At the surface of the Sun, the enhanced convection results in a set of phenomena which involve all the layers of the atmosphere. In the photo sphere convective heat transfer is increased, and the moving masses equalize the temperature difference between the upper and lower layers. There fore in a region of activity the upper layers of the photosphere are hotter than in other places, and at the edge of the solar disk an active region appears as a large bright patch, known as a facula. The faster motions generate a strong flux of waves which travel from the photosphere into the chromo sphere and the corona. Consequently in an active region the chromosphere heats up more, and layers of the same density have a higher temperature than they do in unperturbed regions of the Sun.
The chromosphere emits radiation in distinct lines, the intensity of emission depending on the temperature and density. In the regions of activity the emission of the principal lines is intensified, and thus if the Sun is photographed through a light filter transmitting the ultraviolet line of ionized calcium or the red H , hydrogen line, the active regions show up as brights spots (Figures 4 and 5). These spots are known respectively as calcium and hydrogen flocculi. In the corona active regions appear as condensations, since a strong wave flux heats up to coronal temperature denser gas than does a weak flux. The magnetic field emerging from active regions into the corona imparts to the coronal formations a distinctive structure, which is visible on photographs.
In the neighborhood of large groups of spots, chromospheric flares are sometimes observed, which are the most "active" formations on the Sun. They are most clearly seen in hydrogen spectroheliograrns, where they appear as intensely bright fine details. A flare may last from several minutes to some tens of minutes. During a flare large amounts of cosmic rays are produced, which are detectable after some time in the upper layers of the Earth's atmosphere. A high-velocity jet of gas shoots up from the flare, rising to a great height and then falling back again. This surge is apparently caused by cosmic-ray pressure. Part of the gas does not fall back to the Sun, but escapes with a velocity of more than 1000km/sec and reaches the Earth after a day or two, where it gives rise to magnetic storms, aurorae, and other phenomena. As the stream moves through the corona it produces a burst of radio emission, whose intensity is sometimes millions of times greater than the thermal radio emission of the corona.
The appearance of flares is apparently connected with magnetic forces which strongly compress the gas under certain field configurations. On being compressed the gas heats up, and then nuclear reactions involving deuterium possibly take place, producing fast particles which are further accelerated by impinging against the moving magnetic "walls".
Another phenomenon associated with magnetic forces is that of solar prominences. These are relatively dense gas clouds which rise to considerable heights within the corona. They are clearly visible on the limb in H. light, and when projected against the solar disk they show up as long dark filaments (Figure 5). They sometimes hang almost motionless, and sometimes move at a high velocity. The prominences are mostly formed in the corona. The magnetic forces compress the coronal gas to such density that it starts radiating intensely and quickly cools down. The heavy condensations are supported by the field or they slide down the inclined force lines, as may be often observed in active prominences (Figure 6).
Let us sum up. The Sun draws its energy from the nuclear reactions occurring near its center. As it diffuses upward, this energy causes motions in the gas layers closer to the surface, i.e., it is converted from thermal into mechanical energy. The motions give rise to granulation of the Sun's surface and to a wave flux heating up the chromosphere and the corona. In the presence of magnetic fields convection is suppressed if the field is strong, and intensified if the field is weak. This explains the appearance of spots and active regions. The presence of magnetic forces also explains the formation of flares and prominences. Solar phenomena have an effect on the atmosphere and the magnetic field of the Earth. The ultraviolet radiation, which is especially intense in the regions of activity, ionizes the upper layers of the terrestrial atmosphere. The X-ray emission of the flares penetrates into the deeper layers and causes fading of short radio waves for half an hour after the occurrence of a flare. The cosmic rays due to solar outbursts reach the Earth, and may be responsible to some extent for the radiation belts of fast particles surrounding the Earth. The correlation between solar and terrestrial phenomena has been subjected in the last few years to an extensive research program by scientists all over the world. This joint project was known as the International Geophysical Year and the Year of International Cooperation.