## 1. Introduction

[2] Wave emissions in the whistler frequency range (0.1–0.5Ω_{e}, where Ω_{e} is the electron cyclotron frequency), called ‘lion roars,’ are a characteristic feature of the wave activity in the magnetosheath [*Smith and Tsurutani*, 1976; *Zhang et al.*, 1998; *Baumjohann et al.*, 1999] and magnetosphere [*Baumjohann et al.*, 2000; *Dubinin et al.*, 2007]. It has been shown previously that these emissions could be generated by a cyclotron resonant instability in the magnetosheath plasma with anisotropic electron temperature, *T*_{e⊥} > *T*_{e∥}, where *T*_{e⊥} and *T*_{e∥} are temperatures perpendicular and parallel to the magnetic field, respectively [*Thorne and Tsurutani*, 1981]. The main observed features of the waveforms are coherency and wave packet structure which are difficult to explain using linear theory which predicts a broader frequency range of excited waves with random phases. A scenario for generating nearly monochromatic whistler waves involves wave trapping in a mirror wave cavity which selects a narrow range of wavenumbers [*Treumann et al.*, 2000]. Another mechanism is based on the nonlinear properties of whistler waves where a class of solitary structures with embedded smaller-scale oscillations resembling wave packets has recently been found [*Sauer et al.*, 2002]. These nonlinear stationary waves, termed ‘whistler oscillitons,’ have been investigated using a two-fluid description which is valid for the low electron plasma beta (*β*_{e} ≡ 2*μ*_{o}*nkT*_{e}/*B*^{2} ≪ 1) regime.

[3] As a basic concept for explaining coherent whistler emission it has been suggested that the final stage of temperature anisotropy-driven whistlers has essential signatures of whistler oscillitons. In order to apply this to magnetosheath and magnetospheric plasma observations we must consider the effects of wave-particle interaction and this motivates us to employ electromagnetic particle-in-cell (PIC) simulations. A detailed discussion about this topic on the background of Cluster spacecraft observations is contained in a recent paper by *Dubinin et al.* [2007]. Generally, the appearance of stationary nonlinear waves is related to the existence of a ‘resonance point’ where phase and group velocity coincide. In the cold plasma approximation this point is at (*ω* = Ω_{e}/2, *kc*/*ω*_{e} = 1), where *ω*_{e} is the electron plasma frequency, and according to the kinetic dispersion theory, it shifts to smaller values of *ω* and *k* as *β*_{e} increases. Accordingly, results of PIC simulations are presented for *β*_{e} = 0.01 and *β*_{e} = 1.0.

[4] The paper is organized as follows; section 2 contains the results of PIC simulations starting from an unstable plasma which reaches a quasi-stationary state. In section 3 the dispersion of stationary whistler waves is discussed and using the kinetic approach it is shown how frequency and wavelength vary with parameters, such as *β*_{e}, which allows predictions of the most dominant solitary waves. In addition, the structure of oscillitons is calculated and compared with that of corresponding PIC simulations of a cold plasma. The time evolution of whistlers obtained from PIC simulations for *β*_{e} = 1.0 is found to be in good agreement with Cluster observations in the magnetosphere. Finally, summary and conclusions are given.