Generation and Expansion of Whistler-Chorus Waves in the Earth's Magnetosphere

A new mechanism has been proposed to explain the observed quasi-periodic broad-band whistler-chorus emissions in the inner magnetosphere of the Earth.[1] We show that the gyro motion and bounce motion of the substorm injected energetic electrons can emit broad-band backscattering chorus waves with frequencies below the electron gyrofrequency. The energy expansions of the chorus waves are studied based on their group-velocity distributions. The group-velocity distribution of the whistler-chorus waves can explain the frequency gap near half of the electron gyrofrequency. Our results indicate that the frequency gap is not a result of Landau damping but a consequence of the highly field- aligned expansion of chorus waves at the specific frequencies.