This paper investigates the relationship between measured field strengths, observed over two midlatitude long-distance paths, and total adiabatic invariants calculated for all possible propagation channels, for equinoxes and for low and high solar activity. Communication channel invariants or channel volumes of all types of channels have been calculated for a frequency of 15 MHz using the EMI-81 ionospheric model for both simple channels (F, E, FE) made up of conventional hop trajectories (Fh, Eh) refracted by ionospheric layers, and ricochet, or chordal, trajectories (Fr, Er, FE) which propagate in stratification ducts within the ionospheric layers, or combinations of these channels. It is shown that under night and twilight (day-night) conditions the field strength, in general, is proportional to the total channel volume: E ≈ (IΣ) n, where n = 0.5 to 2. This indicates the strong influence of multiple scattering by irregularities on the processes of capture, loss, and radio wave propagation in ionospheric waveguide channels. This is in accordance with the results of using ray diffusion theory and adiabatic approximation, where the horizontal character of the waveguide channel varies slowly in relation to the oscillation of the ray within the duct. Greater field strengths observed during sunset, when the terminator was moving along the path, are explained by the influence of the larger wave channel volumes at this time in comparison with other periods.