First Daytime Red-Line Emission Measurements of the Stable Auroral Red (SAR) Arcs

We present the first ground-based measurements of daytime stable auroral red (SAR) arc using OI 630.0 nm emissions. SAR arc is a direct consequence of heat conduction from the inner-magnetosphere to the ionospheric regions characterized by increased electron temperatures and low electron density in the region of mid-latitude trough. So far, SAR arc emissions have only been reported for nighttime conditions. For the present study, daytime optical measurements were enabled using a high-resolution imaging echelle spectrograph from Boston (42.36°N, 71.05°W, MLAT ≈ 53°). Simultaneous Millstone Hill Incoherent Scatter Radar and Defense Meteorological Satellite Program measurements of electron density and temperature confirm our findings. Forward modeling approach enabled estimation of daytime electron temperatures in 200–650 km altitude during this SAR arc event to be varying between 3,500 and 4,400 K. These observations open numerous possibilities of optical investigations of magnetospheric-ionospheric interactions during daytime, when the upper atmosphere is dynamic with large gradients in ionospheric conductivities, temperatures, and winds.