1 The solar physics

The Sun is a sphere made up of gaseous elements consisting of 80% hydrogen, 19% helium and 1% of all the well-known substances. It has a diameter of 1.39 ■ 109 m and it is located at a distance of 1.495 ■ 1011 m from the Earth. However, this dis­tance may vary by ±1.7% during the year because of the orbit’s eccentricity.

The Sun is characterized by two motions: a motion of revolution around the centre of the galaxy, which has a linear speed of 300 km/s and takes 200 million years to complete, and a motion of rotation around the axis, which lasts about 4 weeks.

Inside the Sun, numerous fusion reactions take place. The heat produced by these reactions spreads from the inner layers to the outer layers by convection, conduction and radiation. From the outer layers, the heat is transmitted to the surrounding space by radiation. Among the nuclear reactions that occur in the Sun, the most important is the one which converts hydrogen into helium; the mass of a helium nucleus is smaller than that of the four original protons and this mass defect is converted into energy.

The mass of the Sun is roughly 2 ■ 1030 kg. The areas at the centre of the Sun reach temperatures of about 8-40 million kelvin and a density 100 times greater than that of water. However, the density is extremely lower in the outer layers.

It is believed that the region between 0 and 0.23R (R = solar ray), which con­stitutes 40% of the solar mass, produces 90% of the solar energy. The area between 0.7 and 1R is called the convective envelope (temperature 5000 K, den­sity 10-5 kg/m3), because of the importance of convective processes in this layer. The photosphere, the outer layer from the convective envelope, is composed of strongly ionized gases, which are capable of absorbing and emitting through a continuous spectrum of radiation. Over the photosphere, there is the inversion layer, which is hundreds of kilometres wide and is made up of cold gases. Out­side the inversion layer, there is the chromosphere, which is 10,000 km wide, and the corona, characterized by a very low density and high temperatures (106 K) (Fig. 1) [1].

The electromagnetic radiation emitted by the Sun extends over a wide wave­length interval: from 0.1 nm to 104 m; however, the greatest part of that energy falls in the interval between 0.2 and 4 |jm. In particular, 95% of the energy which reaches the Earth is included between 0.3 and 2.4 |jm. The spectrum of the solar radiation is similar to a black body’s spectrum at a temperature of 5780 K, since temperatures at the surface of a star fluctuate between 4000 and 6000 K. Therefore, it is right to assume that the behaviour of the Sun with regard to radiation is similar to the behaviour of a black body at a uniform/regular temperature (Fig. 2). This temperature of 5780 K is calculated using the Stefan-Boltzmann law [1, 3].

Analysing the spectrum more carefully, one can notice that the greatest part of the radiation falls in (1) the ultraviolet band, which extends from 0.20 to 0.38 pm; (2) the visible light band, from 0.38 to 0.78 pm; and (3) the near infrared band until about 4 pm. Only 8-9% of all the solar energy which reaches the Earth falls in the ultraviolet band; 46-47% falls in the visible band while the remaining 45% falls in the infrared band [3].