Time-varying Excess Earth-glow Backgrounds in the WFC3/IR Channel ABSTRACT

We investigate the cause of strong time-dependent variations of the background levels observed in a variety of filters of the infrared (IR) channel of the Wide-Field Camera 3. These variations appear within an exposure in the F105W and F110W filters and both IR grisms (G102 and G141), with background count rates reaching 5 e − /s/pix in extreme cases (c.f. the zodiacal background is typically ∼1 e − /s/pix.). We use IR spectra from both grisms to identify the source of this excess background to be a strong, spatially-diffuse, emission line at 10,830 ˚A from metastable helium atoms in the solar-illuminated upper atmosphere. The He background contribution is negligible in Earth’s shadow but increases sharply when the telescope is outside of the shadow. The He background increases with decreasing limb angle but can also be significant even high (>40 ◦ ) above the Earth limb, affecting, to variable extent, a majority of observations that use the spectral elements sensitive to the feature. 1