Plasma diagnostics aboard the S3-4 satellite have provided direct measurements of high-latitude F region ionospheric density irregularities in the wavelength domain of tens of meters to hundreds of kilometers. The irregularities are found to be ubiquitous, generally populating the auroral oval, the polar cap, and the dayside oval/polar cusp area with varying degrees of intensity and scale size distributions. To investigate the causal mechanisms for various irregularity distributions, including the primary input term from precipitating particles and possible plasma instability processes, we have spectrally analyzed 20 orbits of data during geomagnetically quiet and moderately disturbed conditions (Kp≤3). In the lower F region (Z~200 km), where molecular ions dominate, power spectra of medium-scale plasma density structures (1--50 km) can be characterized by a weak most probable spectral index of 2.8 for the nightside-dayside ovals and the polar cap and an index of 1.4 for the dusk oval. At higher altitudes (Z≥260 km), where O+ is the dominant ion, the analyses show a well-defined most probable spectral index of 1.9 for all the high-latitude regions. Our spectral analyses suggest that at the molecular-ion-dominant altitudes, fast chemical recombination inhibits plasma convection/diffusion and that the observed irregularities are primary the consequence of locally structured particle precipitation patterns. On the other hand, in the O+-dominant F region, particle-produced irregularities are long lived. Such irregularities convect into and across the polar cap and can be a long-lived input term for a number of plasma instability processes.

As a result, the molecular-dominant polar cap F region (Z≤200 km) is generally free of plasma disturbances, while its atomic-dominant counterpart (Z≥260 km) is mostly structured with a broad range of intense plasma density irregularities.