} $ 25%。比较表明,Vlasiator模拟中的前激波可能是由离子-离子右手共振不稳定性产生的,但与线性理论存在差异,尚未得到解释,需要进一步研究。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Journal of Geophysical Research: Space Physics
Journal of Geophysical Research: Space Physics Earth and Planetary Sciences-Geophysics
CiteScore
5.30
自引率
35.70%
发文量
570
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