K. Imai et at. (1992—2002) interpret the Jovian modulation lanes in terms of radiation scattering produced by field-aligned inhomogeneities in the Io plasma torus. However, the lanes with opposite (with respect to the Io torus rotation) drifts in frequency remained an enigma. We show that field-aligned inhomogeneities of the magnetospheric plasma at low altitudes above the Jovian ionosphere generate just the opposite drifts of modulation lanes. A new, more correct, algorithm is used for the comprehensive interpretation of J. J. Riihimaa's (1979) empirical diagram of frequency-time drift rates of the lanes. It is found that all the point clusters of the diagram are explicable in the framework of the proposed model, and the cone half-angle of the decametric radiation is about 70°.