The first simultaneous spectroscopic and monochromatic imaging observations of short-wavelength infrared aurora of N_2^+ Meinel (0,0) band at 1.1 μ m with incoherent scatter radar

This study presents a first simultaneous observation of N_2^+ Meinel (0,0) band (hereafter, N_2^+ (M)) aurora by cutting-edge short-wavelength infrared imaging spectrograph (NIRAS-2) and monochromatic camera (NIRAC) installed at the Kjell Henriksen Observatory (78 ^∘ N, 16 ^∘ E). On January 21 2023, N_2^+ (M) intensification that is associated with a band-shape aurora structure was observed by the NIRAS-2 and the NIRAC having temporal resolutions of 30 s and 20 s, respectively. In addition, the European incoherent scatter Svalbard Radar also observed electron density variations at the same time. Electron density measured at altitude range from 100 km 120 km shows similar variations as of N_2^+ (M) intensity, which implies that a primary source of N_2^+ (M) emissions is direct collisions of N_2 by precipitating electrons penetrating down to around 100 km altitude (up to 10 keV). However, the observation also demonstrated moderate correlations between N_2^+ (M) intensity and electron density above 140 km, which implies that different N_2^+ (M) generation process, N_2 charge exchange with O^+ , may work up to near 160 km and make a non-negligible contribution to N_2^+ (M) emissions. This hypothesis would be verified with further radar observations or stereo imaging observations useful to estimate the vertical distribution of the emission layers. The N_2^+ (M) is a very promising target wavelength for aurora observation because the quality of sensors is highly expected to improve further and further. Continuous observations with our new instruments will undoubtedly provide an important information of N_2^+ (M) characteristics, for future missions of both balloon-borne and satellite-borne imaging. Graphical Abstract