Nonlinear Wave Growth Analysis of Whistler‐Mode Chorus Generation Regions Based on Coupled MHD and Advection Simulation of the Inner Magnetosphere

Abstract

We show the regions where nonlinear growth of whistler-mode chorus waves is preferred to occur in the inner magnetosphere. A global magnetohydrodynamics (MHD) simulation was used to obtain large-scale electric and magnetic fields under the southward interplanetary magnetic field condition. With the electric and magnetic fields obtained by the MHD simulation, we ran a comprehensive inner magnetosphere-ionosphere model to solve the evolution of phase space density of electrons. Hot electrons originating from the tail region drift sunward and penetrate deep into the inner region due to a combination of convection and substorm-associated electric fields. Cold electrons also drift sunward, resulting in a contraction of the plasmasphere. We obtained the following results. (1) The whistler waves can first grow due to the linear mechanism (pitch angle anisotropy) in the premidnight-prenoon region outside the plasmapause, followed by rapid, nonlinear mechanism accompanied with rising-tone chorus elements. (2) When the solar wind speed is high, the whistler waves grow more efficiently due to linear and nonlinear mechanisms over a wider area because of deep penetration of hot electrons and the large contraction of the plasmasphere. This is consistent with the observation that the outer belt electrons increase for the fast solar wind.(3) For slow solar wind, the linear growth is mostly suppressed, but the nonlinear growth can still take place when external seed waves are present. This may explain the persistence of dawn chorus and largeamplitude chorus waves that are often observed in the premidnight-postdawn region in relatively weak geomagnetic activities.

Possible Global Generation Region of Nonlinear Whistler‐Mode Chorus Emission Waves at Mercury

“…The wave properties (e.g., intensity variations as a function of frequency at the Earth's magnetosphere by Yagitani et al, 2014 andat Saturn by Menietti et al, 2013) were characterized very well by the nonlinear growth theory. Not only the wave characteristics at a fixed location, Ebihara et al (2020) showed that the spatial development of the chorus wave generation in the Earth's magnetosphere can be reproduced by applying the nonlinear growth theory referred from the global magnetohydrodynamics and advection simulations in both fast and slow solar wind conditions. They showed that a wider spatial development of chorus generation distributing from in the premidnight-prenoon region outside the plasmapause in the fast solar wind case can be reproduced from a deep penetration of hot electrons into the inner region around up to L = 4 and a steep contraction of the plasmapause in the simulation results.…”

Coupling the Rice Convection Model‐Equilibrium to the Lyon‐Fedder‐Mobarry Global Magnetohydrodynamic Model

“…The one-way coupling methodology has also been widely used in driving a kinetic RC model by a global magnetosphere model with a purpose of reproducing certain physical processes in the inner magnetosphere with a time-evolving global environment (Buzulukova et al, 2010;Ebihara et al, 2020;Ebihara & Tanaka, 2013;Hu et al, 2010;Zaharia et al, 2010;Zhang et al, 2008Zhang et al, , 2009. Unlike the two-way coupling, the RC model does not provide feedback to the global magnetosphere model.…”