In color, we present log₁₀ |(f_WHAMP − f_e.s.)/f_WHAMP|, where f_WHAMP is the frequency obtained by the WHAMP (Roennmark 1982) solver in the general electromagnetic case, and f_e.s. is the frequency obtained by solving the dispersion relation ${\epsilon }_L^{Max.}\left(k_{\perp },k_{\Vert },f\right)=0$ in the electrostatic limit Eq. (3), for the fast magnetosonic (resp. upper-hybrid) dispersion surfaces in the left (resp. right) panel. In gray are the regions where no solution for the dispersion relation could be found. The plasma parameters used are the same as in Fig. 3.