  We analyze a high-resolution simulation of magnetopause reconnection observed
by the Magnetospheric Multiscale (MMS) mission and explain the occurrence of
strongly localized dissipation with an amplitude more than an order of
magnitude larger than expected. Unlike symmetric reconnection, wherein
reconnection of the ambient reversed magnetic field drives the dissipation, we
find the annihilation of the self-generated, out-of-plane (Hall) magnetic field
plays the dominant role. Electrons flow along the magnetosheath separatrices,
converge in the diffusion region, and jet past the X-point into the
magnetosphere. The resulting accumulation of negative charge generates intense
parallel electric fields that eject electrons along the magnetospheric
separatrices and produce field-aligned beams. Many of these features match MMS
observations.
