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 fce ${f}_{ce}$ and wave number of kde $k{d}_{e}$ ∼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 (<∼0.1fce ${f}_{ce}$) falling into suprathermal energy range. Our study can promote the understanding of generation of whistler waves in magnetic reconnection. Plain Language Summary: Whistler waves are one of the most important plasma waves in magnetic reconnection. Their frequency generally decreases as the energy of resonant electrons increases. Although magnetic reconnection can produce high‐energy electrons, the study on low‐frequency whistler waves with power peaked at ∼0.1fce is still quite scarce. Here, we present study of such whistler waves, which are excited by the electron butterfly distributions. The energy of resonant electrons is calculated as 1.06∼4.14 keV, the parts corresponding to lower frequency (<∼0.1fce) falling into suprathermal energy range. Key Points: Whistler waves with power peaked at ∼0.1 fce ${f}_{ce}$ were observed in a turbulent reconnection outflowThe whistler waves were excited through cyclotron resonance by the electron butterfly distributionsThe resonant energy is calculated as 1.06∼4.16 keV, part of them falling into suprathermal energy range [ABSTRACT FROM AUTHOR]

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