THEMIS Observations of Magnetosheath‐Origin Foreshock Ions

Abstract

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.

Magnetosheath Ion Field‐Aligned Asymmetry and Implications for Ion Leakage to the Foreshock

“…In the magnetosheath near the bow shock (Figure 2b), diffuse, suprathermal ions (dark blue) transported from the foreshock are seen. The thermal ions (red to green), on the other hand, show very clear asymmetry toward the parallel direction, consistent with observations by Liu et al (2024). Such an asymmetry toward the parallel direction could enhance magnetosheath ion leakage and possibly contribute to the field-aligned foreshock ions observed in Figure 2a.…”

“…In the statistical study, we show the weak correlation between the magnetosheath ion heat flux and the foreshock ion quantities. Because the field-aligned heat flux is nearly always toward the upstream direction which persists across the magnetosheath, we believe that the magnetosheath ion asymmetry is the cause based on the spatial relationship and supported by the case studies (Liu et al, 2024). However, there is also a possibility that the foreshock ions are the cause or at least partly contribute, for example, through providing a seed population for the magnetosheath heating (such as due to bow shock reformation (Liu et al, 2021)).…”

“…In the foreshock, the heat flux was extremely large (out of panel) due to the counter streaming between the foreshock ions and solar wind ions. This suggests that the directional field-aligned asymmetry of magnetosheath ions observed by Liu et al (2024) is not only close to the bow shock; it persists everywhere in the magnetosheath.…”

“…Figure S6 in Supporting Information S1 shows that faster solar wind speed is not correlated with larger speed deviations and thus cannot be the origin of the observed trend in Figure 8a. Case studies in Liu et al (2024) show that stronger field-aligned asymmetry of magnetosheath ions can result in larger foreshock ion density. Consistent with these case studies, Figure 6b shows a weak trend that larger heat flux favors larger foreshock ion density normalized to the solar wind density with Spearman's coefficient of 0.24.…”

“…In simulations and models, the magnetosheath ions are commonly considered to be isotropic with respect to the field-aligned direction. Case studies in the accompanying paper (Liu et al, 2024) demonstrate that the magnetosheath ions near the bow shock sometimes exhibit strong field-aligned asymmetry toward the upstream direction, which is likely responsible for a significant enhancement of magnetosheath leakage. Thus, the role of magnetosheath leakage in populating the foreshock region may have been previously underestimated.…”