On the origin and spatial extent of high‐latitude F region irregularities

Abstract

Further evidence is presented that structured soft‐electron precipitation is an important source of large‐scale (λ ≳ 10 km ) F region ionization irregularities in the high‐latitude ionosphere. We show that large amplitude, 20‐km to 80‐km structured plasma exists in both the dayside and nightside auroral oval. In the latter case, the structured F layer plasma has been observed to convect into and through the field of view of the Chatanika radar. Here, and in a companion paper, we hypothesize that this soft electron precipitation is the primary source of high‐latitude structure and that convection acts to distribute the irregular plasma throughout the polar ionosphere. Indeed, at the observed production scale (20–80 km), irregularities can easily convect throughout the high‐latitude region with negligible decay from classical or anomalous diffusion, including the effects of a conducting E region. Our convection/decay model also explains the following observed features of published high‐latitude irregularity data: (1) The steep gradient in irregularity intensity at the equatorward edge of the nighttime magnetospheric convection zone. (2) The existence of irregularities throughout the polar cap. (3) The reduction of irregularity intensity in the central polar cap. (4) The dawn‐dusk asymmetry in the equatorward boundary of the high‐latitude irregularity zone. On the other hand, if classical theory is applied to a 100‐m scale structure, diffusion should rapidly limit the irregularities to an area within a few degrees of the production zone, a prediction not upheld by experimental data. Thus, plasma instabilities must operate on the large‐scale structures to produce the observed intermediate scale (100 m < λ < 10 km) power law irregularity spectra. Calculations of the instability growth rate in its generalized form, which includes field‐aligned currents (the current convective instability), shows that growth should occur. The universal drift instability should then operate on these intermediate scale features. We suggest that this latter instability maintains the shorter scale plasma structure at the expense of a more rapid decay of the intermediate scale irregularities in a cascade‐like process. Enhanced, turbulent diffusion can reduce significantly the lifetime of the intermediate scale irregularities but not of the larger structures, which can therefore still transit the polar cap.