Electron concentration irregularities are often observed within Polar-cap plasma Patches, as evidenced by VHF scintillation and HF radar back-scatter observations. The current consensus theory of the formation of such irregularities is that a primary Gradient Drift Instability driven by the quasi-D.C. component of the mapped magnetospheric electric field, combined with the geo-magnetic field and a plasma concentration gradient, initiates a long-to-short wavelength cascade leading to variations in plasma concentration on the appropriate scale to explain the observations. However, other mechanisms have been proposed. A comprehensive statistical-combinatorial analysis of all hypothesised linear mechanisms, using Dynamics Explorer 2 satellite data, is presented and reveals that along with the GDI, the Current Convective Instability and Kelly-Kintner Turbulence (KKT) are processes that cannot be neglected, whilst the primary Kelvin-Helmholtz Instability is found not to occur. It is also shown that even in the non-linear regime, the KKT process is often dominant over the GDI.