Vertical wave numbers for the 2F model as functions of height for all wave periods studied in this work. From top to bottom, the wave periods are 1 s, 3 s, and 5 s. Blue and green solid lines: upward and downward fast mode. Red and black solid lines: Upward and downward normal mode (different from the fast mode). The left column represents the real part and the right represents the imaginary part. The fast mode lines are multiplied by an arbitrary factor for the representation. The vertical dashed line indicates the equipartition layer β_plasma = 1.