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Abstract We analyze Magnetospheric Multiscale Mission observations of whistler waves and associated electron field‐aligned crescent distribution in the vicinity of the magnetotail near‐Earth X‐line. The whistler waves propagate outward from the X‐line in the neutral sheet. The associated field‐aligned streaming electrons exhibit a crescent‐like shape, with an inverse slope (d f /d| v || |>0) at 1–5 keV. The parallel phase velocity of the waves is in the range (1–5 keV) of the inverse slope of the field‐aligned crescents in the velocity space. We demonstrate that the observed whistler waves are driven by the electron field‐aligned crescents through Landau resonance. The cyclotron resonance is at the high‐energy tail with negligible free energy of pitch angle anisotropy in these events.
Ren et al. (Fri,) studied this question.