Identify the nonlinear wave-particle interaction regime in rising tone chorus generation

Nonlinear wave-particle interaction during chorus wave generation was assumed to be in the adiabatic regime in previous studies, i.e., the particle phase-space trapping timescale (tau(tr)) is considered to be much smaller than the nonlinear dynamics timescale tau(NL). In this work, we use particle-in-cell simulations to demonstrate that tau(tr)similar to tau(NL), i.e., the interaction regime during chorus generation is in the nonadiabatic regime. The timescale for nonlinear evolution of resonant particle phase-space structures is determined by making the time-averaged power exchange plot, which clearly demonstrates that particles with pitch angle near 80 degrees make the most significant contribution to wave growth. The phase-space trapping timescale is also comparable to the amplitude modulation timescale of chorus, suggesting that chorus subpackets are formed because of the self-consistent evolution of resonant particle phase-space structures and spatiotemporal features of the fluctuation spectrum.