Transverse structuring of the fundamental transverse eigenmode. Top: real part of the fundamental transverse eigenmode as a function of transverse and longitudinal coordinates, x and y, respectively, from which the basic mode with longitudinal wave number m = 1 and transverse wave number n = 0 is subtracted, . The physical parameters are the same as for Fig. 2. Bottom: normalised transverse wave profile of the derivative of the fundamental transverse eigenmode, df/dx, as a function of x for three positions in y along the loop: 0 (red), y₀ + Δy (blue), and L/2 (black).