Electrostatic oscillations due to filamentary structures in the magnetic-field-aligned flow: The ion-acoustic branch

Recent space missions such as FAST and Freja report highly structured plasm a hows along the magnetic field. Electrostatic fluctuations that can be sup ported by such inhomogeneous parallel hows are investigated. It is found th at even a small transverse gradient in parallel flow can significantly redu ce the critical value of the relative ion-electron field-aligned drift for the current-driven electrostatic ion acoustic modes. It is also shown that the shear-modified ion acoustic mode can be excited without any relative fi eld-aligned drift provided that the flow gradient is sufficiently strong. T he instability mechanism can be described in a local limit. The new shear-m odified ion-acoustic modes considered in this paper are shown to be differe nt from both the nonresonant mode due to a velocity shear in the parallel f low [D'Angelo, 1965] and the resonant classical current-driven ion-acoustic mode [Fried and Gould, 1961]. The new modes discussed in this paper can be excited for typical ionospheric conditions and a wide range of ion-electro n temperature ratios. The reported results may explain ionospheric observat ions of low-frequency ion-acoustic-like waves, especially for ion-electron temperature ratios of the order of unity and larger when the critical curre nt for the homogeneous current-driven ion acoustic mode is significantly ab ove the observed values. Other possible applications of our results to spac e plasmas are also discussed.