Velocity shear and current driven instability in a collisional F-region

Abstract. We have studied how the presence of collisions affects the behavior of instabilities triggered by a combina-tion of shears and parallel currents in the ionosphere under a variety of ion to electron temperature ratios. To this goal we have numerically solved a kinetic dispersion relation, using a relaxation model to describe the effects of ion and electron collisions. We have compared our solutions to expressions derived in a fluid limit which applied only to large electron to ion temperature ratios. We have limited our study to thresh-old conditions for the current density and the shears. We have studied how the threshold varies as a function of the wave-vector angle direction and as a function of frequency. As expected, we have found that for low frequencies and/or elevated ion to electron temperature ratios, the kinetic disper-sion relation has to be used to evaluate the threshold condi-tions. We have also found that ion velocity shears can sig-nificantly lower the field-aligned threshold current needed to trigger the instability, especially for wave-vectors close to the perpendicular to the magnetic field. However the current density and shear requirements remain significantly higher than if collisions are neglected. Therefore, for ionospheric F-region applications, the effect of collisions should be in-cluded in the calculation of instabilities associated with hori-zontal shears in the vertical flow. Furthermore, in many situ-ations of interest the kinetic solutions should be used instead of the fluid limit, in spite of the fact that the latter can be shown to produce qualitatively valid solutions