The master equation for the poloidal spectrum of electromagnetic energy in a toroidal plasma is derived. This equation provides a theoretical framework for a global description of the propagation and absorption of lower hybrid waves (or any weakly damped small wavelength mode) in present-day Tokamaks. Resonant toroidal couplings between the unperturbed cylindrical modes are the basic processes involved in this description. They lead to the destruction of correlations above a stochasticity threshold for the toroidal perturbation. A general method for computing the coupling coefficients is proposed and applied under typical current generation conditions. This normal mode energy diffusion provides a simple theory to address the so-called 'spectral gap' problem.