Low‐Frequency Whistler Waves Excited by Electron Butterfly Distributions in Turbulent Reconnection Outflow

Abstract Whistler waves, leading to electron scattering and energy transport, are frequently observed in magnetic reconnection. High‐energy electrons produced by magnetic reconnection are expected to excite low‐frequency whistler waves. However, the study on low‐frequency whistler waves in magnetic reconnection is still quite scarce. Utilizing high‐resolution data from the Magnetospheric Multiscale (MMS) mission, we provide observations of low‐frequency whistler waves in a turbulent reconnection outflow. The quasi‐antiparallel propagating whistler waves have power peaked at ∼0.1 and wave number of ∼0.43 in the plasma rest frame. It can be excited through the cyclotron resonance by the electron butterfly distributions, which can be interpreted by a model comprising the addition of electron beams hosting perpendicular anisotropy to electron isotropy distributions. The energy of resonant electrons is calculated as 1.06∼4.16 keV, the parts corresponding to lower frequency (<∼0.1) falling into suprathermal energy range. Our study can promote the understanding of generation of whistler waves in magnetic reconnection.