  We present a model for the spontaneous onset of fast magnetic reconnection in
a weakly collisional plasma, such as the solar corona. When a current layer of
macroscopic width undergoes collisional (Sweet-Parker) reconnection, a narrow
dissipation region forms around the X-line. This dissipation region naturally
becomes narrower during the reconnection process as stronger magnetic fields
are convected toward the X-line. When the dissipation region becomes thinner
than the ion skin depth, resistive magnetohydrodynamics breaks down as the Hall
effect becomes important and the Sweet-Parker solution ceases to exist. A
transition to collisionless (Hall) reconnection ensues, increasing the
reconnection rate by many orders of magnitude in a very short time. Predictions
of the model are consistent with constraints set by observations of solar
flares.
