Generation and propagation of chorus emissions observed by Geotail in the dayside outer magnetosphere

S. Yagitani, I. Nagano, H. Matsumoto, Y. Omura, W. R. Paterson, L. A. Frank, and R. R. Anderson
Proc. of ISAP '96, Chiba, Japan, 717-720, 1996.

Many chorus emissions have been observed by the GEOTAIL spacecraft, mainly in the dayside outer magnetosphere. The Plasma Wave Instrument (PWI) onboard GEOTAIL makes it possible to examine not only the overall activities of the emissions but also a detailed analysis of wave normal and Poynting directions of each rising/falling frequency tone. During the chorus activities, energetic electrons responsible for the cyclotron generation of the emissions have also been measured by the Comprehensive Plasma Instrument (CPI) with high energy and time resolutions. Comparison between these high-quality in-situ wave/particle data is expected to give experimental evidence to understand complicated nonlinear wave-particle interaction involved in the generation mechanism of the chorus emissions. In this paper, we analyze generation and propagation of chorus emissions observed by the PWI and compare them with simultaneously observed energetic electrons by the CPI, in the dayside outer magnetosphere (near the magnetopause), on October 17-18, 1992. Computing the cyclotron growth rates from the velocity distributions of energetic electrons observed by the CPI, we have shown that the chorus emissions observed by the PWI are most likely generated by those electrons.