The corona shines brightly in x-rays because of its high temperature. On the other hand, the “cool” solar photosphere emits very few x-rays. This allows us to view the corona across the disk of the Sun when we observe the Sun in X-rays. To do this we must first design optics that can image x-rays and then we must get above the Earth’s atmosphere.
Image and caption courtesy of NASA/MSFC Solar Physics.