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

Abstract

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.