Comparison of dark pixels observed by VIS and UVI: in dayglow images

Frank and Sigwarth [1997a] claim that the dark pixels observed in dayglow images obtained by the Earth sensor of the Visible Imaging System (VIS) are due to bombardment of Earth by 20 to 40 ton cosmic snowballs. We have independently studied the VIS data and compared the dark pixels from the VIS images to those obtained from the overlapping images from the Ultraviolet Imager (UVI). We find the occurrence distributions of the dark pixels, single and multiple, from VIS and UVI are nearly identical. The distributions also do not show any altitude dependence. A search for evidence of spacecraft "wobble" motion, whose presence would indicate that the source is external to the camera, has found that pairs of dark pixel clusters are uniformly distributed in orientation and no preference is observed in the wobble direction. Instrument artifacts as the source of the dark pixels is the most likely explanation for these results. The conclusion of this study is that neither VIS nor UVI provide any scientific evidence that the dark pixels are geophysical.