Spectral composition of solar radiation

Practically all of the radiation from the Sun received at the Earth originates in the photosphere, a thin layer surrounding the convective mantle of high

Figure 2.2. Variation in the solar radius as a function of time (bottom), together with selected milestones in the development on Earth, associated with the building-up of oxygen in the Earth's atmosphere (top). The rapid development of phytoplankton in the upper layers of the oceans at a relative oxygen concentration of 10-2 is associated with the formation of an ozone shield in the atmosphere, cutting down the ultraviolet part of the solar spectrum. When the oxygen level has reached 10-1, the atmospheric ultraviolet absorption is strong enough to allow life on land. (Based on Herbig, 1967; Berkner and Marshall, 1970; Cloud and Gibor, 1970).

Figure 2.2. Variation in the solar radius as a function of time (bottom), together with selected milestones in the development on Earth, associated with the building-up of oxygen in the Earth's atmosphere (top). The rapid development of phytoplankton in the upper layers of the oceans at a relative oxygen concentration of 10-2 is associated with the formation of an ozone shield in the atmosphere, cutting down the ultraviolet part of the solar spectrum. When the oxygen level has reached 10-1, the atmospheric ultraviolet absorption is strong enough to allow life on land. (Based on Herbig, 1967; Berkner and Marshall, 1970; Cloud and Gibor, 1970).

opacity. The depth to which a terrestrial observer can see the Sun lies in the photosphere. Owing to the longer path-length in the absorptive region, the apparent brightness of the Sun decreases towards the edges. The photosphere consists of atoms of varying degree of ionisation, plus free electrons. A large number of scattering processes take place, leading to a spectrum similar to the Planck radiation (see section 2.A) for a black body in equilibrium with a temperature T ~ 6000 K. However, this is not quite so, partly because of sharp absorption lines corresponding to the transitions between different electron configurations in the atoms present (absorption lines of over 60 elements have been identified in the solar spectrum), and partly because of the temperature variation through the photosphere, from around 8000 K near the convective zone to a minimum of 4300 K at the transition to the chromosphere (see Fig. 2.3). Yet the overall picture, shown in Fig. 2.4, is in fair agreement with the Planck law for an assumed effective temperature Teff ~ 5762 K, disregarding in this figure the narrow absorption lines in the spectrum.

Figure 2.3. Schematic picture of solar layers, starting from the centre of the Sun to the left. The solar radius is defined by the bottom of the visible Sun. All distances are in metres, all temperatures T are in K and all densities p are in kg m-3. The solar corona continues into an accelerating stream of particles, the solar wind. At the Earth's distance from the Sun (centre to the right, note the two changes of scale), the solar wind gives rise to magnetic induction and aurorae, and the extension of the corona is seen near the horizon as zodiacal light, in the absence of direct or scattered light from the solar photosphere. The solar wind is also responsible for the comet tails being directed away from the Sun (whereas the radiation pressure can only move the lightest material in the tail). The tails of a comet usually have an ion part and a dust part, with the latter moving more slowly and being deflected as a result of the Sun's rotation (a period of around 25 days). The inner part of the Earth is only sketched. The mantle is believed to consist of an outer part (silicates of Mg and Fe) and an inner part (oxides of Mg and Fe). Similarly, the core has an outer part (probably liquid FeS) and an inner part (a solid iron-nickel alloy).

Figure 2.3. Schematic picture of solar layers, starting from the centre of the Sun to the left. The solar radius is defined by the bottom of the visible Sun. All distances are in metres, all temperatures T are in K and all densities p are in kg m-3. The solar corona continues into an accelerating stream of particles, the solar wind. At the Earth's distance from the Sun (centre to the right, note the two changes of scale), the solar wind gives rise to magnetic induction and aurorae, and the extension of the corona is seen near the horizon as zodiacal light, in the absence of direct or scattered light from the solar photosphere. The solar wind is also responsible for the comet tails being directed away from the Sun (whereas the radiation pressure can only move the lightest material in the tail). The tails of a comet usually have an ion part and a dust part, with the latter moving more slowly and being deflected as a result of the Sun's rotation (a period of around 25 days). The inner part of the Earth is only sketched. The mantle is believed to consist of an outer part (silicates of Mg and Fe) and an inner part (oxides of Mg and Fe). Similarly, the core has an outer part (probably liquid FeS) and an inner part (a solid iron-nickel alloy).

The structure of the solar surface

The turbulent motion of the convective layer underneath manifests itself in the solar disc as a granular structure interpreted as columns of hot, vertical updrafts and cooler, downward motions between the grain-like structures (supported by observations of Doppler shifts). Other irregularities of the solar luminosity include bright flares of short duration, as well as the sunspots, regions near the bottom of the photosphere with lower temperature, ap pearing and disappearing in a matter of days or weeks in an irregular manner, but statistically periodic with an 11-year period. The sunspots first appear at latitudes at or slightly above 30°, reach maximum activity near 15° latitude, and end the cycle near 8° latitude. The "spot" is characterised by churning motion and a strong magnetic flux density (0.01-0.4 Wb m-2), suggesting its origin from vorticity waves travelling within the convective layer, and the observation of reversed magnetic polarity for each subsequent 11-year period suggests a true period of 22 years.

Figure 2.4. Frequency spectrum of solar radiation received on a unit area at the Earth's mean distance from the Sun, the area facing the Sun. Solid line: measured values (smoothed out over absorption lines, based on NASA, 1971). Dashed line: Planck law corresponding to an effective temperature of 5762 K, normalised to the experimental curve.

Figure 2.4. Frequency spectrum of solar radiation received on a unit area at the Earth's mean distance from the Sun, the area facing the Sun. Solid line: measured values (smoothed out over absorption lines, based on NASA, 1971). Dashed line: Planck law corresponding to an effective temperature of 5762 K, normalised to the experimental curve.

Above the photosphere is a less dense gas with temperature increasing outwards from the minimum of about 4300 K. During eclipses, the glow of this chromosphere is visible as red light. This is because of the intense Ha line in the chromospheric system, consisting primarily of emission lines.

After the chromosphere comes the corona, still less dense (of the order of 10-11 kg m-3 even close to the Sun), but of very high temperature (2 x 106 K, cf. Fig. 2.3). The mechanism of heat transfer to the chromosphere and to the corona is believed to be by shock waves originating in the turbulent layer

(Pasachoff, 1973). The composition of the corona (and chromosphere) is believed to be similar to that of the photosphere, but owing to the high temperature in the corona, the degree of ionisation is much higher, and e.g. the emission line of Fe13+ is among the strongest observed from the corona (during eclipses). A continuous spectrum (K-corona) and Fraunhofer absorption lines (F-corona) are also associated with the corona, although the total intensity is only 10-6 of that of the photosphere, even close to the Sun (thus the corona cannot be seen from the Earth's surface except during eclipses, owing to the atmospheric scattering of photospheric light). Because of the low density, no continuous radiation is produced in the corona itself, and the K-corona spectrum is due to scattered light from the photosphere, where the absorption lines have been washed out by Doppler shifts of random nature (as a result of the high kinetic energy).

The corona extends into a dilute, expanding flow of protons (ionised hydrogen atoms) and electrons, known as the solar wind. The increasing radial speed at increasing distances is a consequence of the hydrodynamic equations for the systems (the gravitational forces are unable to balance the pressure gradient; Parker, 1964). The solar wind continues for as long as the momentum flow is large enough not to be deflected appreciably by the magnetic fields of interstellar material. Presumably, the solar wind is penetrating the entire solar system.

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