  Several mechanisms have been proposed to account for the formation of solar
prominences or filaments, among which direct injection and
evaporation-condensation models are the two most popular ones. In the direct
injection model, cold plasma is ejected from the chromosphere into the corona
along magnetic field lines; In the evaporation-condensation model, the cold
chromospheric plasma is heated to over a million degrees and is evaporated into
the corona, where the accumulated plasma finally reaches thermal instability or
non-equilibrium so as to condensate to cold prominences. In this paper, we try
to unify the two mechanisms: The essence of filament formation is the localized
heating in the chromosphere. If the heating happens in the lower chromosphere,
the enhanced gas pressure pushes the cold plasma in the upper chromosphere to
move up to the corona, such a process is manifested as the direct injection
model. If the heating happens in the upper chromosphere, the local plasma is
heated to million degrees, and is evaporated into the corona. Later, the plasma
condensates to form a prominence. Such a process is manifested as the
evaporation-condensation model. With radiative hydrodynamic simulations we
confirmed that the two widely accepted formation mechanisms of solar
prominences can really be unified in such a single framework. A particular case
is also found where both injection and evaporation-condensation processes occur
together.
