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Figure 1.1 The solar spectrum, and the spectra of ideal thermal sources at 5770 K and 4000 K (1nm = 10-9m).

at longer wavelengths. Also, the power emitted by this source is a lot less. The power shown corresponds to the assumption that the 4000 K source has the same area as the source at 5770 K, and thus brings out the point that the temperature of an ideal thermal source determines not only the wavelength range of the emission, but the power too. Note that 5770 K is a representative temperature of the Sun's photosphere; the local temperature varies from place to place.

At a finer wavelength resolution than in Figure 1.1 the solar spectrum displays numerous narrow dips, called spectral absorption lines. These are the result of the absorption of upwelling solar radiation by various atoms and ions, mainly in the photosphere, and therefore the lines provide information about chemical composition. Further information about the Sun's composition is provided by small rocky bodies that continually fall to Earth. They are typically 1-100 cm across, and constitute the meteorites (Section 3.3). At 5770 K significant fractions of the atoms of some elements are ionised, and so it is best to define the composition at the photosphere in terms of atomic nuclei, rather than neutral atoms. In the photosphere, hydrogen and helium dominate, with hydrogen the most abundant - all the other chemical elements account for only about 0.2% of the nuclei. Outside the Sun's fusion core (Section 1.1.3) about 91% of the nuclei are hydrogen and about 9% are helium.

Plate 1 shows that the most obvious feature of the photosphere is dark spots. These are called (unsurprisingly) sunspots. They range in size from less than 300 km across to around 100 000 km, and their lifetimes range from less than an hour to 6 months or so. They have central temperatures of typically 4200 K, which is why they look darker than the surrounding photosphere. Sunspots are shallow depressions in the photosphere, where strong magnetic fields suppress the convection of heat from the solar interior, hence the lower sunspot temperatures. Their number varies, defining a sunspot cycle. The time between successive maxima ranges from about 8 years to about 15 years with a mean value of 11.1 years. From one cycle to the next the magnetic field of the Sun reverses. Therefore, the magnetic cycle is about 22 years.

Sunspots provide a ready means of studying the Sun's rotation, and reveal that the rotation period at the equator is 25.4 days, increasing with latitude to about 36 days at the poles. This differential rotation is common in fluid bodies in the Solar System.

1.1.2 The Solar Atmosphere

Above the photosphere there is a thin gas that can be regarded as the solar atmosphere. Because of its very low density, at most wavelengths it emits far less power than the underlying photosphere, and so the atmosphere is not normally visible. During total solar eclipses, the Moon just obscures the photosphere, and the weaker light from the atmosphere then becomes visible. In Plate 2 the atmosphere just above the photosphere is not visible, whereas in Plate 3 the short exposure time has emphasised the inner atmosphere. The atmosphere can be studied at other times, either by means of an optical device called a coronagraph that attenuates the radiation from the photosphere, or by making observations at wavelengths where the atmosphere is brighter than the photosphere.

Figure 1.2 shows how the temperature and density in the solar atmosphere vary with altitude above the base of the photosphere. A division of the atmosphere into two main layers is apparent, the chromosphere and the corona, separated by a thin transition region.

The chromosphere

The chromosphere lies immediately above the photosphere. It has much the same composition as the photosphere, so hydrogen dominates. The density declines rapidly with altitude, but the temperature rises. The red colour that gives the chromosphere its name ('coloured sphere') is a result of the emission by hydrogen atoms of light at 656.3 nm. This wavelength is called Hw ('aitch-alpha').

The data in Figure 1.2 are for 'quiet' parts of the chromosphere. Its properties are different where magnetic forces hold aloft filamentary clouds of cool gas, extending into the lower corona. The filaments are the red prominences above the limb of the photosphere in Plate 3. Prominences are transitory phenomena, lasting for periods from minutes to a couple of months.

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