Dispersion relations of the global MTI eigenmodes. Top: growth rates of the first six MTI modes as a function of the azimuthal order, m. The curve σ = M₁ is indicated in dotted dark line. It seems to be a common asymptote to all curves, σ_n(m). Inset: same growth rates, σ_n, again (one over two for the sake of visibility though), together with their WKBJ approximations, Eq. (30), in dashed lines. Bottom: growth rates of the MTI eigenmodes with m = 100 and m = 50 as a function of the mode number, n, in plain lines. Their WKBJ approximations, Eq. (31), are shown too, in dashed lines. The curve σ = M₂ is indicated in dotted grey line.