The outermost part of the Sun’s atmosphere, the corona, is highly inhomogeneous. Its building blocks are magnetic structures (also called coronal loops) that confine very hot and tenuous gas with temperatures above 1 million degrees. Ensembles of coronal loops, which connect points of opposite magnetic polarity in the photosphere, form so-called active regions. Due to the high temperatures of the gas confined in the loops, the corona emits mainly in the ultraviolet (UV) and X-ray bands.