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.
Image Copyright: Álvaro Ibáñez Pérez
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.
The chromosphere is a place of high solar activity. In the course of a few minutes we can observe changes in the ejecta and prominences, in the path of the filaments, and as matter flows following very characteristic arches. Chromosphere is also visible in the light emitted by the ionized calcium, in the violet part of the solar spectrum in a wavelength of 393.4 nm. This light comes from calcium atoms that have lost an electron.