Generation of high‐frequency broadband electrostatic noise: The role of cold electrons

Abstract

Broadband electrostatic noise (BEN) is commonly observed in the plasma sheet boundary layer in association with ion beams. Here we investigate the generation of these waves in a plasma consisting of an ion beam and a background of hot ions, hot electrons, and cold electrons. The cold electrons are of ionospheric origin. A complete, systematic study of electrostatic ion beam instabilities, including cold electrons, has been done, and it is shown that for the plasma configuration described, four instabilities can be excited. These are the (1) ion acoustic, (2) Buneman, (3) beam resonant, and (4) electron acoustic instabilities. A low and high beam temperature division is shown to exist that separates when different instabilities can be excited. For typically observed parameters in the plasma sheet boundary layer, the ion beams lie in the high‐temperature regime. In this regime, the beam resonant and electron acoustic instabilities are excited, and these instabilities can account for the high‐frequency (> 500 Hz), low‐power portion of the BEN spectrum. In the absence of cold electrons, no such wave growth occurs.