Whistler waves generated by nongyrotropic and gyrotropic electron beams in asymmetric guide field reconnection*

 

We study electron beams and wave activity in the magnetospheric side in asymmetric guide field reconnection, using a 2D particle-in-cell (PIC) simulation. The guide field strength is 30% of the average asymptotic field. Strong whistler wave activity is observed along the separatrix in the magnetospheric side with a faster electron outflow speed, and waves are propagating almost anti-parallel to the magnetic field with propagation angles between 140 to 170 degrees. We investigate whistler wave generation using the fast Fourier transform and the linear dispersion analysis. Examining electron velocity distribution functions (VDFs), we separate fast electron beams and slow electron beams. The linear numerical dispersion analysis shows that there are two modes of the whistler waves in the separatrix. One is a temperature anisotropy mode due to the fast electron beam, and the other is a Landau resonance mode due to the slow electron beam. Outside the electron diffusion region (EDR), whistler waves are strong near the separatrix and become weaker in regions away from the separatrix. The reduction of the wave intensity away from the separatrix occurs due to the enhancement of the population with zero parallel velocity in VDFs. Inside the EDR, nongyrotropic electrons are found in the VDFs with crescent shapes in the perpendicular velocity plane. Wave intensities in the separatrix near the X line depend on the population density of crescent-shaped nongyrotropic electrons.