The density of the upper layers of the corona is lower than that of the lower portion, and thus they radiate even less. At this point, however, the temperature of the corona no longer grows, but rather declines. This is explained by the fact that in the outer corona a new process comes into play, which causes cooling. Part of the protons in the outer corona attain velocities at which they are able to escape from the Sun completely. The resultant emission of fast particles is similar to evaporation; it cools the corona and prevents its temperature from rising beyond a certain limit. The particles escaping from the corona form a weak stream, or wind, which manifests itself in certain geophysical phenomena and is directly measurable by means of space rockets.
Let us now consider the formations on the surface of the Sun. A cons picuous feature on photographs are the sunspots (Figure 3). These are relatively dark, because their temperature is almost a thousand degrees lower than that of the photosphere. Their dark central core is surrounded by a less dark area, the penumbra, in which the temperature is somewhat higher than in the central umbra. The number of sunspots varies from year to year, with a recurrence period of about 11 years. This period is known as the solar-activity cycle. The spots are distributed in belts which slowly move toward the equator in the course of a cycle. The sunspots usually form groups, in which there are one or two main spots.
A salient feature of the sunspots is that they display a strong magnetic field, whose intensity may reach 3 or 4 thousand oersteds.
Apart from the sunspots, the Sun exhibits many other phenomena associated with the solar-activity cycle, in all of which a magnetic field is prominent, though not as strong as that of the sunspots. By means of modern instruments it is possible to detect on the Sun very weak fields, considerably weaker than the field of the Earth. Investigations have shown that there are on the Sun extensive magnetic regions, in which the field intensity goes from some fractions of an oersted to several hundred oersteds. These magnetic regions exist for several months, sometimes up to a year, and then they disappear. The magnetic field in extended masses of the con ducting gas cannot actually just vanish; it only penetrates into the interior or else is carried off into space. Conversely, a magnetic region appears when the field emerges from the interior.
By analyzing the motion of the zones containing magnetic regions and the nature of the field within them it has been possible to construct a model, for the time being empirical, describing the magnetic field in the interior.
In each hemisphere under the surface of the Sun there are two moving systems of magnetic force-field tubes of opposite polarity which cover the Sun along parallels. One system of tubes rises up in the middle latitudes and moves under the surface toward the equator. At the same time the other system moves in the interior, from the equator to the middle latitudes. When it approaches the equator, the first system penetrates to the interior and starts moving toward the middle latitudes, while the second system rises toward the surface and moves to the equator. An identical process takes place in the other hemisphere.
The reason for this circulation, or the mechanism generating the field, is not yet clear. The circulation has a period of 22 years, during which time two cycles of reversed polarity are completed.
Under certain conditions a tube traveling close to the surface may rise and emerge to the outside, thus forming a magnetic region. Where the intensity of the field is very high a spot is formed, and in other places an active region appears, producing a number of phenomena. The formation of the spots and active regions is caused by the effect exerted by the field on the motions in the conducting medium, and specifically on convective motions. The field inhibits motions in the medium across the force lines, if its energy is comparable with the kinetic energy of the motion. At field intensities of more than 500 oersteds the magnetic forces arrest any motion in the upper portion of the convection zone. Now since convection is the main means of heat transfer in these layers, the photosphere overlying a stagnant area receives less energy, its temperature drops, and a cool spot is formed. In regions of lower intensity the energy of the field is not sufficient to check the motion of convective currents, but it changes the pattern of motion. Under normal conditions the large-scale motion is turbulent, i. e. , it is accompanied by ripples and eddies which are super imposed on the main flow. These ripples cross over from the rising current into the sinking current and thus exert a drag on the motion. This drag, known as eddy viscosity, acts on the currents rising from granules. A weak magnetic field cannot arrest the main flow, but it inhibits the ripples whose energy is considerably lower than that of the main flow. A reduction of the turbulence, and therefore of the eddy viscosity, leads to an increase in the velocity of the currents. Consequently, a weak field helps rather than hinders convection.