Radiation pressure coefficients associated with the Mg ii-k line (dotted red line) and the Mg ii-h line (dashed red line), as a function of the Mg⁺ ion radial velocity in the star reference frame. Their sum β⁺ characterizes the total radiation pressure on an ionized magnesium particle (solid red line). β⁺ is larger than 1 (horizontal dotted black line) in the line core and at high radial velocities in the wings of the line.