Collisionless shocks can be nonstationary with periodic reformation shown in many simulation results, but direct observations are still tenuous and difficult to conclusively interpret. In this study, using Magnetospheric Multiscale (MMS) observations, we report direct observational evidence of Earth's oblique bow shock reformation driven by the foreshock Ultralow‐Frequency (ULF) waves. When the four MMS spacecraft were in a string‐of‐pearls formation roughly along the bow shock normal, they observed that when each period of foreshock ULF waves encountered the bow shock, a new shock ramp formed. Meanwhile, in the magnetosheath, the old bow shock's remnants were observed periodically convecting downstream. We propose that the reformation mechanism of the oblique bow shock is the variation of the upstream conditions by the periodic ULF waves as they encounter the bow shock. We also examine the nature of reflected ions during the reformation process.

中文翻译：

---

地球前倾超低频波对地球斜向弓形冲击的磁层多尺度观测

无碰撞冲击在许多模拟结果中显示为周期性重定可能是不稳定的，但是直接观察仍然是微不足道的，并且难以最终解释。在这项研究中，利用磁层多尺度（MMS）观测，我们报告了由前震超低频（ULF）波驱动的地球斜向弓形激波再形成的直接观测证据。当四个MMS航天器大致沿船首冲击法线成珍珠状排列时，他们观察到，每一次前震ULF波遇到船首冲击时，都会形成一个新的冲击坡道。同时，在磁石场中，定期观察到旧弓激波的残余物在下游对流。我们提出，斜向弓形激波的重整机制是上游ULF波在遇到弓形激波时上游条件的变化。我们还检查了重整过程中反射离子的性质。