Sky-projected view of the split-fan beams typical of the X-mode curvature radiation from narrow plasma streams. The continuous beam shown on the left can be decomposed into a sequence of lobe pairs, emitted quasi-instantaneously from different points along the stream (shown on the right-hand side), e.g. from the points “a” and “b” in the bottom right corner. The horizontal line marks the passage of the line of sight. The stream’s polarisation angle Ψ_B is detectable at two pulse longitudes (corresponding to the lobes a_L and b_T) on the leading and trailing side of the stream.