Propagation of energy carried by pure inertial waves at the frequency ω = f. The two allowed group velocity vectors, v_{g, ±}, form an angle λ_± = ± (π/ 2 − Θ) with respect to the rotation axis, represented in red. Thus, the energy propagates along two characteristics, represented in blue. At the frequency ω = f, one of the characteristics is perpendicular to the vertical axis, so that the energy propagates in the local horizontal direction.