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 By 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.

Mass Loading the Earth's Dayside Magnetopause Boundary Layer and Its Effect on Magnetic Reconnection

“…These counterstreaming populations form even when the reconnection in the two hemispheres is not simultaneous (Fuselier et al, 2014). Alternatively, the counterstreaming O + populations could form through the process described in Nykyri et al (2019) when the IMF had a large B Y component. Assuming these velocities are constant and assuming a field line length of 10 R E , ionospheric ions take~5-10 min to propagate to the subsolar region.…”

“…In this event, a sustained interval of northward IMF mass loads the boundary layer through the aforementioned process. Although the MMS orbit does not go into the cusp proper, the spacecraft occasionally observe cusp-like cavities at midlatitudes away from the subsolar region (Nykyri et al, 2019). Although very high O + densities are observed in the boundary layer and the magnetospheric ion source rivals that in the magnetosheath, reconnection is still observed at the low-latitude magnetopause after the IMF rotates southward.…”

“…The counterstreaming O + in the dayside boundary layer is likely due to the fact that, for dual-lobe reconnection, previously open flux tubes poleward of the cusp now form closed flux tubes with O + streaming along the field from both high-latitude hemispheres. Alternatively, counterstreaming O + was recently observed in a cusp-like diamagnetic cavity for weakly southward IMF with strong IMF B Y (Nykyri et al, 2019). Under these conditions, ions flow out of the ionosphere and are locally trapped between the ionosphere and a magnetic field maximum in the subsolar region.…”

“…Figure 2b shows that the O + ion population in the boundary layer is counterstreaming with velocities of the two peaks centered around ±150 to ±200 km/s (parallel and antiparallel to the magnetic field). Alternatively, counterstreaming O + was recently observed in a cusp-like diamagnetic cavity for weakly southward IMF with strong IMF B Y (Nykyri et al, 2019). Ionospheric O + outflow is energized to several kiloelectron volts by the rapid sunward motion of field lines that are reconnected poleward of the magnetospheric cusps.…”

Statistical Study of Solar Wind, Magnetosheath, and Magnetotail Plasma and Field Properties: 12+ Years of THEMIS Observations and MHD Simulations

“…Cluster observations revealed 90° pitch angle electrons in the cavity, strongly suggestive of a local acceleration mechanism (Nykyri et al., 2012; Walsh et al., 2010). Recently, MMS observation suggested that low‐latitude component reconnection can also trigger similar structure near the cusp region (Nykyri et al., 2019).…”