Whistler wave excitation mechanisms in magnetopause reconnection diffusion regions observed by MMS

Whistler waves are commonly observed in magnetopause reconnection. Possible wave excitation mechanisms include the perpendicular anisotropy or loss cone of hot magnetospheric electrons, the perpendicular anisotropy of electrons mainly in the intermediate energy range between magnetosheath and magnetospheric energies that are possibly associated with the energization during reconnection, and the anisotropy or the drift of magnetosheath electron beams. The first mechanism is well identified close to the magnetospheric separatrix, while the importance of the latter two mechanisms has not been clear. We investigate whistler waves in magnetopause reconnection diffusion regions. Electron distributions concurrent with the wave are analyzed with the linear instability analysis to identify the possible wave excitation mechanisms. A preliminary result shows that the overall anisotropy of electrons in the reconnection exhaust, when present, is a dominant mechanism for whistler excitation. Magnetosheath electron beams may not be as important when superposed on such a background anisotropic population. However, the beam can be unstable to whistler due to either its drift or anisotropy, and we try to discuss the dependence of the beam parameter regimes