Dependencies of the Generation Process of Whistler-Mode Emissions on the Kinetic Energy of Anisotropic Electrons in the Earth's Inner Magnetosphere

Abstract:

Whistler-mode chorus emissions are electromagnetic plasma waves commonly observed in planetary magnetospheres. In the Earth's inner magnetosphere, chorus emissions are observed mostly on the dawn side and are enhanced during geomagnetically disturbed periods. Chorus emissions appear in the typical frequency range from 0.2 to 0.8 fce0 with a gap at the half fce0, where fce0 represents the electron gyrofrequency at the magnetic equator. Recent in situ observation in the magnetosphere revealed the presence of whistler-mode hiss-like emissions, whose wave amplitude is comparable to those of chorus emissions. By a series of electron hybrid simulations, we study dependencies of the generation process of whistler-mode chorus and hiss-like emissions on temperature anisotropy of energetic electrons. The generation process of chorus has been reproduced in electron hybrid simulations and has been explained by the nonlinear wave growth theory [see review by Omura et al., 2012]. The generation mechanism of hiss-like emissions is also explained by the nonlinear wave growth theory and has been reproduced by simulations [Katoh and Omura, 2013]. In the present study, by an improved electron hybrid code with OhHelp library [Nakashima et al., 2009], we conduct a series of electron hybrid simulations for different temperature anisotropy (AT) of the initial velocity distribution function of energetic electrons. We vary AT in the range from 3 to 9 with changing the number density of energetic electrons (Nh) so as to study whether distinct rising-tone chorus emissions are reproduced or not in the assumed initial condition. Simulation results reveal that Nh required for the chorus generation decreases as AT of energetic electrons increases and that reproduced spectra becomes hiss-like for large Nh cases. These simulation results clarify the validity of the nonlinear wave growth theory and suggest that the nonlinear wave-particle interactions play crucial roles in the generation process of whistler-mode chorus and hiss-like emissions in the magnetosphere.