Chromospheric evaporation velocities in solar flares

We show that an upper limit to chromosphere evaporation velocities in solar flare loops is approximately 2.35c/sub s/, where c/sub s/ is the sound speed in the evaporated material. Observed upward velocities greater than this limit would imply the existence of additional nonhydrodynamic forces. Hydrodynamic calculations of collisional heating by nonthermal energetic electrons (thick-target model) yield upward velocities near the upper limit at large (> or =3 x 1010 ergs s-1 cm-2) energy flux values. At more modest fluxes (109-1010 ergs s-1), both ''thermal'' and ''thick-target'' models yield maximum velocities that are between 10% and 30% of the upper limit. Observations of postulated evaporation velocities from the Solar Maximum Mission (Antonucci and Dennis) appear to be between 10% and 25% of the upper limit. When both temperature and velocity of the evaporated plasma are considered, the ''thermal'' model with energy fluxes between 3 x 109 and 1010 ergs s-1 cm-2 is found to be most consistent with these observations