Libmonster ID: BY-1585
Author(s) of the publication: Sergei YAZEV

by Sergei YAZEV, Dr. Sc. (Phys. & Math.), Director of the Astronomic Observatory of the Irkutsk State University, senior researcher of the Institute of Solar-Terrestrial Physics, RAS Siberian Branch (Irkutsk)

The year 2009 marked an outstanding event in the history of astronomy, i.e. the 400th anniversary of the first telescope-based observations of the starlit sky. The pioneers were Galileo Galilei in Italy, Johann Goldschmidt (Fabricius) in Holland, Thomas Harriot in England and Christoph Scheiner in Germany. Owing to optical devices back in early 17th century, it was managed to discover black spots on the Sun and confirm that the formations on the Sun and not planets in their orbital motion happened to be on the Sun-Earth line. Long after it has been found that such spots are concerned at least with three levels of the solar activity organization. One of them has become a subject of our discussion.

SPOTS AND ACTIVE LONGITUDES

During a period of four centuries scientists studied in detail the phenomenon of solar spots. In early 20th century the American astronomer George Ellery Hale made two notable discoveries in regard of these grandiose formations according to terrestrial standards. First, spectral observations have proved that the temperature in the spots is less than in the surrounding photosphere (visible radiating surface layer) of the Sun, and the difference makes 1,000-1,500 K and more. Secondly, the spots have strong magnetic fields, which are actually their basis. By hindering convection (heat transfer by rising plasma fluxes) the magnetic fields result in a decrease of energy exit from the star depths and, correspondingly, to a temperature decrease and relative reduction of brightness of these formations. The large spots reveal a central and darkest part or "a shadow" surrounded by a lighter circular area or "a penubra".

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The magnetic fields are mainly vertical in shadows and quasi-horizontal in penubras.

By means of routine observations it has been managed to reveal a set of essential properties of the solar spots and, in particular, establish that they appear, as a rule, not one by one but in groups. The latter markedly differ in area, shape and number of individual spots, though rapid and essential changes of all parameters of the group are possible. According to statistics mainly such spots do not exist for long, 90 percent of them exist less than 11 days and more than half of them exist less than two days. The spots appear at least 35-40° from the star equator, while the first groups of a new 11-year cycle of solar activity are born close to a high-latitude border of the said zone. The following groups are formed near the equator as the cycle develops.

The term "spot group" is seldom used in modern literature. As a rule, more widespread is an "active region" (AR). It includes not only spots but also a complex of phenomena connected with them such as bright flares on photosphere and flocculi in chromosphere*, a zone of the disturbed structure of the latter surrounding a spot group and also coronal structures (systems of magnetic loops) running high to the corona. It is well known that, with rare exceptions, solar flares occur just in active regions. The physical basis of AR is its magnetic field, sometimes having a very complex structure.

Let's get down now to distribution of spot groups according to longitude, which looks chaotic at first

* Flocculi are bright formations in the Sun chromosphere. Chromosphere is a hot rarified layer of the Sun atmosphere of about 10,000 km thick located over the photosphere.--Ed.

sight. It seems to be quite natural as a priori it is beyond reason to imagine that any longitude of the Sun should differ from others. The plasma sphere of the Sun appears axially symmetrical, and all longitudes should be equal from this point of view, in any case, at time averaging essentially more than an average lifetime of an individual spot group.

But in practice the situation is different. When we sum up the number of spot groups or their total area for a long period of time, i.e. for one, two and more cycles, we reveal that at individual intervals of longitudes the spots systematically occur in larger quantities (or in large areas) than in the neighborhood. Such intervals are called "active longitudes". This phenomenon was studied for many years by Yuri Vitinsky, Cand. Sc. (Phys. & Math.) from the Pulkovo Astronomic Observatory, Russian Academy of Sciences.

The active longitudes as such are known for long though they arouse some perplexity as before. Strictly speaking, the cause of their existence is not clear yet. One hypothesis suggested in 1997 by a group of Finnish astrophysicists and developed by Leonid Kichatinov and Alexander Mordvinov, both Drs. Sc. (Phys. & Math.) from the Institute of Solar-Terrestrial Physics of the RAS Siberian Branch lies in the fact that the Sun's radiant core has a relict nonaxisymmetric magnetic field. By interacting with an axially symmetrical magnetic field it should lead to the observed phenomena of the north-south asymmetry of the solar activity and altitude variability of the neighboring cycles. This interesting hypothesis is not generally recognized. Another version assumes the existence of a nonaxisymmetric

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oscillator (oscillating system) in a spot generation zone, which provides successive activation of two pairs of antipodal active longitudes.

Thus, traditionally two main levels of solar activity organization are singled out: active regions as its basic component and active longitudes as long-living zones of predominant formation of active regions. Now when properties of the said levels are briefly described let us proceed to the description of one more, intermediate, level.

THE THIRD LEVEL

It was discovered by researchers in the 1960s, but was ignored for a long time, and data about it were not systematized. The gist of this phenomenon is that continuous spot formation can be observed in the same area of the solar surface for a long time (during several months). In addition, the matter is not a single long-living active region: here some of them appear and others disappear, but in general a system of the long-living magnetic field exists much longer than a typical time of life of an individual active region. These individual active regions, appearing both successively and simultaneously, cannot be regarded as independent in full measure as their local magnetic fields are included in a single system of the common field.

Such large-scale (though less than active longitudes) formations are called complexes of activity (CA), and this notion was introduced in 1965 by the Czech astrophysicist Václav Bumba (foreign member of the USSR Academy of Sciences from 1988) and the astronomer Robert Howard (USA). It should be noted that many researchers had already paid attention to some areas of the solar surface, where the activity was resumed regularly in the form of spots. The phenomenon was considered from different viewpoints and defined by various terms. For example, in 1948 Moris Eigenson, Dr. Sc. (Phys. & Math.) from the Lvov University, Ukraine, called this phenomenon "a pulse of activity". Other authors defined it as "nests of spots", "pulsating centers of spot formation", etc.

Later the properties of complexes of activity were considered in detail by many researchers both foreign and national from the Pushkov Institute of Earth Magnetism, Ionosphere and Radio-Wave Diffusion (Troitsk), the Shternberg State Astronomic Institute, the Main Astronomic Laboratory, the Institute of Solar-Terrestrial Physics, the Ulugbek Astronomic Institute (Uzbekistan) and others. They offered different approaches for description of this phenomenon. In this paper we shall consider the results obtained by the Irkutsk school of heliophysicists from the Institute of Solar-Terrestrial Physics (RAS SB) and the Astronomic Observatory of the Irkutsk State University. The fundamentals of this approach were laid down by ValEry Banin (1930-1998), Dr. Sc. (Phys. & Math.) from the Institute of Solar-Terrestrial Physics jointly with the author of this paper.

By analysis results of the Sun images in H-alpha flare a complex of activity can be imagined in the form of a giant spot in which one or several active regions united by a common system of magnetic field represent its central part or "the core". A spot with the so-called delta configuration is the closest analog. Within one "penumbra" several fragments of "a shadow" can coexist, and what is more, of both magnetic polarities. Both in the spot shadow and in the core of the complex of activity magnetic polarities are mixed, and the magnetic field (examined at a large spatial scale) is directed mainly vertically. The core proper is surrounded by a wide circle of fibrils located quasi-horizontally and mainly radially to it. Fibrils demonstrate direction of

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the magnetic field just as metal filings in the experiment at a school physics lesson.

Such "superspot" (complex of activity) can exist long enough, from three months to a year or longer. Its core is revealed in the form of a zone of permanent spot formation. Inside it dozens of active regions can occur and disintegrate successively or simultaneously.

Depending on their "architecture" complexes of activity are divided into single-core and multi-core ones. Within a common "penumbra" up to 3-4 cores of CA can be observed, which are interconnected by high arches of the magnetic field. Sometimes similar giant CA cover almost a half of the Sun along the longitude. In some instances it is managed to reveal a "branch" of the core, i.e. an active region existing relatively not long but also connected with the core by the said arches.

The statistics based on the data on the current 24th cycle of Solar activity (started in January of 2009) proves that spot groups as a part of CA make up on average around a half of the total number of the former.

Many researchers including Yevgeny Ivanov, Dr. Sc. (Phys. & Math.), from the Institute of Earth Magnetism, Ionosphere and Radio-Wave Diffusion and the author of this paper believe that the active regions can be divided into two classes depending on the depth of magnetic field formation in the Sun's depths. Most of the short-living active regions probably originate not deeply enough (several thousand kilometers) under the photosphere level. As regards the active regions forming the core of the complex of activity, their magnetic fields most probably are generated in the depths of the Sun's convective zone (up to 200,000 km under photosphere). It cannot be ruled out that CA are connected with giant convective cells supposedly existing on the Sun.

PORTRAIT IN THE INTERIOR OF THE STAR

The Institute of Solar-Terrestrial Physics together with the Astronomic Observatory of the Irkutsk State University is conducting continuous monitoring of complexes of activity. It also developed a method allowing to identify their cores to distinguish complexes of activity, for example, from long-living individual active regions. Besides, a special index, "capacity of CA core", determined by a three-point scale was suggested for assessment of appropriate parameters. Developed by the Irkutsk heliophysicists, the catalog of CA cores numbers more than 370 such structures observed on the Sun from 1980. The analysis of these data revealed specifics of occurrence and development of complexes of activity and also a number of their properties.

A typical CA evolution proceeds in the following manner. The first groups of spots in an incipient core happen to be large enough. Moreover, as a rule, from two to four closely spaced groups appear at once. CA reaches a maximal stage of development in the second or third month of its existence. Short-living structures (up to four months) make up 54 percent of the total number of CA cores. Nevertheless, long-living formations are also observed. For example, in the 23rd cycle of Solar activity (1996-2008) a unique complex of activity of almost one and a half years was registered. It

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should be noted that as regards long-living CA a secondary splash of their activity is typical in the sixth or seventh month of their existence.

At the CA destruction stage the magnetic structure of active regions in the core is simplified. A large-scale torchlight platform remains instead of the disappeared spots. There is often formed a coronal hole due to an increasing in size unipolar (single-pole) region of the magnetic field, from where power lines are carried away by the flow of solar wind particles to the interplanetary space. If we consider that such hole is a complex of activity at a late (spotless) stage of development, we can conclude that the total length of existence of a typical CA in different forms (including a spotless phase) approaches a year.

Several points of interest are revealed in the distribution of complexes of activity on the Sun's surface. First, it is north-south asymmetry. If in the northern hemisphere a complex of activity is observed, in the southern hemisphere at the same heliographic longitudes and in the same period of time its "twin", as a rule, is not formed. It can be formed only at another interval of longitudes, but there where no complexes of activity are found at that time in the northern hemisphere. Besides, sometimes the northern and southern hemispheres form them in different ways. For example, in the 22nd cycle of Solar activity (1986-1996) the CA number in each hemisphere was equal (52 objects each), while in the next 23rd cycle the southern hemisphere prevailed with 77 against 69 CA in the northern hemisphere. In the current 24th cycle (started in January of 2009) the northern hemisphere has an obvious advantage. Secondly, there has been revealed the effect of a peculiar "relaxation" when after a long existence of a multi-core complex of activity no new cores appear in this place for 8-10 months. Thirdly, it was managed to find the effect of longitudinal drift of CA. This means that sometimes a new core appears near the already existing one several months later. Then on the same side of the core another core is formed 4-5 months later, and so on. A chain of the new CA cores in the 23rd cycle was traced for 80 months. This effect leads to a quasi-linear displacement of the active zone of the development of multi-core complexes of activity along longitude at a speed of the order of 10° in a month.

THE SCHWABE-WOLF CYCLE AND POWERFUL FLARES

In the middle of the 19th century it was found that the "sunspot activity" varies with a high amplitude. A period between the neighboring minima of activity, when the number of spot groups drops almost to zero, makes up on average about 11 years. A decisive contribution to the establishment of this regularity was made by the German observer Heinrich Schwabe and Swiss researcher Rudolf Wolf.

Spot groups as a part of complexes of activity are also subjected to this global regularity. The curves describing time changes in the classical index "Wolfs number" (relative number of sunspots) and index "CA core capacity" in the 24th cycle are well correlated. The maximal correlation factors for numbers of cores of CA and Wolf comprise 0.85, for the total core capacity of CA and the Wolf number is 0.86. This fact allows to consider a hypothesis that it is just spots in complexes of activity that determine a progress in the development of Schwabe-Wolf cycles.

The analysis has proved that core indices of complexes of activity change quasi-periodically, by pulses (splashes), during a cycle when in the northern or southern hemispheres several cores appear almost at one time. During a cycle there are registered 6-8 such events, which last for 6 to 14 months.

In the course of studying variations of properties of complexes of activity during the 23rd cycle a marked depression was registered for values of core indices of CA in a period of its maximum. Afterwards the level of its activity increased again to pass over to a phase of its final decay. It was revealed that during depression at the phase of maximum there prevail short-living and not very powerful CA, while a share of spot groups unrelated to CA increased in this period. The conclusion suggests itself: bimodality of the 23rd Schwabe-Wolf cycle known to heliophysicists is connected with a respective behavior of CA cores.

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Many specialists long ago paid attention to the fact that a great majority of spot groups with the strongest Solar flares is a part of complexes of activity. The studies of localization of the most powerful flares, which produce proton streams with energy more than 10 MeV in the quantity over 10 particles per second per 1 cm2 on the Earth orbit, proved that a share of powerful flares in the active regions located in CA cores made up 90 percent for a period of 1980-2012.

A more detailed analysis has enabled us to get convinced that in the first and second months of the existence of CA cores there is registered 41 percent of flares of the population under study. A great number of them is also observed in the third and fourth months, 18 percent and 15 percent respectively of the total selection. An important point is that a chance to produce a flare of such class increases for CA cores in proportion with duration of their existence. For example, only 10 per-

стр. 9

cent of similar flares occur in those cores which live three months, while 100 percent of them occur in "seven-month old" CA cores (data according to the 23rd Schwabe-Wolf cycle).

The contribution of complexes of activity to statistics of another class of flares is also evident. It has proved that the following two parameters demonstrate the high correlation: the number of flares with a prolonged attenuation of the flare in the soft X-ray band (the so-called LDE-event) and the total capacity of CA cores. The following scheme can be offered, which explains this correlation. Due to interconnection of all active regions within a complex of activity disturbances of magnetic fields cover it entirely or its substantial part. These extraordinary events are accompanied by flares, coronal mass ejection and also splashes of X-ray emission in systems of high magnetic loops rising to the corona. Most of the disturbances are connected with rising to the surface of a new magnetic flow from the Sun's depths, which begins interacting with the already existing "old" flow. In such (and only such) systems development of lengthy "excesses" is possible and therefore also exis-

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tence of X-ray splashes with a prolonged attenuation (LDE-events). Thus, we come to a hypothesis that the mere fact of existence of the class of LDE-flares is connected with their development within the limits of a complex of activity.

How do the described events affect the processes occurring on the Earth? The relation of the most geoeffective Solar flares and CA cores is beyond doubts. Emissions of coronal substance is a powerful geoeffective agent in this case. Clots of solar plasma reaching the Earth environs cause geomagnetic storms and other variations of geophysical parameters on our planet. As powerful Solar flares are closely associated with complexes of activity, one should admit that it is just the latter which are mainly responsible for these events.

In some instances it has been proved that coronal holes on low latitudes have a genetic link with CA cores. After decay of spots in such cores magnetic fields of active regions diffuse and form a large-scale bipolar structure, and an isolated coronal hole is formed in one of magnetic cells. As coronal holes serve as sources of a geoeffective flow of solar wind, they can be regarded as

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a manifestation of a complex of activity at the final stage of its evolution.

Consequently one may state that it is just complexes of activity which are main sources of all types of geoeffective radiations of the Sun and also, in places, of generation of major proton flares and emissions of coronal substance (at the development stage of spots in complexes of activity), and at the same time, in places, of flowing of a high-speed solar wind (at the decay stage of complexes of activity).

Studies of complexes of activity on the Sun by Russian scientists are going on. Next thing on the agenda is formation of effective algorithm for predicting the development of complexes of activity, which will allow to implement long-term forecasts of geoeffective Solar events.

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