The ion-ion mode is excited in plasmas whenever two cold ion populations drift relative to one another. Such conditions occur in the high-latitude auroral region associated with discrete auroral arcs due to the existence of parallel electric fields. These fields accelerate cold ionospheric ions (primarily H+ and O+ and on occasion He+) upward while accelerating plasma sheet electrons downward. In a previous study, two distinct regimes were found for the parallel propagating ion-ion mode depending upon the relative ion drift: (1) two-stream and (2) transition. Generalized analytic stability conditions are derived for the ion-ion mode when fast (light mass, e.g., H+) and slow (heavier mass, e.g., O+) ion distributions are present. In particular, marginal stability conditions are derived for the parallel propagating ion-ion waves in both regimes and peak growth conditions for the waves in the two-stream regime. These conditions are compared to numerical results. The effects of varying relative ion concentration, relative ion mass, and relative ion-electron temperature on the characteristic regimes of the ion-ion and ion acoustic modes are investigated. A study of oblique propagation is also presented. Results show that for parallel waves (1) the threshold of the instability occurs at a smaller drift for O+ than for He+ interactions when a finite ion temperature is assumed, and (2) the maximum growth rate, as well as the range of ion drift over which enhanced growth occurs, varies with ion concentration and species. For obliquely propagating waves it is found that whenever the ion-ion mode is in the two-stream regime for parallel propagation, peak growth occurs on axis.

When in the ion-ion transition and ion acoustic regimes, however, peak growth occurs off axis. Furthermore, the range of wave normal angles excited depends upon ion composition, relative ion drift, and ion concentration. These results are used to suggest that ion temperature relative to H+ drift energy remains approximately constant throughout the acceleration region. In addition, the nature of the wave emissions and occurrence of differential ion heating are discussed in connection with this mode. ¿ American Geophysical Union 1988