Generation Mechanism of Low-Frequency Waves in the Plasma Sheet Boundary Layer:
A Consequence of the Reconnection Jet Observed by Geotail

T. K. Takada (Nagoya Univ.), K. Seki, M. Hirahara, 
M. Fujimoto, Y. Saito, H. Hayakawa, and T. Mukai

The plasma sheet boundary layer (PSBL) of the Earth's magnetosphere connects 
to the nightside reconnection region along the field line. The reconnection 
jets are often observed as an ion and electron beams in the PSBL and propagate 
earthward from the reconnection region. On one hand, the broadband 
electrostatic and electromagnetic waves are also observed in this region. 
Although a variety of beam-related instabilities have been considered as the 
generation mechanism of these waves, the dominant wave generation mechanism in 
PSBL still remains to be clarified. 
In this study, we examine the statistical properties of low-frequency 
electromagnetic waves and ion beams in the PSBL using Geotail spacecraft data 
taken from 1996 to 2000. Utilizing the observed ion distribution functions as 
inputs, the linear electromagnetic dispersion relations are solved and the 
dominant mechanisms of wave generation are discussed in detail. The results 
suggest the importance of the ion cyclotron anisotropy instability (due to the 
temperature anisotropy, Tpara < Tperp) modified by the existence of cold-core 
ions for the generation of low-frequency large-amplitude electromagnetic waves 
in the PSBL.