English: , IRIS will observe the lowest part of the sun's atmosphere: the chromosphere, an expanse of ionized gas or plasma lying just above the sun's surface, and the transition region, where the chromosphere transitions into the even hotter corona above. This interface region lies at the core of many outstanding questions about the sun's atmosphere, such as how the sun creates giant explosions like solar flares or coronal mass ejections (CMEs), or how solar material in the corona reaches millions of degrees, several thousand times hotter than the surface of the sun itself. Understanding the interface between the photosphere and corona remains a fundamental challenge in solar and heliospheric science.