Observations of limb prominences often show material falling along vertical threads and moving with an acceleration smaller than that of free-fall. A simple partially ionized plasma model is used to investigate these vertical downflows, which are generated by a mass condensation in the corona. From our calculations, the mass injection gives rise to the formation in ~200 s of a dense blob that falls with a roughly constant acceleration of 15–70 m s$^{−2}$ and a density of 4–10 × 10$^{-11}$ kg m$^{−3}$.

For a fully ionized plasma it is found that the gas pressure gradient is responsible for reducing the effect of solar gravity. In the absence of charged particles, neutrals behave like a fully ionized gas. In a partially ionized gas the mass condensation gives rise to the formation of a neutral and a charged blob that, in the absence of friction between these two species, fall with different accelerations. The friction force, however, plays a prominent role since it quickly couples charged particles and neutrals and makes them fall jointly with a constant acceleration.