| Abstract | Nighttime zenith sky spectral brightness in the 3.3–20 μm wavelength region is reported for an observatory site nearby Eureka on Ellesmere Island in the Canadian High Arctic. Measurements are derived from an automated Fourier-transform spectrograph that operated there continuously over three consecutive winters. During that time, the median through the most transparent portion of the Q window was 460 Jy arcsec^-2, falling below 32 Jy arcsec^-2 in the N band, and to sub-Jansky levels by M and shortward, reaching only 36 mJy arcsec^-2 within L. Nearly six decades of twice-daily balloonsonde launches from Eureka, together with contemporaneous meteorological data plus a simple model, allows characterization of background stability and extrapolation into K band. This suggests that the study location has dark skies across the whole thermal infrared spectrum, typically sub-200 μJy arcsec^-2 at 2.4 μm. That background is comparable to South Pole and more than an order of magnitude less than estimates for the best temperate astronomical sites, all at much higher elevation. Considerations relevant to future facilities, including for polar transient surveys, are discussed. |
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