  The plasma heating during collisionless magnetic reconnection is investigated
using particle-in-cell simulations. We analyze the time evolution of the plasma
temperature associated with the motion of the reconnecting flux tube, where the
plasma temperature is defined as second-order moment of velocity distribution
function in the simulation frame/in the center of the flux tube frame, and we
show that the plasma heating during magnetic reconnection can be separated into
two distinct stages: the nonadiabatic heating stage, in which the magnetic
field lines are just reconnecting in the X-type diffusion region, and the
adiabatic heating stage, in which the flux tube is shrinking after two flux
tubes merge. During the adiabatic heating stage, the plasma temperature $T$ can
be approximated by $TV^{\gamma-1}=const.$, where $\gamma=5/3$ is the specific
heat, and $V$ is the volume of the flux tube. In the nonadiabatic heating
stage, we found numerically that the ratio of the increment of the ion
temperature to that of the electron temperature can be approximated by $\Delta
T_i/\Delta T_e \approx (m_i/m_e )^{1/4}$, where $m_i$ and $m_e$ are the ion and
electron mass, respectively. We also present a theoretical model based on a
magnetic-diffusion-dominated reconnection to explain the simulation result.
