Four-mode mixing model. Top row: signal from emission region ℳ₁; middle row: signal from emission region ℳ₂; and bottom row: incoherent superposition of ℳ₁ and ℳ₂. Upper two rows: from right to left, the right-most panels depict the OPM pairs as linearly polarised states in the x − y plane orthogonal to the line of sight of the antenna; the middle panels depict the OPM pairs in the Poincaré sphere; and the left-most panels depict the modulated time series produced by the OPM pairs over two drift cycles, where the dashed lines represent the positive OPM state, the dash-dot lines represent the negative OPM state, the upper thick lines represent the total intensity, and the lower thick lines represent the superposed polarisation state. Bottom row: the right panel depicts the four modes in the equatorial plane of the Poincaré sphere and the left panel depicts the modulated time series produced by the incoherent superposition the OPM pairs over two drift cycles (colours and line types are the same as for the upper two rows, and the resultant total intensity is shown in red.