Magnetic spectral signatures in the terrestrial plasma depletion layer: Hybrid simulations

The electromagnetic ion cyclotron waves in the terrestrial plasma depletion layer (PDL) are characterized by three different spectral categories. They are LOW, where the ion cyclotron waves have a continuous spectrum with main power below 0.5Ω p (Ω p is the proton gyrofrequency); CON, where the main power in the continuous spectrum of the waves can extend from ∼0.1 up to 1.0Ω p ; and BIF, where a diminution around 0.5Ω p occurs between two activity peaks in the spectrum. These magnetic fluctuations in the PDL are considered to be the combined effects of two types of ion cyclotron waves: proton cyclotron waves and helium cyclotron waves, which are excited by the H + and He 2+ temperature anisotropies, respectively. In this paper, with one‐dimensional (1‐D) hybrid simulations we investigate the nonlinear evolution of the ion cyclotron waves excited by the H + and He 2+ temperature anisotropies. The proton cyclotron waves with the dominant frequency (the amplitude at that frequency is largest in the spectrum) larger than 0.5Ω p are first excited, and then the helium cyclotron waves with the dominant frequency smaller than 0.5Ω p are excited. The frequencies of the proton cyclotron waves decrease in their corresponding nonlinear stage. For β ∥ p = 0.1 (where β ∥ p is the parallel proton plasma beta), the dominant frequency of the proton cyclotron waves remains around 0.62Ω p because of the He 2+ absorption around the helium gyrofrequency. Therefore, after the helium cyclotron waves with the dominant frequency around 0.25Ω p are excited, there exist two activity peaks in the spectrum. At the quasi‐equilibrium stage, the amplitude of the proton cyclotron waves is very small because of the He 2+ absorption, and a continuous spectrum with main power below 0.5Ω p is formed. When β ∥ p = 0.3, the effect of the He 2+ absorption is very small and can be neglected. The frequencies of the proton cyclotron waves decrease in their nonlinear evolution. After the helium cyclotron waves are excited, their frequency band merges with that of the proton cyclotron waves and forms a continuous spectrum with main power extending above 0.5Ω p . At the quasi‐equilibrium stage, with the decrease of the frequencies of the ion cyclotron waves a continuous spectrum with main power below 0.5Ω p is formed. The relations between the three different spectral categories observed in the PDL and the spectra during the evolution of the ion cyclotron waves are also discussed. The results on the spectrum evolution of the ion cyclotron waves excited by the H + and He 2+ temperature anisotropies may help to explain the magnetic spectral signatures observed in the PDL.