Of considerable interest is the large group of sunspots observed between 9 and 20 July. On 14 July it passed through the central meridian of the Sun. On 9 July the group comprised 16 spots, their number increasing to 41 by 15 July; by 20 July only 5 spots remained. The surface area occupied by this spot group increased rapidly. On 12 July it reached a high of 4.7 billion i. e. , more than 9 times as great as the entire surface of our planet. The largest spot of the group then had an area of 3.9 billion But this enormous sunspot was still far from breaking the record: on 7 April 1947, near a solar activity maximum, astronomers observed a sunspot with an area of over 180 billion Prominences are more difficult to detect than sunspots, but they are one of the most interesting and spectacular kinds of solar phenomena. These unusual "fountains" of incandescent gas rise to a height of up to a million kilometers above the surface of the Sun, moving in some cases with a velocity of 600km/sec. One of these giant prominences was observed on 23 July 1961 by V. G. Banin and V. F. Chistyakov, at the Far-East Astronomical Station.
Comparatively recently, little over 100 years ago, were discovered the chromospheric flares—truly remarkable and most powerful of the solar phenomena. These mostly originate in the middle layer of the solar atmosphere, the chromosphere, whence their name. Flares generally appear near spot groups and are related to them.
The brightest chromospheric flares can be seen even in daylight. Carrington and Hodgson, on 1 September 1859, were the first observers.
Ordinarily, however, flares are not much brighter than the surrounding regions of the solar surface, so that as a rule they are detectable only with spectroscopic apparatus. The chromospheric flares appear particularly bright in lines of the spectrum emitted by hydrogen atoms. In this light the flares are one order of magnitude brighter than the surrounding parts of the Sun.
The summer of 1961 was rich in chromospheric flares. In July, over twenty flares were recorded at the Crimean Astrophysical Observatory of the Academy of Sciences of the USSR alone. Each flare generally persists for no more than half an hour. The flare of 18 July, however, was observed for 1 hour 22 minutes. Its area was over 7.5 billion i.e. , 15 times as great as the entire surface area of the Earth.
The chromospheric flares emit most of their energy in the ultraviolet and the X-ray regions of the spectrum. These emissions disturb the ionospheric layers of the Earth's atmosphere, which act as a kind of mirror for the reflection of short radiowaves. These disturbances result in the partial breakdown of short-wave radio communication. Since X-ray and ultraviolet radiations propagate with the velocity of light, the ionospheric disturbances produced by these emissions set in immediately after the flare.
Electrically charged particles, corpuscules, are also emitted from the flare focus, their velocities reaching 1-2 thousand km/sec. After a little over 24 hours they hit the Earth. When the Earth meets the stream of solar corpuscules, complex electromagnetic phenomena occur. Polar aurorae are observed. Prolonged breakdown of short-wave radio communication takes place, and magnetic storms arise, making it impossible to use a compass.
As they pass through the solar corona, the corpuscular streams make the charged coronal particles (electrons) oscillate. Radiowaves are emitted owing to these oscillations. This may enhance the radioemission of the Sun a thousandfold, and during very powerful flares even a millionfold. In the middle of July 1961 the solar radioemission was intensified by a factor of 50 on meter waves.
Flares sometimes give rise to streams of cosmic rays too. Hitting the Earth's atmosphere, cosmic rays destroy molecules and atoms of gases and thus disappear themselves. Showers of fragments of broken atomic nuclei reach the surface of the Earth from the atmosphere. Particularly intense cosmic-ray showers were observed following the chromospheric flare of 23 February 1956. After the chromospheric flare of 12 September 1959, Soviet researchers observed an increase by a factor of 12 in the count of heavy nuclei reaching a counter.
Another phenomenon linked with chromospheric flares was discovered a few years ago. After the prodigious flare of 23 February 1956, French astronomers headed by Prof. A. Danjon, director of the Paris Observatory, recorded a certain slow-down of the Earth's rotation. While before the flare the Earth was in the middle of its seasonal acceleration, which started on 27 September and shortened the period of rotation by 7.2 microseconds per day, after the flare the period of rotation started increasing by 2.5 micro seconds per day. According to Danjon, the deceleration of the Earth's rotation occurred a few hours after the flare.
The discovery was so unexpected that at first many astronomers questioned the actuality of the phenomenon observed. Only much later, when artificial satellites and space rockets led to the discovery of radiation belts around the Earth was the deceleration of terrestrial rotation attributed to the interaction of charged particles of these belts with the corpuscules emitted by the Sun.
Some observers, however, expressed their doubts of the validity of the results obtained by Danjon. The question of the deceleration of the rotation of our planet as a result of flare effects therefore still remains open.
It is remarkable that in July 1961, after the appearance of the magnificent flares, the American artificial satellite Echo-I, according to the data of French astronomers, accelerated its revolution around the Earth.
The nature of chromospheric flares has not yet been fully studied. These flares are enormous explosions, immeasurably more powerful than A- and H-bomb explosions. They are set up by interactions of electric and magnetic forces. In all probability these are unusual compression explosions produced by the solar magnetic field.
The processes occurring on the face of the Sun have a significant influence on many terrestrial phenomena and even on the behavior of our planet as a whole. As we progress in our understanding of the rules governing solar activity, we shall improve our ability to predict its undesirable consequences, e. g. , magnetic storms and radio breakdown. The enormous achievements of science in the study of the cosmos make us confident that the secret of the origin of solar activity will be disclosed in the near future.
S.I. Isaev