By means of particle simulation, we study the excitation mechanism of low‐freqeuncy (0.01–0.05Hz) upstream hydromagnetic waves. Initially, we observe excitation of the right‐hand polarized waves propagating parallel to the field‐aligned ion beam, which is given as the free energy source. In the nonlinear stage we observed the phase space bunching of beam ions by the excited waves. We apply this bunching effect for the explanation of “gyrophased bunched” ions observed in the fores