THEMIS Observations of Magnetosheath‐Origin Foreshock Ions.

The ion foreshock, filled with backstreaming foreshock ions, is very dynamic with many transient structures that disturb the bow shock and the magnetosphere‐ionosphere system. It has been shown that foreshock ions can be generated through either solar wind reflection at the bow shock or leakage from the magnetosheath. While solar wind reflection is widely believed to be the dominant generation process, our investigation using Time History of Events and Macroscale Interactions during Substorms mission observations reveals that the relative importance of magnetosheath leakage has been underestimated. We show from case studies that when the magnetosheath ions exhibit field‐aligned anisotropy, a large fraction of them attains sufficient field‐aligned speed to escape upstream, resulting in very high foreshock ion density. The observed foreshock ion density, velocity, phase space density, and distribution function shape are consistent with such an escape or leakage process. Our results suggest that magnetosheath leakage could be a significant contributor to the formation of the ion foreshock. Further characterization of the magnetosheath leakage process is a critical step toward building predictive models of the ion foreshock, a necessary step to better forecast foreshock‐driven space weather effects. Key Points: Using observations of bow shock crossings by Time History of Events and Macroscale Interactions during Substorms mission, we investigate the magnetosheath‐origin foreshock ionsForeshock ion density, velocity, phase space density, and distribution shape are consistent with non‐adiabatic magnetosheath leakageMagnetosheath ion field‐aligned anisotropy could cause leakage to become a dominant source of foreshock ions [ABSTRACT FROM AUTHOR]