Chorus emissions are triggered from the linear cyclotron instability driven by the temperature anisotropy of energetic electrons (10-100 keV) in the magnetosphere. Chorus emissions grow as an absolute nonlinear instability near the magnetic equator because of the presence of an electromagnetic electron hole in velocity space. The transition process from the linear wave growth at a constant frequency to the nonlinear wave growth with a rising tone frequency is due to formation of a resonant current -JB antiparallel to the wave magnetic field. The rising-tone frequency introduces a phase shift to the electron hole at the equator and results in a resonant current component antiparallel to the wave electric field -JE, which causes the nonlinear wave growth. To confirm this triggering mechanism, we perform Vlasov hybrid simulations with JB and without JB. The run without JB does not reproduce chorus emissions, while the run with J B does successfully reproduce chorus emissions. The nonlinear frequency shift ω1 due to JB plays a critical role in the triggering process. The nonlinear transition time TN for the frequency shift is found to be of the same order as the nonlinear trapping period, which is confirmed by simulations and observation. The established frequency sweep rate is ω1/TN, which gives an optimum wave amplitude of chorus emissions. Copyright 2011 by the American Geophysical Union.