Airglow Patches in the Polar Cap Region: A Review

Polar cap airglow patches have been known as regions of enhanced 630.0 nm airglow detected by ground-based all-sky imagers at the polar cap latitudes well inside the main auroral oval. Although they were already recognized almost four decades ago as counterparts of polar cap (plasma density) patches, such airglow observations had not been utilized extensively for the studies of ionospheric structures and/or magnetosphere-ionosphere coupling processes in the polar cap. In the last two decades, following the development of highly-sensitive airglow imagers equipped with cooled CCD (Charge Coupled Device) cameras, it has become possible to visualize the dynamical temporal evolution and complicated spatial structure of airglow patches with improved signal-to-noise ratio. Such a progress has enabled us not only to use airglow patches as tracers for plasma convection in the polar cap but also to understand the generation of small-scale plasma irregularities in the ionospheric F region. In addition, recent observations demonstrated a case in which an airglow patch was accompanied by an intense flow channel and corresponding field-aligned current structure along its edges. This implies that airglow patches can signify magnetosphere-ionosphere coupling process in the region of open field lines at the polar cap latitudes, serving as a remote sensing tool just like auroras do. Further studies showed an association of airglow patches with the intensification of aurora on the nightside (Poleward Boundary Intensification: PBI and/or streamer) leading to the expansion phase onset of substorms. This paper reviews such recent progresses in the researches of airglow patches obtained by combining data from all-sky airglow imagers, radars and low-altitude satellite observations in the polar cap.