Nonlinear Damping of Oblique Whistler Mode Waves Via Landau Resonance

Nonlinear trapping of electrons via Landau resonance is an important mechanism of oblique whistler mode wave-particle interactions. Electrons trapped by Landau resonance gain energies from waves. The Landau resonance velocity becomes very close to the group velocity of nearly parallel whistler mode waves at frequencies around half the electron gyrofrequency, resulting in a long interaction time and possible wave damping. We perform test particle simulations with parameters at L = 5 and a small wave normal angle 10 degrees to study the wave-particle interactions via the Landau resonance. Analyzing the wave electric fields and the resonant currents formed by electrons undergoing Landau and cyclotron resonances, we show that effective wave damping occurs near half the electron gyrofrequency. This nonlinear wave damping is contributed by Landau resonance rather than cyclotron resonance. Furthermore, we confirm that this damping is dominated by perpendicular components of the wave electric field and perpendicular resonant currents. The simulation results indicate that nonlinear damping via Landau resonance is one of the mechanisms dividing chorus emissions into the upper band and the lower band. Plain Language Summary We solve a long-standing problem of the formation of the gap at half the electron gyrofrequency of whistler mode chorus emissions by test-particle simulations. The Landau resonance velocity of obliquely propagating whistler mode waves at half the electron gyrofrequency is very close to the wave group velocity, resulting in effective wave-particle interactions. Space scientists who investigate whistler mode waves mainly focus on parallel propagation. This study reveals that even a small obliquity can make the phenomena very different. We found that Landau resonance is a process dividing chorus emissions into the upper band and the lower band. The cyclotron resonance, which is a typical wave-particle interaction of whistler mode waves, does not contribute to the wave damping for the gap.