The solar atmosphere: photosphere, chromosphere and corona
2.5 The solar atmosphere: photosphere, chromosphere and corona
When the Sun is observed (taking great care to use the appropriate fi lters) it appears to have a sharp edge but there is, of course, no actual ‘surface’. We are, in fact, just seeing down through the solar atmosphere to a depth where the gas becomes what is called ‘optically thick’. This deepest visible layer of the atmosphere is called the photosphere (as this is where the photons that we see originate from) and is about 500 km thick (Figure 2.8). The temperature falls from ∼6500 K at its base to ∼4400 K at its upper region. As was derived earlier, the effective temperature of the photosphere is ∼5800 K. The convective transport of energy from below gives rise to a mottling of the surface – solar granulations that are about 1000 km across. Each granulation cell last about 5–10 min as hot gas, having risen from below the surface radiates energy away, cools and sinks down again.
Figure 2.8 The photosphere of the Sun showing granulations and sunspot groups. Image: SOHO
60 Introduction to Astronomy and Cosmology
The region, about 2000 km thick, above the photosphere, is called the chromosphere . The gas density in this region falls by a factor of about 10 000 and the temperature increases from ∼4400 K at the top of the photosphere to about 25 000 K. Above this is the transition region , in which the temperature rises very rapidly over a distance of a few hundred kilometres to a temperature of ∼1 million K. The transition region leads into the outer region of the Sun called the Solar Corona where temperatures reach in excess of 2 million K (Figure 2.9). Its form and extent depends strongly on the solar activity that varies through what is called the Sun Spot Cycle but, typically, extends for several solar radii into what is called the heliosphere . At the time of solar minima when activity is low it usually extends further from the Sun at its equator and the pattern of the Sun’s magnetic fi eld is often well delineated near its poles. At solar maxima, the overall shape is more uniform and has a complex structure.
The density is very low, ∼10 14 times less than that at the Earth’s surface, and its brightness at visible wavelengths is about a million times less than that of the photosphere. It can thus only be observed during a total eclipse of the Sun, or by using a special type of telescope, called a coronagraph, that can block out the light from the solar disc. How the Corona can reach such high temperatures is still somewhat of a mystery, but it is thought that energy might be transported into it by magnetic fi elds. The million degree temperatures give rise to X-ray emission that may be observed from space.
Our Solar System 1 – The Sun
2.5.1 Coronium
The spectrum of the corona – the coronal spectrum – contains emission lines. When the emission lines observed in the Solar Corona during a solar eclipse were correlated with emission lines from known elements on Earth, a green emission line was found that had no match. It was thought to relate to an unknown element and, as it had fi rst been observed in the Solar Corona, it was provisionally named Coronium. However, in the 1930s, Walter Grotrian and Bengt Edlén, discovered that this spectral line was due to highly ionized iron – its high level of ioniza- tion being due to the extreme temperature of the Solar Corona. The resulting, very high energy, photons strip the outer electrons from atoms so giving them a
positive charge. These are called ions .