Observational evidence of generation mechanisms for very oblique lower band chorus using THEMIS waveform data

Abstract

Chorus waves are intense coherent whistler mode waves with frequency chirping which play a dual role in both loss and acceleration of radiation belt electrons in the Earth's magnetosphere. Although the generation of parallel chorus waves has been extensively studied by means of theory, simulations, and observations, the generation mechanism of very oblique chorus waves still remains a mystery. In this study, we have analyzed hundreds of very oblique discrete (rising or falling tone) lower band chorus events collected from 7 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) waveform data to investigate their potential generation mechanisms. Comparisons between wave normal angles directly measured onboard THEMIS in the dawn-day sector at L = 5–9 and inferred from theoretical models on the basis of measured wave characteristics (frequency sweep rate, mean frequency, and amplitude) show that these very oblique waves are more commonly generated through cyclotron resonance with anisotropic electron streams. However, a second generation mechanism via Landau resonance with low-energy electron beams seems to be also operating on the nightside at L < 6.7 and at all local times at L > 8.5. Moreover, very oblique lower band chorus waves with large frequency chirping rates or small magnetic field amplitudes are more likely excited via cyclotron resonance, while waves with small frequency chirping rates or large magnetic field amplitudes are preferentially generated through Landau resonance. This comprehensive statistical study provides interesting insight into the possible generation mechanisms of very oblique lower band chorus waves in the Earth's magnetosphere.