On the detached green proton auroral structure during Canada Thanksgiving storm

A geomagnetic storm, also-known-as the Canada Thanksgiving storm, occurred on 12 October 2021 night. A number of subauroral optical structures were seen over Canada, and aroused keen interest and heated discussions among citizen/professional scientists. In this study, we focus on a proton auroral arc structure that appeared to detach from the main auroral oval and move equatorward into subauroral region during ~0245-0345 UT on the Canada Thanksgiving storm night. Using a comprehensive set of optical instruments, including the multi-wavelength all-sky imager (ASI) at Athabasca, Canada, the Four-Eight-Six-One (FESO) photometer at Athabasca, the Transition Region Explorer (TREx) RGB ASI at Lucky Lake, Canada, and the TREx spectrograph at Lucky Lake, we perform a detailed investigation on the evolution and spectrographic properties of such a detached proton auroral arc structure. The detached arc features intense Hb (486.1 nm) emissions of up to a few hundred Rayleigh, indicating strong energetic proton precipitation. CARISMA Magnetometer data registered pronounced electromagnetic ion cyclotron (EMIC) waves when the detached proton auroral arc passed over the station, strongly suggesting an inherent link between the EMIC wave and the proton precipitation. The 557.7 nm green-line constituted the strongest emission line of the detached arc, so that the arc was visually greenish. Via careful examination of the spectral intensities and the structural shapes of the 557.7, 427.8, and 486.1 nm emissions based on the spectrograph data, we conclude that the 557.7 nm emissions of the detached arc were unlikely to owe their main source to energetic electron precipitation from the magnetosphere, but were mostly the byproduct of the proton precipitation, e.g., excited by secondary electrons produced in the energetic proton precipitation, and/or led by the impact of fast hydrogen atoms. This conclusion is corroborated by the observation from a DMSP satellite passage in the conjugate southern hemisphere, which showed the presence of energetic proton precipitation but no counterpart electron precipitation when the satellite traversed the conjugate region of the detached proton arc. The emission intensities of various lines inferred form the spectrograph data are compared with the outcome of proton transport models.