Two WKB basis elements in the polar (R,φ)-plane. Each basis element is located around a central radius R0, on a region of size σ. The winding of the spirals is governed by the radial frequency kr. The number of azimuthal patterns is given by the index kφ, e.g. kφ = 1 for the interior dark gray element, whereas kφ = 2 for the exterior light gray one.
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