Electromagnetic ion-beam-plasma instabilities

It is well known that ion beam-plasma interactions can destabilize right and left-hand polarized electromagnetic waves. However, these instabilities have been studied for frequencies less than the ion-beam gyrofrequency, with the exception of Goldstein and Wong (1987), and Brinca and Tsurutani (1988), who studied also the region of high frequencies (omega > Omega(i), where Omega(i) is the ion-beam gyrofrequency), in connection with pickup cometary ions. It has been recently demonstrated that in the cold plasma theory, left and right-hand modes can be destabilized in a rather large range of frequency and wavenumber values. It has also been shown that, in some cases, the unstable spectrum has a band structure. We show here that for large beam velocities the spectrum has two unstable regions of very nearly monochromatic waves. This particular result seems to be in agreement with observations in the Earth's foreshock as reported by Thomsen et al., 1985. When thermal effects are considered the low-frequency unstable region remains unchanged, while the high-frequency instability is strongly affected. It is also shown that under some conditions a positive beam thermal anisotropy increases the growth rate of the high-frequency right-hand waves.