Velocity-shear origin of broadband electrostatic noise

Summary form only given, as follows. Multiple, independent eigenmodes of an inhomogeneity-driven instability are observed simultaneously with large and comparable amplitude. This is in sharp contrast to the usual case in plasmas in which one eigenmode dominates or, for conditions far above the excitation threshold, a collection of harmonics appears. The significance is that the spectral features are not separated by the ion gyrofrequency ω/sub ci/, or any other normal-mode frequency of the plasma. This has implications regarding the spectral width of the fluctuations, which, in some cases, can exceed 50% of center frequency. Furthermore, the spectral features are Doppler downshifted, in some cases to frequencies associated with ion-acoustic modes. For example, ω/ω/sub ci/=0.25 are readily observed (recall that T/sub i//T/sub e/ is of order unity). These facts are particularly unexpected because the plasma conditions correspond to large excitation thresholds for current-driven electrostatic ion-cyclotron (CDEIC) and ion-acoustic (IA) waves. The results have direct relevance to recent SCIFER and AMICIST rocket experiments in the Earth's cleft ion fountain where electrostatic waves (ω<ω/sub ci/) are observed and are strongly correlated with transverse ion heating and reduced plasma densities, but with the unexplained characteristic that the spectral features are not separated by ω/sub ci/.