What drives chorus wave frequency chirping?

For over 50 years, chorus wave frequency chirping has been regarded as a nonlinear phenomenon driven by the hot electron current density aligned with the wave magnetic field (δjB). However, recent theoretical models have challenged the crucial role played by δjB. Here, using modified first-principles particle simulations, we demonstrate that chirping can occur even in the absence of δjB. This result not only clarifies the physical mechanism governing chorus wave evolution but also has broader implications for nonlinear frequency chirping in other wave modes and plasma environments.