Radio signals traversing polar-cap plasma patches and other large-scale plasma structures in polar regions are prone to scintillation. This implies that irregularities in electron concentration often form within such structures. The current standard theory of the formation of such irregularities is that the primary Gradient Drift Instability drives a cascade from larger to smaller wavelengths that manifest as variations in electron concentration. The electric field can be described as the sum of a quasi-D.C. and an A.C. component. Whilst the effect of the quasi-D.C. component has been extensively investigated in theory and by modelling, the contribution of the A.C. component has been largely neglected. This paper investigates the relative contributions of both components, using data from the Dynamics Explorer 2 satellite. It concludes that the contribution of the A.C. electric field to irregularity growth cannot be neglected. This has consequences for our understanding of large-scale plasma structures in polar regions (and any associated radio scintillation) as the A.C. electric field component varies in all directions. Hence, it effect is not limited to the trailing edge of such structures, as it is for the quasi-D.C. component. This raises the need for new experimental and modelling investigations of these phenomena.