This image shows the main sunspot of active region 11823 as observed on August 121, 2013 at the Dunn Solar Telescope with the IBIS (Interferometric Bidimensional Spectrometer) and ROSA (Rapid Oscillations in the Solar Atmosphere) instruments.
In the presence of strong magnetic fields, convective heat transport is supressed and the solar plasma cools down. For this reason, sunspots are dark. The darkest region of a sunspot, called the umbra, is about 2000°C cooler than its granular vicinity. The penumbra encircling the umbra has a filamentary structure caused by the more horizontal orientation of the magnetic field lines. In the image, contours are used to identify the umbral and penumbral areas.
When the same sunspot is observed in different spectral lines and wavelength positions by spectroscopic techniques, as indicated on the left of the image, its appearance changes with increasing height in the solar atmosphere. In the photosphere (the deepest part of the atmosphere), the sunspot shows its familiar dark umbral and filamentary penumbral structure. But towards the chromospheric layers above, the filamentary magnetic field lines start to dominate the appearance of the sunspot. In the presence of the sunspot field, acoustic waves from the solar interior are transformed and channeled to the upper atmosphere. Eventually, those waves develop into shocks that can be observed as bright firework-like events in the umbral chromosphere.