A recently developed theory of force-free electromagnetic oscillations (Osherovich, 1985, 1986), is employed to intepret the diffuse plasma resonances stimulated by topside ionospheric sounders at the frequencies fDn. The main prediction of the theory, namely, that the spectrum of frequencies should be proportional to n1/2, n=1, 2, 3, ⋅⋅⋅, is checked against observations. It is found that fDn≈ 0.95(fSfH)1/2n1/2 is a fair approximation for diffuse ionospheric resonances, where fN and fH are the plasma and electron gyrofrequencies, respectively.