  Magnetospheric Multiscale (MMS) encountered the primary low-latitude
magnetopause reconnection site when the inter-spacecraft separation exceeded
the upstream ion inertial length. Classical signatures of the ion diffusion
region (IDR), including a sub-ion-Alfv\'enic de-magnetized ion exhaust, a
super-ion-Alfv\'enic magnetized electron exhaust, and Hall electromagnetic
fields, are identified. The opening angle between the magnetopause and
magnetospheric separatrix is $30^\circ\pm5^\circ$. The exhaust preferentially
expands sunward, displacing the magnetosheath. Intense pileup of reconnected
magnetic flux occurs between the magnetosheath separatrix and the magnetopause
in a narrow channel intermediate between the ion and electron scales. The
strength of the pileup (normalized values of 0.3-0.5) is consistent with the
large angle at which the magnetopause is inclined relative to the overall
reconnection coordinates. MMS-4, which was two ion inertial lengths closer to
the X-line than the other three spacecraft, observed intense electron-dominated
currents and kinetic-to-electromagnetic-field energy conversion within the
pileup. MMS-1, 2, and 3 did not observe the intense currents nor the
particle-to-field energy conversion but did observe the pileup, indicating that
the edge of the generation region was contained within the tetrahedron.
Comparisons with particle-in-cell simulations reveal that the electron currents
and large inclination angle of the magnetopause are interconnected features of
the asymmetric Hall effect. Between the separatrix and the magnetopause,
high-density inflowing magnetosheath electrons brake and turn into the outflow
direction, imparting energy to the normal magnetic field and generating the
pileup. The findings indicate that electron dynamics are likely an important
influence on the magnetic field structure within the ion diffusion region.
