Chorus intensification in response to interplanetary shock

Abstract

On 3 September 2009, the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites observed a significant intensification of chorus in response to the interplanetary shock in the Earth's dayside plasma trough. We analyze the wave‐particle interaction and reveal that the chorus intensification can be caused by the gyroresonance between the chorus and the energetic electrons. When the electrons are scattered from resonance points to low‐density regions along the diffusion curves, a part of their energy can be lost and then transferred to amplify the chorus. During the compression of the magnetosphere, the temperature anisotropy of electrons is enhanced. This makes the electron diffusion and chorus intensification very effective. The maximum growth rate after the shock is about 50% greater than that before the shock. The lower‐energy (15–25 keV) electrons contribute more to the growth of chorus due to the larger density gradient along the diffusion curve. The <10 keV electrons are almost isotropic, so they contribute little to the amplification of chorus. We investigate the free energy for the chorus intensification and find that it can be generated through the local betatron acceleration and radial diffusion processes. The local betatron acceleration results from the shock‐induced compression of the magnetosphere. The linear and nonlinear growth rates are also compared. We find that the linear diffusion process works well for the present case.