A new mechanism for exciting the kinetic ion cyclotron waves in the presence of a nonuniform electric field perpendicular to the external magnetic field is given. Application of this instability to various space plasmas is discussed. The new instability mechanism may provide a more efficient agent for perpendicular ion heating than other EIC generation processes, since the linear growth rate is insensitive to the temperature ratio.

Ion-cyclotron turbulence has been observed with shocks and double layers in the magnetosphere where strongly localized electric fields perpendicular to the magnetic field are present. Theoretical analysis suggests that electrostatic waves with frequencies of the order of the ion-cyclotron frequency can be destabilized as a result of the coupling of regions of positive and negative-energy ion waves. The nonlocal theory for a smooth profile of transverse inhomogeneous electric fields shows that localized ion waves grow in the region where the electric fields are present. Using a spatially two-dimensional electrostatic code, we investigate this instability in plasma conditions characterized by a localized transverse electric field L≪Lx, where Lx is the simulation length in the x direction; and distinguish it from the transverse kinetic Kelvin–Helmholtz instability. The simulation results show that the growing ion waves are associated with the small vortices in the linear state, which evolve into a nonlinear stage dominated by large vortices with lower frequencies.

Using a spatially two‐dimensional electrostatic particle simulation code, we examine the stability of a plasma equilibrium characterized by a localized transverse dc electric field and a magnetic‐field‐aligned electron drift for L ≪ Lx, where Lx is the simulation length in the x direction and L is the scale length associated with the dc electric field. It is found that the dc electric field and the field‐aligned current can together play a synergistic role to enable the excitation of electrostatic waves even when the threshold values of the field‐aligned drift and the E×B drift are individually subcritical. The simulation results indicate that a broadband turbulence is associated with such an equilibrium. This is in contrast to the current‐driven ion cyclotron instability, which is characterized by a coherent spectrum around the ion cyclotron harmonic. Further, the growing ion waves are associated with small vortices in the linear stage, which evolves to a nonlinear state dominated by larger vortices with lower frequencies.

The stability of a stratified shear layer is investigated using an exponential density profile and a laminar shear flow with a continuous velocity distribution. It is shown that an exact stability boundary can be obtained for an inhomogeneous inviscid fluid under the action of gravity without the need to impose the Boussinesq approximation. The stability boundary is given by J=k̂2(1−β2/4−k̂2), where β is the ratio of the velocity and density gradient scale sizes, J is the Richardson number, and k̂ is the perpendicular wavenumber normalized to the velocity gradient scale size; this reduces to the stability boundary derived by Drazin [J. Fluid Mech. 4, 214 (1958)] in the limit β=0. The solution allows for c=β/2, where c is the normalized phase velocity.

Observations from the DE 2 satellite indicate the existence of a broadband turbulence associated with sheared ion flows. A recent attempt [Basu and Coppi, 1988, 1989] to explain the observed spectrum by electrostatic waves that are driven by a sheared ion flow along the magnetic field ignores the stronger shear in the ion flow transverse to the magnetic field. We show that when the shear in the transverse ion flow is also considered the modes described by Basu and Coppi [1988, 1989] are easily stabilized and their theory breaks down even when the shear in the parallel flow exceeds the shear in the transverse flow. Hence, the explanation of the DE 2 observations based on these modes is questionable.