Whistler waves driven by field-aligned streaming electrons in the magnetic reconnection

We analyze MMS 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 X-line in the neutral sheet. The associated field-aligned streaming electrons exhibit a crescent-like shape, with an inverse slope (df/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 high-energy tail with negligible free energy of pitch-angle anisotropy in these events.