Estimation of energy of precipitating electrons causing pulsating aurora and omega band: Digital camera observations from ISS

Abstract:

Astronauts have been taking a large number of auroral photographs from the International Space Station (ISS) using digital single lens reflex cameras. We have introduced a method for projecting the photographs of aurora from ISS onto the geographic coordinate system using city lights in the images as markers [Nanjo et al., 2020]. The FOV of the ISS photography sweeps through a long distance (4-5 hours in local time) in a short time interval (~10 min). In addition, because the images are taken in the rim direction of the Earth, they allow us to visualize the height structure of the aurora, which is difficult to be revealed by optical observations from the ground and space.

The purpose of this study is to estimate the energy of the precipitating electrons from the color of auroras in the ISS images. For pulsating auroras (PsAs) which are known to have a narrow range of emission altitude, we tried to infer the average energy from the B/G (blue to green) ratios of the images. This idea is based on the fact that the 557.7 nm emission should contribute to the G channel and the band emission of molecular nitrogen ions should contribute to the B channel. During the two cases of PsA, the B/G ratio tended to be higher on the morning sector than on the midnight sector. This tendency has always been reported in a number of studies, indicating that the B/G ratio from the ISS images can be a qualitative proxy for the energy of precipitating electrons.

We also investigated the colors of aurora during so-called omega bands and identified unique wall-like aurorae in the western boundary of torches. There wall-like aurorae are tall in altitude and thin in the longitudinal direction; thus, we call them "Great Walls". To quantitatively validate the energy of the electrons precipitating into the Great Wall, we compared the field-aligned altitude profiles of luminosities of the RGB channels with those of the 557.7 nm and 630.0 nm obtained from the GLOW model. The Great Walls were found to extend for more than 200 km in altitude and contain greenish and reddish colors respectively at the bottom and top of them, which is inconsistent the previous study showing only 2-5 keV electron precipitations on the western flank of the torches [Amm et al., 2005]. In the presentation, we will discuss the detail of the analysis and introduce a future plan aiming at the quantitative use of B/G ratios.