Generation Mechanisms of Hiss-Like Chorus in the Earth's Magnetosphere

Whistler mode chorus waves appear either as a series of discrete elements (rising or falling tones) or as banded hiss-like emissions in the dynamic spectrogram. Although the generation of rising-tone chorus waves has been extensively studied, few studies have focused on the generation of hiss-like emissions. In this paper, using the one-dimensional (1-D) general curvilinear particle-in-cell (gcPIC) simulation model, we have thoroughly studied the generation mechanisms of hiss-like emissions. Based on the spectrogram, we first classify hiss-like emissions into four types (I, II, III, and IV). We find that the type I hiss-like waves are linearly excited by the anisotropic electrons, while other types are excited through the nonlinear process. For the type I waves, they do not undergo a frequency chirping process and have a relatively broadband spectrum. The high proportion of hot electrons or the large temperature anisotropy of hot electrons will decrease the threshold for nonlinear growth and cause the wave amplitudes to exceed the threshold, leading to the nonlinear generation of type II hiss-like waves. The fresh hot electrons continually injected into the system with a large drift velocity significantly reduces the time interval between the rising tone elements to form a continuous spectrum, which are the type III waves. The intense and long-lasting hiss-like emissions, that is, type IV, which are the most frequently observed, should be generated due to the combined effect of the rapid injection and the high proportion of hot electrons (or the large temperature anisotropy). Our findings provide a comprehensive understanding of the generation of hiss-like chorus waves in the Earth's magnetosphere.