our another motivation is to prove the intriguing optical Magnus effect can be observed even in the beam propagation direction. In this work, we want to explore what role of the transverse momentum currents play in the optical Magnus effect in the free space. First, we introduce the Whittaker scale potentials to describe the vector field structure for different polarization models. In the frame of classical electrodynamics, it is the rotating wavepacket but not the spin photon acting as the spin ball, which is significantly different from the previous works. Then, we uncover how the centroid of wavepacket evolves, and how the polarization state affects its rotation of the centroid trajectory. Finally, we examine what roles the spin and orbital currents play in the optical Magnus effect. A relation between transverse momentum currents and optical Magnus effect is obtained. We believe that our findings may provide insights into the fundamental properties of optical currents in beam propagation. In order to reveal the role of the transverse momentum currents in optical Magnus effect, we begin to establish a general beam propagation model to describe the vector field structure. Figure 1 illustrates the geometry of the vector field structure in the Cartesian coordinate system. Following the standard procedure, the electric field E ( r , ω ) is obtained by solving the vector Helmholtz equation where r = x e$_{x}$ + y e$_{y}$ + z e$_{z}$ and k = ω/c is the wave number in the free space. The vector Helmholtz equation can be solved by employing the angular spectrum representation of the electric field as The transverse condition ∇ · ˜ E ( k$_{x}$, k$_{y}$ ) = 0 implies that the components of the angular spec- trum satisfy the relation k · ˜ E ( k$_{x}$, k$_{y}$ ) = 0. In principle, due to the longitudinal component, the Jones vector is not sufficient to describe the vectorial properties of a finite beam. Hence it is necessary for us to define two mutually orthogonal vectors I$_{1}$ = I$_{2}$ × k | I$_{2}$ k | , I$_{2}$ = k × u | k u | , (3) E ( r ) = ∫ dk$_{x}$dk$_{y}$ ˜ E ( k$_{x}$, k$_{y}$ ) × exp[ i ( k$_{x}$x + k$_{y}$y + k$_{z}$z )] . (2) ( ∇ 2 + k $^{2}$) E ( r , ω ) = 0 , (1) 3 II. VECTOR FIELD MODEL