Top panel: growth rates of an unstable m = −3 MHD mode in three instability windows. Adequate numerical values consistent with the observational data are obtained at k_za = 0.696245, located in the third instability window, for which value of the dimensionless wave number the normalised mode growth rate is equal to 0.232117. Bottom panel: dispersion curves of unstable m = −3 MHD mode for three values of the twist parameter ε = 0.025, 0.1, and 0.2. The normalised phase velocity at k_za = 0.696245 is equal to 1.45.