  Recent Magnetosphere Multiscale (MMS) observations found 238 high energy (>
40 keV) electron "leaking" events in the the magnetosheath [Cohen et al.,
2017]. While several sources have been proposed, the dominant mechanism or
origin of these particles is not well understood. We have analyzed MMS
locations during these these events and found that most of these electron
leaking events were observed close to the southern high-altitude cusp and
associated boundaries, so these events may have a high-latitude source. Here we
present a new case study of observations of a MMS encounter of southern
magnetospheric cusp boundaries. In the magnetosheath side of the southern cusp,
MMS observes both parallel streaming energetic ions, as well as a population
with 90 degree pitch angles. Subsequently, MMS detects plasma flows and
magnetic field rotation consistent with magnetic reconnection (operating above
and dawn-ward of the MMS spacecraft), finally entering a depressed magnetic
field region with nearly stagnant flow. The depressed field region is occupied
by high-fluxes of trapped high-energy electrons and protons, which resemble the
Cluster observations of cusp diamagnetic cavities at the northern hemisphere
[Nykyri et al., 2011a]. However, the local magnetic field geometry during the
prevailing IMF orientation is not favorable for magnetic reconnection to
directly create a cavity in this location. We show that the plasma flows that
were observed prior to cavity observation satisfy the onset condition for the
Kelvin-Helmholtz instability for 50 percent of all available k-vector
directions.
