First MMS Observation of Energetic Particles Trapped in High‐Latitude Magnetic Field Depressions

We present a case study of the Magnetospheric Multiscale (MMS) observations of the Southern Hemispheric dayside magnetospheric boundaries under southward interplanetary magnetic field direction with strong B-y component. During this event MMS encountered several magnetic field depressions characterized by enhanced plasma beta and high fluxes of high-energy electrons and ions at the dusk sector of the southern cusp region that resemble previous Cluster and Polar observations of cusp diamagnetic cavities. Based on the expected maximum magnetic shear model and magnetohydrodynamic simulations, we show that for the present event the diamagnetic cavity-like structures were formed in an unusual location. Analysis of the composition measurements of ion velocity distribution functions and magnetohydrodynamics simulations show clear evidence of the creation of a new kind of magnetic bottle structures by component reconnection occurring at lower latitudes. We propose that the high-energy particles trapped in these cavities can sometimes end up in the loss cone and leak out, providing a likely explanation for recent high-energy particle leakage events observed in the magnetosheath. Plain Language Summary Understanding the origin, acceleration mechanisms, and ultimately predicting the space radiation environment of high-energy particles is critical for safe unmanned and manned space operations. In this article we present the first observations of the generation mechanism of a new kind of magnetic bottle at the high-latitude region of the Earth's magnetic shield with high-fluxes of high-energy particles (> 100 keV) using data from National Aeronautics and Space Administration's Magnetosphere Multiscale mission and numerical simulations. The Magnetosphere Multiscale composition measurements of He++, O+, H+, and e- as well as sensitivity to different energies allows us to calculate the size of the magnetic bottle, which for the present event was created by magnetic reconnection about 8 R-E from the spacecraft. We show the first observations of the counterstreaming O+ of ionospheric origin in the magnetic field depressions. We show that particles from these bottles can sometimes "leak out" to the magnetosheath providing a likely explanation for recent high-energy particle leakage event observations.