  Plasmoid instability is usually accounted for the onset of fast reconnection
events observed in astrophysical plasmas. However, the measured reconnection
rate from observations can be one order of magnitude higher than that derived
from MHD simulations. In this study, we present the results of magnetic
reconnection in the partially ionized low solar atmosphere based on 2.5D
magnetohydrodynamics (MHD) simulations. The whole reconnection process covers
two different fast reconnection phases. In the first phase, the slow
Sweet-Parker reconnection transits to the plasmoid-mediated reconnection, and
the reconnection rate reaches about 0.02. In the second phase, a faster
explosive reconnection appears, with the reconnection rate reaching above 0.06.
At the same time, a sharp decrease in plasma temperature and density at the
principle X-point is observed which is associated with the strong radiative
cooling, the ejection of hot plasma from the local reconnection region or the
motion of principle X-point from hot and denser region to cool and less dense
one along the narrow current sheet. This causes gas pressure depletion and the
increasing of magnetic diffusion at the main X-point, resulting in the local
Petschek-like reconnection and a violent and rapid increase in the reconnection
rate. This study for the first time reveals a common phenomenon that the
plasmoid dominated reconnection transits to an explosive faster reconnection
with the rate approaching the order of 0.1 in partially ionized plasma in the
MHD scale.
