In visible light sun spots show up darker than the solar disc because they are cooler. In X-ray UV images why do they show up as brighter? In these images does brighter mean cooler and darker mean hotter relative to the solar disc?
1 Answer
Brighter images always mean hotter.
Explanation:
Sun spots are darker than the surrounding photosphere because they are cooler. Sun spots are caused by strong magnetic fields. These magnetic fields locally slow the convection process which is constantly mixing the hotter material deep inside the Sun with the outer layers.
Charged particles get accelerated along the strong magnetic field lines. This causes them to emit light at ultra violet and X-ray frequencies which are invisible to the eye.
When images are taken of sunspots at UV and X-ray frequencies the sunspot is still darker that the surrounding area. At these frequencies however the light emitted from the magnetic fields surrounding the sunspot become visible and are brighter than the surrounding areas.
