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Abstract Behavior of unstable plasma waves generated by the Farley‐Buneman instability (FBI) and the gradient drift instability (GDI) is analyzed in the transitional valley region near 120 km in altitude. The analysis is based on the expression for the FBI/GDI growth rateγthat has been recently generalized to include ion inertia effects for arbitrary altitude and wavelength, within the limits imposed by the fluid and local approaches. It is found that the ion inertia leads to a different instability behavior when the convection component is between the two critical values determined by the ion acoustic speedC_sand the ratior_ibetween the ion collision and gyrofrequency. The most interesting case occurs near 120 km, just below wherer_i=1. From analysis of electron density gradientsG=∇n/nthat result in marginal instability conditionγ=0 (i.e., critical gradientsG₀), there exists a critical scale whereG₀=0 and below which all waves are unstable to FBI. Above this scale,G₀>0 and gradients need to be sufficiently strongG>G₀for the plasma to become unstable through GDI. There also exists a maximum in dependence, which refers to the least unstable scale and gradient. For convection outside of the specified range, no critical or least unstable scale exists, which is a typical situation outside of the transitional valley region. Overall, this analysis shows that the FBI convection thresholds and the GDI critical gradients are modified by the ion inertia and that the effects are most pronounced in the transitional valley region near 120 km.