Design of runaway electron mitigation strategies for reactor scale machines relies on present-day experiments. Therein, analysis is aided by the availability of diagnostics that are sensitive to the spatio-temporal runaway energy distribution. Filtered visible light camera imaging of synchrotron emission in Tokamak à configuration variable (TCV) [1] was previously used to infer the most strongly radiating part of the distribution [2]. The inferred large pitch angles (0.5 rad) could not be explained just considering electric field acceleration, particle collisions and radial transport [1]. Herein, we demonstrate that these TCV observations are consistent with runaway pitch angle scattering through resonant interaction of their gyromotion with the toroidal magnetic field ripple. A pitch angle diffusion operator [3] is introduced in fluid-kinetic DREAM simulations [4], predicting strong scattering at a resonant momentum (p/(me c)~45) reached by the most energetic runaways. Synchrotron power losses increase with pitch angle, yielding net deceleration of the scattered runaways after which these are re-accelerated to the ripple resonance. Ripple interaction can thus act as an energy limiter. The ripple hypothesis is tested experimentally in TCV by exploiting the decrease in p during a toroidal magnetic field ramp down. Multi-wavelength multi-view imaging data from MANTIS indicates momentum-space behaviour in agreement with modelling. This highlights the importance of accounting for runaway-ripple interaction in interpreting present-day tokamak runaway experiments, and demonstrates the explanatory and predictive power of state-of-the-art kinetic runaway models. [1] M. Hoppe et al. Nuclear Fusion 9, 60 (2020). doi: 10.1088/1741-4326/aba371 [2] T.A. Wijkamp et al. Nuclear Fusion 4, 61 (2021). doi: 10.1088/1741-4326/abe8af [3] B. Kurzan et al. Physical Review Letters 25, 75 (1995). doi: 10.1103/PhysRevLett.75.4626 [4] M. Hoppe et al. Computer Physics Communications, 268 (2021). doi: 10.1016/j.cpc.2021.108098