Time evolution of the normalized spherical harmonic expansion coefficients for different (ℓ,m) modes computed for the shock surface R_s(θ,φ) (left) and for the surface mass density D(θ,φ) (Eq. (2); right) of models W15-2 (top) and L15-2 (bottom). The polar axis of the coordinate system chosen for the expansion is aligned with the final NS kick direction. Note the dominance of the (ℓ,m) = (1,0) mode for model W15-2 with the highest kick velocity, while this dominance is absent in the case of model L15-2.