Unraveling the Velocity Evolution of High‐Speed Jets in Earth's Magnetosheath

Abstract High‐speed jets (HSJs) are transient phenomena characterized by significant enhancement of magnetosheath dynamic pressure. They are capable of traversing the magnetosheath and impinging upon the magnetopause, triggering a diverse array of geoeffects. However, the evolution of HSJs during their propagation from the bow shock to the magnetopause still remains unclear. Leveraging multi‐satellite data from MMS (2015–2023), THEMIS (2008–2023), and Cluster (2001–2020), we have compiled a comprehensive data set of nearly 40,000 HSJs to statistically study the velocity evolution of HSJs from the bow shock to the magnetopause for the first time. It is shown that the occurrence rate of HSJs depends on their relative positions within the magnetosheath, peaking in the middle region. Typically, in the GSE coordinates, as HSJs penetrate into the magnetosheath, the component gradually decreases until they reach the magnetopause, where HSJs are deflected and their and components are enhanced. Meanwhile, some HSJs are redirected backwards owing to rebounce of the magnetopause, resulting in the sunward flow. Notably, the velocity‐direction distribution of HSJs in the ‐plane is largely isotropic overall yet exhibits a subtle dusk‐favored asymmetry. This distribution aligns with the background flow throughout the evolution of HSJs, a feature that is consistent with the outcomes of 3‐D global simulations. This strong consistency implies HSJ transverse velocity tends to align with the ambient magnetosheath flow even early in their evolution. Our study provides some new insights in better understanding the evolution of HSJs within the magnetosheaths of Earth and other planets.