From: NASA HQ
Posted: Saturday, April 2, 2011
Two topics need to be covered this week. We''ll do one today. What happened while SDO exited the eclipse on March 29? The pictures at the left show images in four AIA wavelengths (304, 171, 193, and 335) at about 07:14:50 UT Tuesday. The edge of the Earth is the ragged line across the southern hemisphere of the Sun. Where is the sharp line seen in the HMI movie from the FirstLight gallery or the edge of the moon in a lunar transit?
The answer is a combination of atmospheric absorption and color tables. The sharp line in HMI is in the visible spectrum and emphasizes the thinness of the troposphere that we live in. Light at the EUV wavelengths of AIA is completely absorbed much higher in the EarthaO(TM)s atmosphere (at an altitude of about 300 km at the limb). Even small amounts of atmosphere remove the light from the picture.
Then the image is processed into an image we can see by changing to a log intensity and scaling to a color table. The log intensity makes the bright bits and dim bits visible in the same image but over-emphasizes the dim background. Even a little absorption is enough to cause a dim region to drop to below the minimum intensity allowed in the color table, hence the irregular border that traces out the dimmer regions seen in the following uneclipsed image.
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