  Ellerman bombs (EBs) with significant H$\alpha$ wing emissions and
ultraviolet bursts (UV bursts) with strong Si IV emissions are two kinds of
small transient brightening events that occur in the low solar atmosphere.We
numerically investigated the magnetic reconnection process between the emerging
arch magnetic field and the lower atmospheric background magnetic field. We aim
to find out if the hot UV emissions and much colder H$\alpha$ wing emissions
can both appear in the same reconnection process and how they are located in
the reconnection region. The open-source code NIRVANA was applied to perform
the 2.5D magnetohydrodynamic (MHD) simulation. We developed the related
sub-codes to include the more realistic radiative cooling process for the
photosphere and chromosphere and the time-dependent ionization degree of
hydrogen. The initial background magnetic field is 600 G, and the emerged
magnetic field in the solar atmosphere is of the same magnitude, meaning that
it results in a low- $\beta$ magnetic reconnection environment. We also used
the radiative transfer code RH1.5D to synthesize the Si IV and H$\alpha$
spectral line profiles based on the MHD simulation results. Magnetic
reconnection between emerged and background magnetic fields creates a thin,
curved current sheet, which then leads to the formation of plasmoid instability
and the nonuniform density distributions. The mix of hot tenuous and much
cooler dense plasmas in the turbulent reconnection region can appear at about
the same height, or even in the same plasmoid. The turbulent current sheet is
always in a dense plasma environment with an optical depth larger than
6.5$\times$10$^{-5}$ due to the emerged magnetic field pushing high-density
plasmas upward.
