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

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 $f_{ce}$ and wave number of $k{d}_e$ ∼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$f_{ce}$) falling into suprathermal energy range. Our study can promote the understanding of generation of whistler waves in magnetic reconnection.