Square of the frequency as a function of the Alfvén speed in the thread. In this plot R = 3.3  ×  10²   km, λ = 1.2  ×  10⁵   km and . The same notation as in the previous figures has been used. The long dashed line corresponds to the frequency (P = 4   min) derived from the observations of Okamoto et al. (2007). The dashed lines correspond to the C ₊  solution while the solid lines represent the C ₋  solution.