magnetic force F=l i*B  (* represents cross product)

now angle b/w i and B=90 degrees

therefore F=liBsin90

F=ilb    (where l=2(pi)a)

F=2(pi)aiB  

and to know the direction put your fingers in the direction of current and curl them in direction of magnetic field. thumb will show the direction of force.

in this case it is pointing downwards.