Thermal Infrared Sky Background for a High-Arctic Mountain Observatory

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 derive from an automated Fourier-transform spectrograph which operated continuously there 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 N band, and to sub-Jansky levels by M and shortwards; reaching only 36 mJy arcsec −2 within L. Nearly six decades of twicedaily 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 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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