Light from singly-ionized calcium ions in the Sun’s upper photosphere and chromosphere (up to 2000 km altitude). Because the blue Calcium K Line (393.3 nm) is sensitive to magnetic fields, magnetically active structures show up in high contrast against the surrounding chromosphere. Places where moderate magnetic fields exist show up bright whereas images of high magnetic fields are dark.
In this CaK image, you typically see brightness along the edges of large convection cells called supergranules and in areas called plages. Dark sunspots are also visible.
Spots on the sun are areas of high magnetic field which appear dark to their surroundings (5,800K) due to their cooler temperature of around 3000-3500K. Spots consist of a dark central region (umbra) and are surrounded by an annular region of dark and bright filaments called the penumbra. Within a developing active region (sunspot group) tiny spots form initially without a developed penumbra and are called pores. These are usually relatively short lived or can develop a penumbra and become a fully developed spot.