Dayside Dispersive Field‐Aligned Warm Ion Flux Enhancements (DFAWIFEs) Driven by Transient Magnetopause Intrusions

Abstract We present an event of Dispersive Field‐Aligned Warm Ion (∼10–1,000 eV) Flux Enhancements (DFAWIFEs) observed by multiple satellites in the dayside equatorial plasma sheet ( r > ∼10 R E ) driven by transient (∼5–10 min) magnetopause inward movements (intrusion). The dispersion, which appeared a few minutes after a magnetopause intrusion, exhibited a feature of decreasing energy over a timescale of several tens of minutes. Comparing the observed dispersion profiles with predictions indicates that these warm ions were dominantly H + ions coming from a warm ion source in the topside ionosphere (∼1 R E altitude). DFAWIFEs were observed at locations closer to the magnetopause where shear Alfvén waves generated by the intrusion were more intense, suggesting a connection between the wave powers and the warm ion source. Additionally, preexisting cold (<10 eV) ions were observed in the plasma sheet, suggesting cold ion outflows in the topside ionosphere as the seed population for the warm ion source. We establish a scenario to connect these observations to explain DFAWIFEs. A magnetopause intrusion can generate shear Alfvén waves propagating toward the ionosphere, providing the energy for generating a transient warm ion source by energizing the preexisting cold outflow in the topside ionosphere. The larger wave powers closer to the magnetopause are more likely to provide the needed energy for generating such a source. The warm ions from the source then travel along magnetic field lines to the equatorial plasma sheet with higher energy ions arriving first, resulting in the observed dispersion signature.