A rough ball is thrown such that it rotates clockwise about a horizontal axis and simultaneously moves right in viscous air as shown. The force on the ball due to magnus effect acts
Since the (resultant) air speed around the top half of the ball is less than the air speed around the bottom half of the ball, the pressure is greater on the top of the ball. This causes a net downward force (F) to act on the ball. This is due to Bernoulli's principle which states that when air velocity decreases, air pressure increases (and vice-versa).
Hence, The force on the ball due to magnus effect acts downward.