In plasmas where the thermal energy density exceeds the magnetic energy density ($beta_parallel > 1$), the aperiodic ordinary mode (O-mode) instability is driven by an excess of parallel temperature $A = T_perp /T_parallel < 1$ (where $parallel$ and $perp$ denote directions relative to the uniform magnetic field). When stimulated by parallel plasma streams the instability conditions extend to low beta states, i.e., $beta_parallel <1$, and recent studies have proven the existence of a new regime, where the anisotropy threshold decreases steeply with lowering $beta_parallel to 0$ 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. An hierarchy of these two instabilities is established for both the low $beta_parallel <1$ and large $beta_parallel > 1$ plasmas. The conditions where the O-mode instability can operate efficiently are markedly constrained by the electrostatic instabilities especially in the low-beta plasmas.
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