The Atmosphere

The solar atmosphere is divided into the photosphere and the chromosphere. Outside the actual atmosphere, the corona extends much further outwards.

The Photosphere. The innermost layer of the atmosphere is the photosphere, which is only about 300-500 km thick. The photosphere is the visible surface of the Sun, where the density rapidly increases inwards, hiding the interior from sight. The temperature at the inner boundary of the photosphere is 8000 K and at the outer boundary 4500 K. Near the edge of the solar disc, the line of sight enters the photosphere at a very small angle and never penetrates to large depths. Near the edges one therefore only sees light from the cooler, higher layers. For this reason, the edges appear darker; this phenomenon is known as limb darkening. Both the continuous spectrum and the absorption lines are formed in the photosphere, but the light in the absorption lines comes from higher layers and therefore the lines appear dark.

The solar convection is visible on the surface as the granulation (Fig. 12.3), an uneven, constantly changing granular pattern. At the bright centre of each granule, gas is rising upward, and at the darker granule boundaries, it is sinking down again. The size of a granule seen from the Earth is typically 1", corresponding to about 1000 km on the solar surface. There is also a larger scale convection called supergranulation in the photosphere. The cells of the supergranulation may be about 1' in diameter. The observed velocities in the supergranulation are mainly directed along the solar surface.

The Chromosphere. Outside the photosphere there is a layer, perhaps about 500 km thick, where the temperature increases from 4500 K to about 6000 K, the chromosphere. Outside this layer, there is a transition region of a few thousand kilometres, where the chromosphere gradually goes over into the corona. In the outer parts of the transition region, the kinetic temperature is already about 106 K.

Normally the chromosphere is not visible, because its radiation is so much weaker than that of the photosphere. However, during total solar eclipses, the chromosphere

Fig. 12.3. The granulation of the solar surface. The granules are produced by streaming gas. Their typical diameter is 1000 km. (Photograph T. Rimmele/Richard B. Dunn Solar Telescope, NSO/AURA/NSF)

Fig. 12.3. The granulation of the solar surface. The granules are produced by streaming gas. Their typical diameter is 1000 km. (Photograph T. Rimmele/Richard B. Dunn Solar Telescope, NSO/AURA/NSF)

Fig. 12.5. The solar surface in the hydrogen Ha line. Active regions appear bright; the dark filaments are prominences. Limb darkening has been removed artificially, which brings to light spicules and prominences above the limb. The photograph was taken in October 1997. (Photograph Big Bear Solar Observatory/NJIT)

Fig. 12.5. The solar surface in the hydrogen Ha line. Active regions appear bright; the dark filaments are prominences. Limb darkening has been removed artificially, which brings to light spicules and prominences above the limb. The photograph was taken in October 1997. (Photograph Big Bear Solar Observatory/NJIT)

shines into view for a few seconds at both ends of the total phase, when the Moon hides the photosphere completely. The chromosphere then appears as a thin reddish sickle or ring.

During eclipses the chromospheric spectrum, called the flash spectrum, can be observed (Fig. 12.4). It is an emission line spectrum with more than 3000 identified lines. Brightest among these are the lines of hydrogen, helium and certain metals.

One of the strongest chromospheric emission lines is the hydrogen Balmer a line (Fig. 12.5) at a wavelength of 656.3 nm. Since the Ha line in the normal solar spectrum is a very dark absorption line, a photograph taken at this wavelength will show the solar chromosphere. For this purpose, one uses narrow-band filters letting through only the light in the Ha line. The resulting pictures show the solar surface as a mottled, wavy disc. The bright regions are usually the size of a supergranule, and are bounded by spicules (Fig. 12.6). These are flamelike structures rising up to 10,000 km above the chromosphere, and lasting for a few minutes. Against the bright surface of the Sun, they look like dark streaks; at the edges, they look like bright flames.

The Corona. The chromosphere gradually goes over into the corona. The corona is also best seen during total solar eclipses (Fig. 12.7). It then appears as a halo of light extending out to a few solar radii. The surface brightness of the corona is about that of the full moon, and it is therefore difficult to see next to the bright photosphere.

The inner part of the corona, the K corona, has a continuous spectrum formed by the scattering of the photospheric light by electrons. Further out, a few solar radii from the surface, is the F corona, which has a spectrum showing Fraunhofer absorption lines. The light of the F corona is sunlight scattered by dust.

In the latter part of the 19th century strong emission lines, which did not correspond to those of any known element, were discovered in the corona (Fig. 12.8). It was thought that a new element, called coronium, had been found - a little earlier, helium had been discovered in the Sun before it was known on Earth. About 1940, it

Fig. 12.6. Spicules, flamelike uprisings near the edge of the solar disc. (Photograph Big Bear Solar Observatory)

Fig. 12.6. Spicules, flamelike uprisings near the edge of the solar disc. (Photograph Big Bear Solar Observatory)

Fig. 12.7. Previously, the corona could be studied only during total solar eclipses. The picture is from the eclipse on March 7, 1970. Nowadays the corona can be studied continuously using a device called the coronagraph
Fig. 12.8. The presence of lines from highly ionized atoms in the coronal spectrum shows that the temperature of the corona has to be very high

was established that the coronal lines were due to highly ionized atoms, e.g. thirteen times ionized iron. Much energy is needed to remove so many electrons from the atoms. The entire corona has to have a temperature of about a million degrees.

A continuous supply of energy is needed in order to maintain the high temperature of the corona. According to earlier theories, the energy came in the form of acoustic or magnetohydrodynamic shock waves generated at the solar surface by the convection. Most recently, heating by electric currents induced by changing magnetic fields has been suggested. Heat would then be generated in the corona almost like in an ordinary light bulb.

In spite of its high temperature the coronal gas is so diffuse that the total energy stored in it is small. It is constantly streaming outwards, gradually becoming a solar wind, which carries a flux of particles away from the Sun. The gas lost in this way is replaced with new material from the chromosphere. Near the Earth the density of the solar wind is typically 5-10 particles/cm3 and its velocity about 500km/s. The mass loss of the Sun due to the solar wind is about 10-13 Me per year.

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