  We analyze the local dynamics of magnetotail reconnection onset using
Magnetospheric Multiscale (MMS) data. In conjunction with MMS, the macroscopic
dynamics of this event were captured by a number of other ground and
space-based observatories, as is reported in a companion paper. We find that
the local dynamics of the onset were characterized by the rapid thinning of the
cross-tail current sheet below the ion inertial scale, accompanied by the
growth of flapping waves and the subsequent onset of electron tearing. Multiple
kinetic-scale magnetic islands were detected coincident with the growth of an
initially sub-Alfv\'enic, demagnetized tailward ion exhaust. The onset and
rapid enhancement of parallel electron inflow at the exhaust boundary was a
remote signature of the intensification of reconnection Earthward of the
spacecraft. Two secondary reconnection sites are found embedded within the
exhaust from a primary X-line. The primary X-line was designated as such on the
basis that (1) while multiple jet reversals were observed in the current sheet,
only one reversal of the electron inflow was observed at the high-latitude
exhaust boundary, (2) the reconnection electric field was roughly 5 times
larger at the primary X-line than the secondary X-lines, and (3) energetic
electron fluxes increased and transitioned from anti-field-aligned to isotropic
during the primary X-line crossing, indicating a change in magnetic topology.
The results are consistent with the idea that a primary X-line mediates the
reconnection of lobe magnetic field lines and accelerates electrons more
efficiently than its secondary X-line counterparts.
