Let us now turn back to the present state of the Sun. The energy diffusing from the interior does not produce in most of the solar body any macro scopic motions. However, in the layer underlying the surface, to a depth of about 0.1 solar radii, convective motions arise. The appearance of convection in that particular layer is caused by partial ionization of the hydrogen. The ionization changes with temperature variations of the gas, which is equivalent to an increase in heat capacity facilitating convection. In the deeper layers the hydrogen is fully ionized, but convection may be sus tained by partially ionized helium. At greater depths convection ceases.
In the main part of the convection zone heat transfer is effected not by radiation but by ascending and descending currents. Only in the uppermost, photospheric layers, where the transparency of the gas is higher, the convective energy transfer is sharply reduced.
In the convection zone, at least in its upper layers, individual cells are formed, in which the gas rises along the centerline and sinks along the border. These cells, or rather streams of gas, which move across the photosphere, appear as brilliant patches—granules—which cover the whole solar surface. The granules appear and disappear within a few minutes. Their temperature is several hundred degrees higher than that of the darker areas between them.
The motions in the upper layers of the convection zone produce contrac tions and expansions of the gas. These disturbances are propagated in the form of sound waves into the higher layers of the solar atmosphere.
As the waves advance into a medium of lower density, their amplitude increases and thus the sound waves gradually turn into shock waves. The shock waves damp out fairly rapidly, and their energy is converted into heat. The upper atmosphere of the Sun is continuously heated in this way by the waves coming up from the underlying convection zone. This is responsible for certain anomalous properties displayed by the upper layers of the atmosphere.
The radiation of the visible surface of the Sun has a continuous spectrum with dark absorption lines. When the Sun's disk is eclipsed by the Moon, the chromosphere is visible around it as a thin crescent, whose spectrum is mainly composed of bright lines.
The thickness of the chromosphere is about 10,000km. Above it extends the tenuous corona (Figure 2), from which streamers stretch out to a distance of some tens of solar radii. The great extent of the streamers has been a mystery for a long time, and it now appears that the explanation is connected with the flux of energy from-below.
The temperature that the gas attains by the heating depends also on the process by which the gas is cooled. The solar gas cools down by self radiation; electrons lose thermal energy by exciting atoms which in turn emit radiation quanta into space. The emission of the transparent gas layer depends on its density—the higher the density the more frequently collisions occur, and thus the more intense is the emission of the gas and the stronger the cooling. The rarefied corona radiates very little, and is consequently heated by the waves to a very high temperature, exceeding a million degrees. The upper layers of the chromosphere, being denser, are cooled more, so that the temperature there only goes up to 10,000 or 20,000°K. The lower layers of the atmosphere radiate so intensely that they gain no heat from the waves. Accordingly, the corona is very hot because it is so tenuous and emits little radiation. This is why its high temperature does not affect the Sun's emission in the visible portion of the spectrum, and thus the temperature of the Sun is said to be close to 5800°. If the corona were denser, it would radiate more intensely. But then its temperature would be lower and it would not be a corona.
The statement that the corona is transparent is not quite correct. In certain ranges of the short-wave X-ray region and for radio waves longer than a meter the corona is opaque, and consequently within these ranges it radiates like a black body with a temperature of more than a million degrees.
In the longer-wavelength region, the ultraviolet, the emission of the corona is low, but the chromosphere is still opaque and radiates relatively strongly. Thus, the short-wave and radio regions of the spectrum are emitted not by the relatively cool photosphere, but by the hotter upper layers of the solar atmosphere. If the corona and the chromosphere were absent, the Sun's radiation could not produce any detectable ionosphere on Earth, and our short-wave radio communications would greatly suffer from it. It is true that the upper layers of the terrestrial atmosphere are ionized not only by radiation but also by corpuscular fluxes and other phenomena associated with them, which explains why the ionosphere persists even during the long polar night. But in the middle latitudes the effect of these factors is small, and the main source of ionization remains the radiation of the chromosphere.