Faint Diffuse 557.7‐nm Emissions Equatorward of the Auroral Oval: Conjugate Observations by Ground Airglow Imagers and the Arase Satellite

Abstract

At subauroral latitudes, discrete auroras, such as strong thermal emission velocity enhancement and stable auroral red arcs, have been extensively studied. On the other hand, diffuse emissions at subauroral latitudes have not been studied well. In this paper, we report eight events of conjugate observations of faint diffuse 557.7‐nm emissions with intensities of 300–1,000 rayleighs without discrete structures equatorward of the auroral oval, using the Arase satellite and high‐sensitivity all‐sky airglow imagers in North America. Ground‐magnetosphere conjugate observations of such diffuse emissions have not been previously reported. Diffuse 557.7‐nm emissions were observed during both geomagnetically disturbed and quiet conditions. Preference for magnetic local time was not found in these eight events. Arase observed plasma‐sheet electrons with energies from several hundred eV to beyond 10 keV in the source magnetosphere, which is located inside the plasmapause. Electromagnetic waves, such as plasmaspheric hiss and lower‐band chorus waves, were also observed there by the Arase satellite. For five events, the resonance energies estimated from the observed wave frequencies ranged from 30 eV to 30 keV, suitable for generating the 557.7‐nm emission, though the pitch‐angle diffusion coefficient estimated for one event was not so large. We discuss the possibility that plasma‐sheet electrons in the plasmasphere precipitated into the ionosphere through wave‐particle interaction, forming faint diffuse 557.7‐nm emission equatorward of the auroral oval.