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by Boris KUZHEVSKY, Cand. Sc. (Phys. & Math.), Scientific Research Institute of Nuclear Physics (named after D. Skobeltsyn), Moscow State University
Everything that took place, or is taking place on this planet-the origin and evolution of life, geological and climate changes-has been associated in one way or another with the energy of the sun. This being so, studies of our luminary, of processes within and around it, are of really vital importance.
Since time immemorial people in different parts of the world have been observing some dark spots on the shining surface of "daystar Helios". From time to time these spots change their shape and size and also their number. As we know now, these spots are the visible manifestations of processes occurring on and in the luminary and their observations had led to the establishment of what are known as cycles of solar activity. These manifest themselves as periodic alterations of different parameters of these spots and of the electromagnetic solar emission. The best studied cycle of this activity is the so-called 11-year period.
Another impressive solar phenomenon, discovered more than 100 years ago, are chromospheric solar flares-sporadic bursts into parts of the atmosphere of the sun (its chromosphere) of vast amounts of energy - each equal to the simultaneous blast of 100 mln nuclear bombs of one kiloton each. Subsequent studies led scientists to the conclusion that a chromospheric flare extends far beyond the confines of the chromosphere proper, including the
low and high areas of the atmosphere of the Sun - its photosphere and corona. And the accompanying releases of vast amounts of energy occur in the form of what we call gasodynamic movements of solar plasma and also as electromagnetic emission in the broadest range - from radiowaves to gamma- quanta of hundreds of millions of electron volts and the generation of solar cosmic rays (SCR) of tens of billions of electron volts (SCR energy spectrum).
The relative concentrations of different nuclei in SCR are determined by the following three factors: the chemical composition of the solar shell, peculiarities of the process of acceleration, and nuclear reactions occurring between the fast- flying particles and matter of the solar corona.
To this day specialists have experimentally identified in solar flares of different magnitude a range of nuclei - from hydrogen to iron (SCR charge spec-
tmm). And, naturally enough, the absolute stream of energy particles and the SCR energy spectrum are linked with the "power rating" of this phenomenon. Scientists, however, were also concerned with some other and very important questions: can it be that fluxes of accelerated particles from a solar flare appear all of a sudden? Are they not preceded by some minor fluxes of lower intensity and energy?
The way we see it now, evolution of the active zone is accompanied by growing density of magnetic energy within it. Finally, there occurs a rapid collapse of the complex configurations of this active zone, accompanied by the generation of SCR. And it was really hard to imagine a quiet process of energy accumulation in the active zone up to densities when there begins an "overflow" of one of its kinds into another. Indeed, when scientists were able to station recorders in interplanetary space on a permanent basis they discovered that before an SCR flare relatively low-energy particles begin to be registered for 24 hours on the average. This phenomenon got to be known as "pre-flare buildup".
It was studied in the fullest details by staff of the department of cosmophysical research of our Institute in the early 1980s and also confirmed by observations of Russian satellites in the PROGNOZ series and by the American and European IMP and HELIOS probes. It was established that this flux can either monotonously increase up to the very "arrival" of SCR particles, or increase up to a certain value within a relatively short time and then remain practically unchanged until these particles' appearance. This kind of a "pre-flare" effect has been identified both in the proton and the electron component and also in the flux of helium nuclei. And it must also be present in the flux of heavy nuclei, although in order to detect it one has to have instruments with greater transmission factors than those used earlier. Observations of the pre-flare buildup of low-energy particles can be used for improving the accuracy of predictions of radiation situation in the interplanetary space. The first studies of this kind proved that forecast authenticity increased up to 85-90 percent.
The discovery of this pre-flare phenomenon confirmed our hypothesis to the effect that particle acceleration in the solar atmosphere takes place practically without a break with only the intensity of this process changing. This being so, the notions of a "quiet" or "active" sun are but relative and reflect in no precise terms the "real life" of our luminary.
An analysis of the results of the interactions of high-energy SCR particles with the nuclei of chemical elements of the solar atmosphere, which was conducted in details in early 1967 by our own and NASA experts, changed appreciably the formerly accepted view on the role of this process in the formation of the isotopic and elementary composition of SCR. It became evident that nuclear reactions occurring in the solar atmosphere with the participation of high-energy particles can produce marked changes in the SCR ratios of different isotopes as compared with their initial proportions. This is especially true of the rare earths (lithium, beryllium, boron) whose very presence in flare particles must be due to nuclear interactions occurring
between high-energy primary nuclei of carbon, oxygen, nitrogen and the hydrogen nuclei of the solar atmosphere. But even the isotope composition of such naturally abundant element as helium is not the same in SCR and the solar atmosphere. And there are two reasons for that: nuclear interactions and what we call probability selectivity - the selective nature of the process of particle acceleration. The mechanism of the latter was studied in detail by researchers of the Joffe Physico-Technical Institute (St. Petersburg).
And it has been natural to expect that nuclear transformations will result in the presence among the high-energy particles from a solar flare of not only stable, but also radioactive isotopes. As a matter of fact, among the latter one can observe in a terrestrial orbit only those with a sufficiently long half-life - of no less than several days. But a theoretical analysis has unexpectedly revealed that for some chemical elements the flux of radioisotopes in SCR will exceed that of the stable ones. This situation transpired from calculations carried out at our Institute as early as in 1967, in particular for the nuclei of beryllium and cobalt. A flux with these elements should consist in the main of radioactive Be 7 and Co 56 .
An experimental confirmation of this fact has become possible only now, 34 years later, and from a totally unexpected angle, so to say, - in connection with problems of studies of materials in space conditions.
During gamma-spectrometry studies in 1990 of plates of different materials, which covered a cylinder of 9 m in length and 3 m in diameter (LDEF experiment), American specialists discovered a large concentration of Be 7 . Incidentally, this cylinder remained up in open space for nearly six years at an average distance of 350 km from the earth. And during all that time its position in space was strictly oriented.
As a result, scientists came to the conclusion that the pattern of distribution of radioactive beryllium was unusual. Its high concentrations were observed only on plates which were located on the front side in relation to the velocity vector. And the conclusion was that this chemical element was in a thermal balance with the matter of the upper atmosphere of the Earth and simply stuck, or piled up there. And the natural question was - how this could happen? How did radioactive beryllium find its way into the atmosphere of our planet?
Further studies of the find were conducted jointly by Russian and American specialists. Since 1996 staff of our Department of Cosmophysical Studies, using satellites of the COSMOS and RESURS series, have been conducting constant observations in the upper atmosphere of our planet (at altitudes of about 200 km) of radioactive Be 7 with the view of establishing the nature of its formation. And there is an abundance of hypotheses on that score, with some scientists saying that the element can be produced in the earth atmosphere
as a result of nuclear interactions of galactic cosmic rays with oxygen and nitrogen. In that case its levels must be submitted to temporal variations, typical for galactic cosmic rays, that is, they should be in anti-correlation with solar activity.
Other researchers feel that Be 7 can be produced by nuclear interactions caused by the entry into the earth atmosphere of high-energy particles from solar Hares. In that case there should be direct correlation with increases of solar activity.
In a word, everything seems to be fairly simple - try and establish the temporal correlation of Be 7 concentration in the earth's upper atmosphere and the mechanism of its origin will become clear. Unfortunately, there is a snag in this approach. The thing is that radioactive beryllium appears in the atmosphere of our planet due to the aforesaid causes, that is produced as such, and then comes into thermal equilibrium with the surrounding medium at altitudes which are much lower than the orbits at artificial satellites - of the order of tens of kilometers. And if that is so, there should be some mechanism of "injecting" its nuclei produced at low altitudes into satellite orbits. If and when this kind of a mechanism is discovered, it will radically alter the whole picture, since this process can possess temporal characteristics of its own, either reflecting its complex links with the solar activity cycle, or having none of them at all.
Before picking up one of these hypotheses, one had to recall the two more likely explanations of the presence of radioactive Be 7 in the upper atmosphere of the earth. There must be in SCR radioisotopes generated in nuclear reactions between the medium and heavy nuclei and hydrogen of the solar atmosphere. The energy of the main share of such secondary nuclei should be of up to tens of MeV per nucleon. On the one hand this should be enough for a nucleus to break the barrier of the earth magnetosphere and penetrate deep into the
atmosphere. On the other hand, since it is multiply charged, it will be rapidly decelerated and will enter into a thermal equilibrium with the terrestrial atmosphere at a sufficiently high altitude so that it can reach the altitudes of artificial satellites. And that means that radioactive beryllium registered in the earth atmosphere is of a solar origin. In that case the possibility of effective formation of radioactive elements in SCR, predicted back in 1967, will be given experimental confirmation.
And there are other "traces" of the solar origin of Be 7 found in the earth atmosphere at orbital altitudes. According to calculations by Russian and American scientists, conducted independently, the concentration of beryllium in the solar atmosphere could be determined in general by the Be 7 radioisotope. And if so, it can be present in solar wind, be carried with it to the earth orbit and enter its upper atmosphere through the polar regions.
Choosing between the first and the second groups of hypotheses is no simple matter. To make a confident choice it is necessary to trace in our terrestrial atmosphere an element which can not be produced by nuclear reactions between galactic, or solar, cosmic rays and elements of the earth atmosphere. This function could well be performed by radioactive Co 56 which must also be effectively produced among the secondary SCR isotopes.
And if it turns out that the origin of Be 7 in the earth atmosphere is connected with solar wind, this will confirm our ideas about nuclear interactions between SCR and the matter in the atmosphere of our luminary having a strong impact on the chemical and isotopic composition of the latter. And since the process of acceleration of what we call energetic particles within it can be regarded as quasi-continuous, it turns out in fact that it is there that the constant "reprocessing" of the primary matter is going on. That same matter from which the sun itself and the surrounding planets* had originated billions of years ago.
What is more, during the numerous analyses of the chemical composition of the atmosphere of the sun researchers came across what seems to be a strange fact. If one proceeds from the current notions about the natural occurrence of chemical elements in our environment, then there should be less beryllium on the sun than, say lithium. But this is not so in reality. There is not only no less beryllium in the solar atmosphere than lithium, but the ratio of their concentrations is constantly changing and is doing so in
Example of pre-flare increase of the flux of low-energy particles in interplanetary space, registered by PROGNOZ satellite on October 29, 1972. Shown on the lower part of the drawing is movement along the sun disc of the active zone, involved in the pre-flare build-up, in which the flare occurred. Along the ordinate: rate of count of protons of 1-5 MeV. Proton flux of more than 10 MeV and more than 30 MeV. Heliolongitudes of the active zone.
* See: G. Vasenin, "How the Solar System Was Born?", Science in Russia, No. 5, 1999. -Ed.
Solar gamma-flare. Spectrum of gamma-radiation obtained simultaneously by VENERA-13 (1) and VENERA-14 (2). One can clearly see that in the zone of quanta energy of 0.4-0.6 MeV the usual gamma spectrum pattern is upset. This is linked with the "inclusion" of yet another mechanism of gamma-quanta generation.
accordance with solar activity cycles: when it is high and the number of powerful Hares increases, the ratio of the levels of beryllium and lithium is clearly greater than one; and otherwise it is dwindling. All of these things can be explained only taking into account the possibility of formation of these elements directly in the solar atmosphere.
One should also bear in mind that among the isotopes of beryllium there is a stable one - Be 2 and two radioactive - Be 7 (half-life of about 35 days) and Be 10 (half-life of more than one million years). The most readily produced in nuclear reactions is the lightest one of them - Be 7 . This is primarily due to the fact that during interactions of energetic SCR particles (with the main role being played by hydrogen nuclei) with the nuclei of the most commonly occurring medium and heavy elements comprising the solar atmosphere (carbon, nitrogen, oxygen, iron) the probability of the formation of Be 7 is the greatest among all other beryllium isotopes. Apart from that it is also very important that it can be produced in what are called nuclear synthesis reactions in which the fusion of two lighter nuclei produces one heavier one.
Among such elements one should mention first of all helium whose concentration is second only to that of hydrogen. In nature helium is represented by two isotopes - He 4 (the most common) and He 3 (more rare). An interaction of two nuclei of the first produces isotopes of lithium and Be 7 .
The production of Be 7 in the interaction of the isotopes He 4 and He 3 is important because it is determined by what is called the exothermal reaction. This can take place practically at zero energy of interaction. In other words, the Be 7 isotope is being constantly produced in the atmosphere of the sun. On the other hand, because of its relative short life-time, its concentration will dwindle with changes of solar activity during the 11-year cycle. And it should also be noted that the role of the radioactive Be 7 isotope for the solar atmosphere and interplanetary space, as probably the main "culprit" responsible for the formation of Li 7 isotopes, will be clarified when it is finally registered in the solar wind. And one should add to that that nuclear synthesis reactions at low energies of interaction turn out to be very important not only for the formation of chemical elements in nature, such as in the atmospheres of the stars and on luminary, but that they represent one of the most important sources of gamma emissions.
And one should recall at this point that problems of solar gamma-astronomy began to be investigated from the 1970s by American, Russian (Soviet) scientists and also scientists of Germany, Japan, China, etc. The first successful theoretical analysis of these problems was carried out at our Institute back in 1967* and it received an amazingly accurate confirmation in an American space experiment with observations of gamma-radiation from powerful solar flares in August 1972. Gamma-astronomy studies later confirmed the relative nature of the term "quiet Sun" because even then there are enough possibilities in its atmosphere for the acceleration of nuclei of various chemical elements up to the energies of millions of electron volts. This is enough for synthesis
* See: M. Panasyuk, "Breakthrough into Cosmos", Science in Russia, No. 4, 2000. - Ed.
reactions and the nuclei thus produced are the source of solar gamma emissions in a range of quanta energy from 400 KeV and 3 MeV.
Theoretical and experimental studies of solar gamma radiation have been long and successfully conducted by experts of such Russian research centers as the RAS Institute of Cosmic Research, the Moscow Engineering-Physical Institute, the RAS Joffe Physical-Technical Institute and the Moscow University Research Institute of Nuclear Physics.
It was established that in the interactions of SCR with the matter of the solar atmosphere there occurs a transition into the state of excitation of not only the nuclei of its elements, but of particles moving in SCR at high velocities. This is eliminated by gamma radiation. But since the nuclei also possess considerable velocities, there occurs a "broadening" of the energy band of radiation. This phenomenon is well known as Doppler effect in wave processes of different nature. With reference to the gamma-lines from the 1974 solar Hare, they were identified by researchers of the Astrophysics Department of the Joffe Physical- Technical Institute.
As follows from experimental nuclear physics, reactions of synthesis of elements can occur at relatively low energies of the interacting particles and this often takes place in the solar atmosphere. Proceeding from the results of analyses, pioneered by our Institute scientists, one can say with confidence that synthesis of elements in the solar atmosphere is taking place almost continuously and involves almost all of the known elements in nature.
A synthesized nucleus, as a rule, "manifests" itself by emitting a typical gamma- quantum. But with the present-day possibilities we can observe only those of them which are generated in the collisions of the abundant "initial" elements in the solar atmosphere-hydrogen, helium, carbon, nitrogen, oxygen, neon and iron. In the course of this synthesis there appear vast numbers of gamma- quanta whose energy difference is so small that it takes some special and expensive instruments to trace them. For example, in interactions of nuclei of carbon and oxygen, the synthesized isotopes-silicon, aluminum, neon, magnesium, sodium, etc. - studied in an energy band from 0.4 MeV to 3 MeV there are about 100 quanta of different energy. The density of "compaction" by them of the energy band is so great in this case that it is quite appropriate to speak of the production of a quasi-continuous gamma-spectrum in the process of element synthesis.
Often present among the products of nuclear reactions are neutrons. Originating in the solar atmosphere during various active processes, they rapidly attain thermal equilibrium with the environment after which they are effectively captured, mainly by hydrogen, and deuterium is synthesized as a result. Close to one ton of it is produced on the average in one solar flare. The newly formed deuterium nucleus "settles down", having emitted a gamma-quantum of 2.223 MeV. This process is known as radiation neutron capture.
Particular interest to the registration of gamma-quanta, emitted in the radiation capture of neutron by a hydrogen nucleus, is due to the fact that this takes place deep within the solar atmosphere. And it turns out that gamma-quanta of 2.223 MeV contain some unique information about the conditions of solar plasma at these depths. What is more, the temporal passage of gamma-radiation, coming from the radiation capture of neutron by a hydrogen atom of the solar plasma, carries information about the energy spectra of the generated neutrons and the particles accelerated during the flare. Being able to separate these "channels of information" means getting a new source of data about active processes on the sun. This area of research is now being actively pursued in Russia, China, Japan and the USA.
At the present stage gamma-radiation generated in the radiation capture of a neutron by hydrogen can be easily registered by on-board instruments of space vehicles and satellites. Thanks to that it has been possible to
determine in a number of cases the depth distribution of density of solar plasma. And it became clear that during a flare this factor is different from a standard model. And local compactions are produced in the plasma at such times, appearing under and above the photosphere. Radiation neutron capture, of course, can take place not only on the atoms of hydrogen, but also of heavier elements. But so far it has been impossible to register characteristic gamma emissions.
Incidentally, the neutrons produced in nuclear interactions during active process of different intensity on the sun, extend not only into the depth of its atmosphere. If the energy of a neutron by that time exceeds 2 KeV, it escapes into interplanetary space. And then a neutron corona* appears around the sun. And since a neutron is but an unstable particle (life-time of about 15 min), it is impossible to observe solar corona from the ground and one has to move closer to the luminary One can also supposedly see it on approaches to Mercury.
One should bear in mind that only neutrons of over 100 MeV can cover the distance to the earth without significant losses of the flux. Such particles are generated during major solar flares and in interactions of high-energy galactic cosmic rays with the solar atmosphere. Registration of these neutrons opens up a unique possibility of sounding the space quite near the sun itself - in areas inaccessible to any space vehicles.
* See: B. Kuzhevsky, "Gravitation of Heavenly Bodies and Neutron Fluxes", Science in Russia, No. 5, 2001. -Ed.
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