The image above shows the Sun as a purple disk because it was taken in ionized calcium light. This is light from calcium atoms that have lost an electron. It peaks in the violet part of the spectrum (393.4 nm) and is sensitive to magnetic fields — magnetically active structures are easier to detect in this spectral band. Image © Alvaro Ibanez Perez; Alvaro’s Web site
Moderate magnetic fields show up brightly (hot), whereas high magnetic fields are quite dark (cool). The brightest regions are found in the chromosphere and are known as “plages.” The darkest regions are sunspots on the Sun’s photosphere. These spots are approximately 1,500 – 2,000 K cooler than their surroundings. Over much of the photosphere granulation can also be identified.
On the left edge of the disk (at center), note the bright solar prominence, and on the right edge there’s an incredible solar flare equivalent to 25 Earths in length. This huge flare lasted only about 25 minutes. Image acquired on May 13, 2013.
Photo details: Image made with a Coronado PST CaK solar telescope; bandwidth of 2.2 angstroms centered at 393.4 nm (very dim). QHY5-II camera — monochrome. Summary Author: Alvaro Ibanez Perez