Collisionless magnetic reconnection is studied using a 2 1/2-dimension
al hybrid code including Hall dynamics and electron inertia. The simul
ations reveal that the dissipation region develops a two-scale structu
re: an inner electron region and an outer ion region. Close to the X l
ine is a region with a scale of c/omega(pe), the electron collisionles
s skin depth, where the electron flows completely dominate those of th
e ions and the frozen-in magnetic flux constraint is broken. Outside o
f this region and encompassing the rest of the dissipation region, whi
ch scales like C/omega(pi), the ion inertial length, is the Hall regio
n where the electrons are frozen-in to the magnetic field but the ions
are not, allowing the two species to flow at different velocities. Th
e decoupling of electron and ion motion in the dissipation region has
important implications for the rate of magnetic reconnection in collis
ionless plasma: the ions are not constrained to flow through the very
narrow region where the frozen-in constraint is broken so that ion flu
x into the dissipation region is large. For the simulations which have
been completed to date, the resulting rate of reconnection is a subst
antial fraction of the Alfven velocity and is controlled by the ions,
not the electrons. The dynamics of the ions is found to be inherently
nonfluid-like, with multiple ion beams present both at the X line and
at the downstream boundary between the inflow and outflow plasma. The
reconnection rate is only slightly affected by the temperature of the
inflowing ions and in particular the structure of the dissipation regi
on is controlled by the ion inertial length C/omega(pi) and not the io
n Larmor radius based on the incoming ion temperature.