The Sun - Photosphere and Chromosphere

Image of a sunspot at close range. Granulation can be recognized in the outer area. The black area (umbra) and the ring around it (penumbra) together create the sunspot. The width of the image corresponds to about three times the diameter of the earth. (Source: F. Woeger (KIS), C. Berst, M. Komsa (NSO/AURA/NSF))

The photosphere and chromosphere are for many reasons very interesting areas of the sun. The photosphere, which possesses an effective temperature of 5700 kelvins, can be directly observed because almost all of the total radiation of the sun comes from this layer. The chromosphere is only visible during a solar eclipse, when the much brighter photosphere of the sun is covered by the moon.  Both of these areas are interesting because many interactions between radiation, magnetic fields and convection can be studied there.

The granulation, which can be seen in the image as a honeycombed structure in the entire outer area, is a photospherical phenomenon of the tranquil, magnetically inactive sun. It is an extension of the convection radiation in the interior of the sun, and the honeycombs have a diameter of about 1000 km. The hot plasma, which is transported by convection to the surface, emerges there and cools, in order to again sink into the interior of the sun. The bright “granules” are areas where the plasma flows out, whereas the dark “canals” surrounding them are areas of sinking plasma.

A sunspot (centrally situated in the right image) is, on the other hand, a phenomenon of the magnetically active sun. Through differential rotation, the magnetic field of the sun is coiled in deeper layers of the sun, and magnetic field lines running parallel to the direction of rotation develop. Through instabilities, these can reach the surface, which can lead to a sunspot. This is thus an area of a strongly concentrated magnetic field; whereas in the black area (umbra) the field lines extend vertically out from the surface, in the surrounding ring (penumbra), the exit angle is smaller. The strong magnetic field in this area reduces convection and thus energy transport. The temperature consequently drops to about 2000 kelvins, and the spot appears dark. Sunspots typically stay on the sun's surface for a few weeks before they, together with the magnetic field, disappear from the the surface (photosphere). 

Magnetische Elemente und Granulation. (Quelle: KIS)

In contrast to sunspots, whose diameter is in the area of about 10,000 km, the photosphere also exhibits magnetic phenomena that are smaller than the granules of the tranquil sun.  These are magnetic elements in the range of 100 to 500 km that can be seen as bright points.  One can come across them everywhere on the surface of the sun but can only be observed with a good telescope, because of their small size.  In the image at the right (download: full resolution), one sees the magnetic elements not only as bright points between the granules, but also often as bright, oblong bands.  Magnetic elements that are somewhat larger – at least as large as a granule, i.e. having a dimension of about 1000 km or more – do not appear bright, but rather as dark pores.  The right-hand image shows a plane with a lateral length of 40,000 km and was captured in the CH molecule band (classified by Fraunhofer as the G-band) at 430.5 nm.  This active region shows an active dynamic on a time-scale of minutes.  A short animation shows this.  The animation presents a 15-minute time series of this observation.

Das untere Diagramm gibt das alle elf Jahre auftretende Aktivitätsmaximum an. Das ober Schmetterlingsdiagramm zeigt auf, wo sich die Sonnenflecken während eines Zyklus auf der Sonnenoberfläche befinden.(Quelle: NASA)

The solar activity is subject to an eleven-year cycle, which resembles the sunspot-covered surface of the sun, which can be seen in the temporal change of the area covered by sunspots, as shown in the lower diagram. At the completion of a cycle, the magnetic field of the sun reverses its polarity. During an eleven-year cycle, the regions where sunspots originate move from higher latitudes to the equator (upper diagram). These diagrams are known as  butterfly diagrams because of their shape.  It should be noted that the life of a spot is typically only a few weeks, but a cycle is approximately 11 years long.

Solar magnetism is affected by great regularities: at the beginning of a cycle, the sunspots occur at high latitudes (ca. 30 degrees). In the course of a cycle, the number and size of the sunspots increases, while they more and more can be found at lower latitudes.  At the end of the cycle, the number decreases and the few sunspots appear closer to the equator. A new cycle begins again at a high latitude but with the opposite polarity.  This periodic behaviour has continued in this manner for centuries.

The colourful chromosphere

Während einer Sonnenfinsternis kann die eindrucksvolle Chromosphäre auch mit dem bloßen Auge, ohne spezielle Filter beobachtet werden. (Photo taken by Luc Viatour during the total eclipse of 1999)

The chromosphere, which is located above the photosphere, is not able to be seen by the human eye without further technical aids, since it only radiates in single emission lines and is outshined by the photosphere.  Only during a solar eclipse, when the moon blocks the sun's photosphere, can the dimmer chromosphere be seen, in its typical red colour of the hydrogen line at 656 nm (see image at right).

With special filters, it is possible however to observe the characteristic emission lines of the chromosphere on the apparent solar disc.  The three images below exemplify this for the chromospheric lines of hydrogen at 656 nm (lower left, calcium at 396 nm (lower middle), and helium at 1083 nm (lower right).  The images were captured with ChroTel.