A MODEL FOR SPONTANEOUS ONSET OF FAST MAGNETIC RECONNECTION

We present a model for the spontaneous onset of fast magnetic reconnection in a weakly collisional plasma, such as the solar corona. When sheared magnetic fields in the corona undergo collisional (Sweet-Parker) reconnection, a narrow dissipation region forms around the X-line. This dissipation region dynamically 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.