For future ITER operations it is important to minimize the damaging effect that large amplitude ELMs may have on plasma-facing components, for example by reducing their amplitude and increasing the repetition rate. One possibility in this direction is the triggering and mitigation of ELMs by pellet injection, which was successfully achieved on ASDEX Upgrade. Experiments on JET have demonstrated that ELM triggering and pacing is possible also in large size tokamaks, thus encouraging further investigation of the ELM triggering mechanism by pellets. Experiments with pellet injection in JET plasmas are presented, including Ohmic, L-mode and H-mode regimes. The toroidal mode number spectrum of MHD pellet-triggered events is compared with that of spontaneous ELMs. The method, which combines wavelet spectral analysis with statistical two-point correlation techniques and allows one to extract the coherent part of fluctuations from the incoherent background and follow the evolution of the toroidal mode number spectrum with a time resolution of 0.05-0.1ms.