Top: time evolution of the vertical component of the neutral momentum from Eq. (14) in the θ = 89° AD run. Negative values mean downward motion. The contours of vertical drift momentum and −10^-9 kg m^-2 s^-1 (see Eq. (16)) are overplotted in green and gray, respectively. Bottom: the same for the ion momentum. The size of each snapshot is 250 × 1000 km; the time elapsed is given at the bottom of each panel, the color scheme is constant in time, and is the same in both panels.