Constraints for the aperiodic O-mode streaming instability
In plasmas, where the thermal energy density exceeds the magnetic energy density (β{sub ∥} > 1), the aperiodic ordinary mode (O-mode) instability is driven by an excess of parallel temperature A = T{sub ⊥}/T{sub ∥} < 1 (where ∥ and ⊥ denote directions relative to the uniform magnetic field). When stimulated by parallel plasma streams, the instability conditions extend to low beta states, i.e., β{sub ∥} < 1, and recent studies have proven the existence of a new regime, where the anisotropy threshold decreases steeply with lowering β{sub ∥} → 0 if the streaming velocity is sufficiently high. However, the occurrence of this instability is questionable especially in the low-beta plasmas, where the electrostatic two-stream instabilities are expected to develop much faster in the process of relaxation of the counterstreams. It is therefore proposed here to identify the instability conditions for the O-mode below those required for the onset of the electrostatic instability. A hierarchy of these two instabilities is established for both the low β{sub ∥} < 1 and large β{sub ∥} > 1 plasmas. The conditions where the O-mode instability can operate efficiently are markedly constrained by the electrostatic instabilities especially in the low-beta plasmas.