  Solar flares are rapid energy release phenomena that appear as bright ribbons
in the chromosphere and high-temperature loops in the corona, respectively.
Supra-arcade Downflows (SADs) are plasma voids that first come out above the
flare loops and then move quickly towards the flare loop top during the decay
phase of the flare. In our work, we study 20 SADs appearing in three flares. By
differential emission measure (DEM) analysis, we calculate the DEM weighted
average temperature and emission measure (EM) of the front region and the main
body of SADs. It is found that the temperatures of the SAD front and body tend
to increase during the course of SADs flowing downwards. The relationship
between the pressure and temperature fits well with the adiabatic equation for
both the SAD front and body, suggesting that the heating of SADs is mainly
caused by adiabatic compression. Moreover, we also estimate the velocities of
SADs via the Fourier Local Correlation Tracking (FLCT) method and find that
increase of the temperature of the SAD front presents a correlation with the
decrease of the SAD kinetic energy, while the SAD body does not, implying that
the viscous process may also heat the SAD front in spite of a limited role.
