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Abstract In the ion foreshock, there are many foreshock transients driven by back streaming foreshock ions. When the foreshock ions interact with tangential discontinuities (TDs), hot flow anomalies form if the foreshock ion‐driven current decreases field strength at TDs, but the opposite situation has been paid little attention. Using 2.5‐D local hybrid simulations, we show that a compressional boundary with enhanced field strength and density can form. We examine how the foreshock ions interact with TDs under various magnetic field geometries to drive currents that lead to compressional boundaries. The current driven by the foreshock ions should peak on its initial side of a TD so that the enhanced field strength at the TD in turn increases this current by keeping more foreshock ions on their initial side. Which side the current peaks can be determined by whether the foreshock ions initially cross the TD and/or how their velocity is projected into the local perpendicular direction. Additionally, the foreshock ion‐driven currents from two sides could compete, and whether a compressional boundary forms is determined by the net current profile. Because such compressive structures in the foreshock can drive magneto sheath jets and cause many geoeffects, it is necessary to fully understand their formation.
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Observations of Compressional Structures Driven by Interaction Between Foreshock Ions and Discontinuities
Abstract The ion foreshock is very dynamic, characterized by various transient structures that can perturb the bow shock and influence the magnetosphere‐ionosphere system. One important driver of foreshock transients is solar wind directional discontinuities (DDs) that demagnetize foreshock ions leading to a local current. If this current decreases the field strength at the DD, a hot flow anomaly (HFA) can form. Recent hybrid simulations found that when the current increases the field strength at the DD, a compressional structure forms with enhanced density and field strength opposite to HFAs. Using MMS and THEMIS observations, we confirm this situation. We demonstrate that the current geometry driven by the foreshock ions plays a critical role in the formation. The initial gyrophase of foreshock ions, due to their specular reflection, determines whether they can cross the DD. When many of the foreshock ions cannot cross the DD and the local current they drive increases the field strength at the DD, the enhanced field strength inhibits more foreshock ions from crossing the DD, further enhancing the local current. This feedback loop promotes the growth of the compressional structure. Such foreshock ion‐driven compressional structures can result in dynamic pressure enhancements in the magnetosheath, leading to magnetosheath jets. Our study enables prediction of the location and formation probability of such compressional structures and their potential geoeffectiveness.
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- Award ID(s):
- 2247760
- PAR ID:
- 10542499
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 9
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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