Cartoon model of the drift track evolution: (a) general view on the system (inside traverse geometry, pulsar is rotating clockwise), similar to Fig. 1, but with individual sparks at two different radio frequencies, ν_lo (red) and ν_hi (green). According to the RFM, emission at ν_lo comes fromhigher altitudes, where the opening angle of the dipole field lines is larger. The modfolds at these two frequencies are shown in (b). The carousel configuration corresponding to the moment marked with the dotted horizontal line in (b) is shown in (c). The violet arrows in (a) and (c) mark the direction of the carousel rotation (see text for more details). The black arrows show increasing longitude, ϕ, and the magnetic azimuth, ψ, both measured from the fiducial longitude. At this moment, the LOS sweeps through the spark centres at ν_lo and misses the spark centres at ν_hi. For the leading component (LC), the rotating carousel needs more time to bring the spark centre to the LOS. The drift phase Θ is directly proportional to time (Eq. (16)), thus |Θ_LC(ν_hi)| > |Θ_LC(ν_lo)|. The spherical triangles defined by the spin axis, magnetic axis, and the point where the LOS passes through the emission cone (black lines) are identical to the triangle in the inset of Fig. 1.