On the marginal instability threshold condition of the aperiodic ordinary mode

Abstract

The purely growing ordinary (O) mode instability has recently received renewed attention owing to its potential applicability to the solar wind plasma. Here, an analytical marginal instability condition is derived for counter-streaming bi-Maxwellian plasma particle distribution functions. The derived marginal instability condition as a function of the temperature anisotropy and plasma beta agrees remarkably well with the numerically determined instability condition. The existence of a new instability domain of the O-mode at small plasma beta values is confirmed with the leading A∝β∥−1-dependence, if the counter-stream parameter Pe exceeds a critical value. At small plasma beta values at large enough counter-stream parameter, the O-mode also operates for temperature anisotropies A = T⊥/T∥ > 1 even larger than unity, as the parallel counter-stream free energy exceeds the perpendicular bi-Maxwellian free energy.