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The limb (edge of the sun) of the x-ray corona is usually fainter on the disk than outside of the disk. This is because the line of sight goes through more material above the limb. At the N pole (up), the limb is especially well-defined because of the presence of brighter corona behind it. In the case of a spherically-symmetric corona, there should be a simple factor of two jump in brightness at the limb, due to line-of-sight addition.