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Four invariants of the ray trajectory are found for a ray propagating in a horizontally stratified ionosphere under the density perturbation of HF wave-induced field-aligned irregularities. The reflection height of the ray can then be determined with the aid of those invariants. The results show that the reflection height of the ray varies drastically (namely, strong spread F echoes) in the presence of irregularities that polarize in the magnetic meridian plane. By contrast, the reflection height is not affected (namely, no spread F echoes) by those irregularities that polarize in the direction perpendicular to the meridian plane. Spread F is quite insensitive to the magnetic dip angle θ0 in the region from 20° to 70°. The dependence of spread F on the scale length of the irregularity has also been examined for the case θ0 = 50°. It is found that spread F is not caused by irregularity with scale length less than about 100 m.