  Whistler mode chorus waves are quasi-coherent electromagnetic emissions with
frequency chirping. Various models have been proposed to understand the
chirping mechanism, which is a long-standing problem in space plasmas. Based on
analysis of effective wave growth rate and electron phase space dynamics in a
self-consistent particle simulation, we propose here a phenomenological model
called the "Trap-Release-Amplify" (TaRA) model for chorus. In this model, phase
space structures of correlated electrons are formed by nonlinear wave particle
interactions, which mainly occur in the downstream. When released from the wave
packet in the upstream, these electrons selectively amplify new emissions which
satisfy the phase-locking condition to maximize wave power transfer, leading to
frequency chirping. The phase-locking condition at the release point gives a
frequency chirping rate that is fully consistent with the one by Helliwell in
case of a nonuniform background magnetic field. The nonlinear wave particle
interaction part of the TaRA model results in a chirping rate that is
proportional to wave amplitude, a conclusion originally reached by Vomvoridis
et al. Therefore, the TaRA model unifies two different results from seemingly
unrelated studies. Furthermore, the TaRA model naturally explains fine
structures of chorus waves, including subpackets and bandwidth, and their
evolution through dynamics of phase-trapped electrons. Finally, we suggest that
this model could be applied to explain other related phenomena, including
frequency chirping of chorus in a uniform background magnetic field and of
electromagnetic ion cyclotron waves in the magnetosphere.

Plasma Physics;Space Physics