Ion Acceleration and Corresponding Bounce Echoes Induced by Electric Field Impulses: MMS Observations

Dayside magnetosphere interactions are essential for energy and momentum transport between the solar wind and the magnetosphere. In this study, we investigate a new phenomenon within this regime. Sudden enhancements of ion fluxes followed by repeating dropouts and recoveries were observed by Magnetospheric Multiscale on 5 November 2016, which is the very end of the recovery phase from a moderate geomagnetic storm. These repetitive flux variations display energy-dispersive characteristics with periods relevant to ion bounce motion, suggesting they are corresponding echoes. Alongside the flux variations, bipolar electric field impulses originating from external sources were detected. We traced the source region of the initial injection and found it is located near the spacecraft's position. To elucidate the underlying physics, a test-particle simulation is conducted. The results reveal that radial transport resulting from impulse-induced acceleration can give rise to these echoes. Observations demonstrate dayside magnetosphere interactions are more common than we previously considered, which warrants further research. In spacecraft observations, the quasi-periodical motion of particles in the geomagnetic field can generate repeating signals known as "echoes". Examining the relationship between the periods of echoes and the particle's energy is necessary for determining their types. The present study reports a novel mechanism responsible for generating echoes that arise from the ion bounce motion along magnetic field lines. Observations from both ground-based stations and spacecraft, as well as numerical simulations, suggest these echoes can result from the acceleration and radial diffusion of ions responding to external field variations. These findings extend the current understanding of interactions in the dayside outer magnetosphere. Ion acceleration and corresponding bounce echoes were observed by MMS in the dayside outer magnetosphere Observations and simulations suggest these echoes are driven by the accompanied electric field impulses Impulse-induced ion radial transport with positive phase space density radial gradient leads to the observed pattern