  Magnetic flux emergence has been shown to be a key mechanism for unleashing a
wide variety of solar phenomena. However, there are still open questions
concerning the rise of the magnetized plasma through the atmosphere, mainly in
the chromosphere, where the plasma departs from local thermodynamic equilibrium
(LTE) and is partially ionized. We aim to investigate the impact of the
nonequilibrium (NEQ) ionization and recombination and molecule formation of
hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of
the flux emergence process. Using the Bifrost code, we performed 2.5D numerical
experiments of magnetic flux emergence from the convection zone up to the
corona. The experiments include the NEQ ionization and recombination of atomic
hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar
diffusion term of the Generalized Ohm's Law. Our experiments show that the LTE
assumption substantially underestimates the ionization fraction in most of the
emerged region, leading to an artificial increase in the ambipolar diffusion
and, therefore, in the heating and temperatures as compared to those found when
taking the NEQ effects on the hydrogen ion population into account. We see that
LTE also overestimates the number density of H2 molecules within the emerged
region, thus mistakenly magnifying the exothermic contribution of the H2
molecule formation to the thermal energy during the flux emergence process. We
find that the ambipolar diffusion does not significantly affect the amount of
total unsigned emerged magnetic flux, but it is important in the shocks that
cross the emerged region, heating the plasma on characteristic times ranging
from 0.1 to 100 s. We also briefly discuss the importance of including elements
heavier than hydrogen in the equation of state so as not to overestimate the
role of ambipolar diffusion in the atmosphere.
