Collision frequencies between ions and neutrals (solid blue line), neutrals and ions (solid orange line), ionization rate (dashed red line) and recombination rate (dashed green line), and the hydrodynamic frequency (dotted black line) as a function of height, computed for the equilibrium model atmosphere. For reference, the Brunt-Väisälä frequency is calculated at height z = 0 and is shown as a gray dotted line.