Excitation of Whistler Waves Through the Bidirectional Field‐Aligned Electron Beams With Electron Temperature Anisotropy: MMS Observations

Abstract

AbstractUsing four‐point measurements from the Magnetospheric Multiscale (MMS) mission, we identify whistler waves at the boundary of an ion scale magnetic hole, which should be locally excited rather than propagated from other regions. Based on the measured local plasma parameters, which include those for bidirectional electron beams with electron temperature anisotropy, the frequency range with the growth rate derived from kinetic theory is consistent with the observations, while the growth rate in the absence of such beams is negative; hence, the observed whistler waves are locally excited by the bidirectional electron beams with electron temperature anisotropy. The dispersion relation, derived from one‐dimensional particle‐in‐cell simulations that are conducted using the inputs of the bidirectional electron beams with electron temperature anisotropy, agrees well with the one from kinetic theory, which further supports this excitation mechanism. Our results shed light on a new excitation mechanism for whistler waves.