by Academician N.V. Vasilyev

from the TunguskaEvent Website
 

The term "Tunguska meteorite fall" refers to the cosmic phenomenon that was observed on June 30, 1908, about 7 a.m. of the local time in Central Siberia, over Krasnoyarsk Territory, Irkutsk Region and Yakutiya [I ]. The most remarkable feature of the event was an explosion of a space object of unknown origin which was moving generally SE to NW and was seen in many settlements of the region.

The flight of the object was attended by sound, seismic and electrophonic effects which covered a vast territory [1-4], and it was equal to the whole set of manifestations of a bolide of 22m - 17m of the stellar magnitude. Its brightness was comparable to that of the Sun, but there was no smoky trail characteristic of large iron meteorites. Still, many witnesses noticed a trail of iridescent bands looking like a rainbow [4].

At the moment when the body was flying at the approximate altitude of 5.5 to 8 km over the area with the coordinates 60º 53’ N, 101º 54’ E (70 km to NW from the little trading station of Vanavara, Krasnoyarsk Territory, not far from the Podkamennaya Tunguska river), there occurred an explosion, or, more precisely, explosion-like energy release.* The TNT equivalent of the effect is estimated as 10-40 megatons, the energy being 4.2? 1023 to 1.7? 1024 ergs [5-8]. There is some evidence suggesting that following the explosion-like energy release at least a part of the Tunguska Space Body (TSB) continued to move in the "pre-explosion" direction upwards [9; 10].

The TSB "explosion" gave rise to a seismic wave recorded in Irkutsk, Tashkent, Tbilisi and Jena [1], as well as pressure disturbances which traveled around the globe [3; 11; 12]. In addition, 5.9+0.9 min. (or, according to another estimate, 6.6 + 0.2 min) after the ’’explosion", local magnetic storm began which persisted for more than four hours and was similar to geomagnetic disturbances following nuclear explosions in the atmosphere [13-16].

The shock wave of the Tunguska explosion leveled 2150 + 25 km2 of taiga [9; 17], and the flash burnt vegetation over an area of about 200 km [18; 19]. The Tunguska explosion resulted in a major forest fire covering an area comparable with that of fallen forest [20; 21].

It is noteworthy that the explosion on the Pod-kamennaya (Stony) Tunguska was just the most striking event in the set of natural anomalies which occurred in the summer of 1908 and were probably interrelated. It is now known [22] that beginning on June 23, 1908, atmospheric optical anomalies were observed in many places of Western Europe, the European part of Russia and Western Siberia. They gradually increased in intensity till the 29^th of June and then jumped to their peak in the early morning of the 1^st of July. These anomalies which are described in detail in Refs. [22 ] and [23] included unprecedented active formation of mesospheric (silvery) clouds, bright "volcanic" twilights ("variegated afterglows"), disturbances of the normal travel of the Arago and Babinet neutral points, supposed increase in the emission of the night sky, and unprecedented intense and long solar halos. Later, after July 1, these effects exponentially reduced; still some after-effects took place up to late July of 1908.

The area involved in these phenomena included a considerable part of the Northern Hemisphere and was bounded by the Yenisey river in the East, by the Tashkent - Stavropol - Sevastopol - Bordeaux line in the South, and by the Atlantic coast in the West. In August of the year the Western Hemisphere saw a decrease in the air’s transparency [24 ] which was due, as V.G.Fesenkov believed, to atmospheric circulation of the Tunguska explosion products. It has been shown recently that along with the Tunguska aerosol cloud, there was simultaneous circulation of products of destruction of another large bolide which entered the Earth’s atmosphere in May of 1908 [25]. Superimposition of these effects makes their separate interpretation more difficult.

It should be also noted that the summer of 1908 was quite rich in bright bolides in Siberia, as well as elsewhere [22; 26] (if one compares that year with the years 1907 and 1909 [27]).

The Tunguska meteorite explosion area was found in the 20s by surface explorations of L.A.Kulik, as well as through analysis of geophysical data performed by A.V.Voznesensky [28]. The chronology and findings of subsequent studies of this area are described in Refs. [29-32].
 

1)  accelerated growth of young (post-catastrophic) trees and those which survived the event [49-51]
2)  population-genetic effects, chiefly at the epicenter and along the TSB trajectory [52; 53]

This is a general outline of the Tunguska phenomenon which, upon thorough study, proves to be principally different from other impact phenomena.

The many hypotheses proposed to explain the Tunguska phenomenon can be subdivided into two groups. One will include those based on the concept of transfer of the kinetic energy of the Tunguska body into the shock wave energy. The other group consists of hypotheses emphasizing release of the internal energy of the body, chemical or nuclear.

Among the hypotheses of the former group, worthy of detailed consideration are primarily those involving concepts of asteroidal (an iron or stone asteroid, or a gigantic carbonaceous chondrite) or comet TSB nature. These can be classified as hypotheses based on the classical concepts of the minor bodies of the Solar System.

The hypotheses of the latter group assume a special type of the nature of TSB, different from asteroids or comets. These include the hypotheses of the antimatter nature of the TSB [54], of the Tunguska meteorite as a microscopic black hole [55], of a "solar energophore" [56] and even of technogeneous origin of the TSB [57-59].

The very fact of the existence of the so diverse views suggests a situation where the phenomenon in question is difficult to explain in all its aspects. Indeed, profound analysis of the factual data on the phenomenon evidences its structural complexity and seeming contradictoriness which restrict its interpretation in traditional terms. It is thus suitable to dwell upon certain most serious difficulties which are to be coped with in any attempt to construct an integrated concept of the Tunguska phenomenon.

1.  On the direction of the TSB flight
2.  On some peculiarities of the evidence of eye-witnesses who were close to the Tunguska explosion epicenter
3.  On some specific features of destruction of the forest at the Tunguska explosion epicenter
4.  The energy balance of the Tunguska explosion
5.  On the geophysical effects of the Tunguska catastrophe

 

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1. On the direction of the TSB flight

The first investigators of the Tunguska meteorite (L.A.Kulik, E.L.Krinov, and I.S.Astapovich [1; 2; 3 ]) who analyzed comparatively fresh evidences of the flight of the TSB on the Angara river did not doubt that it had moved generally from the south to the north, though there were three versions of its trajectory (the southern one, proposed by L.A.Kulik, the south-eastern by E.L.Krinov and the south-western by I.S.Astapovich). By the early 60-s it was Krinov’s trajectory, namely 135º east of the true meridian, that was considered the most realistic.

Later however, as more information was accumulated on the vector structure of the fallen forest field [9; 17; 59], a "corridor" of axially symmetric deviations of the vectors of the forest falling from the dominating radial pattern was revealed, and this deviation was interpreted as the track of the ballistic wave. The direction of, the "corridor" which was initially estimated as 111º E from N (114º east of the true meridian) [17] was later found to be 95º E from N (99º east of the true meridian) [10], which roughly coincides with the axis of symmetry of the radiant burn area [19]. In this period of time, V. G.Konenkin [60] and later other investigators [61-63] questioned old residents of the area who had lived in the upper reaches of the Nizhnyaya (Lower) Tunguska in 1908 (where there was no questioning in the 20s and 30s). This resulted in the conclusion that TSB had been observed in the said area as well, the analysis of the data suggesting that the body moved from the ESE to the WNW, i.e. by the path coinciding with the projection of that of the TSB, as found on the basis of analysis of the vector picture of the fallen forest area. This coincidence caused revision of the notion of the TSB path, and since the year 1965 the ESE-WNW (in fact, even E-W) version has been accepted in literature. For some years it was assumed to be finally true.

A grave disadvantage of the calculations of TSB path before the mid-80s was that there were analyzed only some separate groups of eye-witnesses’ accounts obtained by different researchers, in different periods of time, and not the whole body of evidence. Publication of the catalogue of eye-witnesses information [4] enabled analysis of the whole event. This was done in Ref. [56] and corroborated the considerations expressed earlier in Ref. [58] and also by I.S.Astapovich [64]. Two fundamental facts were established in particular:

1.  The total combination of evidence given by "eye-witnesses of the Tunguska fall" contains in fact information on at least two (most likely more) large day-time bolides. It is important that the "images" of the "Angara" and the "Nizhnyaya Tunguska" bolides are quite different and everything seems to indicate that they belong to different objects.

2.  The trajectory calculated on the basis of evidences of witnesses of the "Angara" phenomenon and corresponding most likely to its version proposed by E.L.Krinov [1] deviates considerably from that determined by analyzing of the vector structure of the forest fall area and the radiant burn area [9; 19]. Indeed, evidences of the Angara eye-witnesses, including the report of a district police officer, strongly suggest that the bolide flew "high in the sky", which is hardly consistent with the path 99º E of the true meridian. On the contrary, the data obtained on the Nizhnyaya Tunguska river, though agreeing with the configuration of the destruction area, are in contrast with the Angara observations.

An extra complication is that Nizhnyaya Tunguska data suggest virtually unambiguously that bolide’s flight took place in the afternoon, unlike those of the Angara which refer to the early morning.

Attempts to resolve the conflict between the data face with considerable problems. If the Angara and Nizhnyaya Tunguska observations are due to different bolides, which is most probably so, then with which of them the destruction area originally explored by L.A.Kulik is associated? Judging by the destruction area configuration, the most probable candidate is the eastern (Nizhnyaya Tunguska) bolide. However none of TSB investigators doubts that the explosion at the distance of 70 km from Vanavara occurred in the early hours of the day, not past midday [56]. Moreover, there is no direct proof that the Nizhnyaya Tunguska bolide was observed in the year 1908, inasmuch as this event was not recorded in any official documents, unlike the Angara bolide.

Besides, even assuming the area of the leveled forest, discovered by L.A.Kulik, to be due to the Nizhnyaya Tunguska bolide, it remains unclear where the Angara bolide fell, then. Throughout the Tunguska "meteorite" study there was no doubt the latter had in fact exploded in the Vanavara region...

But if the forest leveling was caused by the Angara bolide, how does it fit the direction of the "corridor" impressed in the area of the fallen forest by the TSB ballistic wave?

In the search of way out of this maze, more than one approach has been tried. Some researchers, preferring direct physical evidence, practically ignored eye-witnesses’ testimonies as an unreliable subjective material. This approach could be agreed with to some extent, if it were a matter of a few inconsistent testimonies, not many hundreds of independent reports. Besides - what is very important - the testimonies of the year 1908 include official documents of the time, whose authors were responsible to the authorities for their trustworthiness. For this reason, the eye-witnesses’ reports should be regarded as a material equal to other data sets or at any rate not to be ignored, even if they do not conform to some speculative arguments.

Other investigators tried their best to combine the Angara evidence, the Nizhnyaya Tunguska data and the geometry of the destruction area [65]. The results were rather dubious, strained, and quite different in this from the sufficiently unambiguous and clear picture which is provided when the two groups of eye-witnesses’ reports are analyzed separately.

Then, F.Yu.Zigel [58] introduced the concept of a "manoeuvre" made by the TSB, assuming it to have moved initially in a path close to that calculated by E.L.Krinov [1] and then, describing an arc, entered the space over the interfluve of the Nizhnyaya and Podkamennaya Tunguskas and took an eastern path which led it to the "explosion".

These contradictions have not been reconciled. It seems probable that the eastern group of eye-witnesses’ evidence is not directly related to the Tunguska bolide and that the latter moved in a trajectory-close to that calculated by E.L.Krinov [1]. The cause of the incompatibility of the bolide path projection with the data of the Angara eye-witnesses’ group remains unclear. Yet it should be borne in mind that the identity of the axis of symmetry of the observed forest destruction pattern with the projection of the bolide path that appears almost self-evident to the Tunguska meteorite investigators is only an assumption of high probability, rather than an established truth. The axially symmetric "corridor" is the trace of the ballistic wave where it touched the earth surface’, it remains essentially an open question what its initial space position was and whether it could change for some reason or other.

However, the problems associated with the TSB path parameters are not only these. Most of the authors who studied this question conclude that the slope of the TSB path was relatively small (some 15º) [1; 66; 67]. Still, modeling experiments [68; 69], as well as results of mathematical simulation of the Tunguska explosion parameters [70; 71] evidence that the slope of the final path section was most probably 40º. The transition of the TSB from the comparatively flat path to the steep one ("the peck") seems to have taken place as the bolide approached the spot of the explosion: it might be due, as is held in Refs. [72; 73], to avalanche breaking of the "meteorite", enlargement of its frontal surface and increasing resistance of the air.

Especially embarrassing is the fact that the "corridor", this impression of the ballistic wave on the forest, is observed, as has been recently shown, even beyond the epicenter of the explosion, as if roughly-continuing the direction of the TSB flight [74]. The most reasonable explanation is assumption of a ricochet of the TSB part which survived the explosion and continued its flight, maintaining, to some degree, the same trajectory. The question however arises if this assumption satisfies the requirements of the theory of the strength of materials.

As was mentioned above, the TNT equivalent of the Tunguska explosion is estimated as 10-40 megatons, the temperature of the center of the fire ball measuring at least tens of thousands of degrees Kelvin [75]. What must have been the characteristics of the TSB substance to withstand this "fiery font" and retain compactness and ability to ricochet? And how does it go with the above concept of the TSB consisting of comet ice or a silicate material, suggested by the first group of hypotheses to account for the Tunguska phenomenon?

Thus, analysis of materials characterizing the TSB path suggests its rather complex nature. It is not improbable that the Tunguska object moved in a non-ballistic trajectory.
 

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2. On some peculiarities of the evidence of eye-witnesses who were close to the Tunguska explosion epicenter

The TSB (or some part of it) exploded over a sparsely populated area, and for this reason only few eye-witnesses of the event found themselves close enough to the scene of the disaster. (The number of the victims of the Tunguska catastrophe was also very limited.) Besides Russian eye-witnesses who lived at the Vanavara trading station (70 km SE of the epicenter), whose reports were collected by L.A.Kulik (see: [1 1], there were within 40 km around the epicenter detached nomad camps of the Evenks (Tungus) on the Nizhnyaya Dilyushma, the Avarkitta (or possibly the Makikta, a right tributary of the Chamba river) and near the mouth of the Yakukta river (40 km to the south of the epicenter, close to where the so-called Kulik’s path intersects the Chamba river). The testimonies of the witnesses from the Nizhnyaya Dilyushma river were published as early as in the 1920-s [76] and contain memories of the fire in the forest and fallen trees. These are well known and have been commented on by specialists more than once. As regards the testimonies of the Evenks who had been on the Avarkitta and the Yakukta, those were published much later and contain some strange details that seem to deserve special attention. These details are definitely queer, and the reader finds himself before the alternative: either deny them as obviously absurd, or - be they believed if to a certain degree - assume that our ideas of the physics of the Tunguska explosion are wrong.

The first of these reports was communicated by the well-known ethnographer and public figure I.M.Suslov who spent many years working in Evenkiya (the territory of the Evenks or Tungus). In 1925, during the "suglan" (kin council), he questioned those people who had seen the Tunguska event [ 76 ]. Much later, in the 60s, being a pensioner, I.M.Suslov informed the Siberian Commission on Meteorites and Cosmic Dust that he had some unknown materials on the problem which he was ready to make public in a collection of papers on the topic of the Tunguska meteorite. Shortly after that he gave them a large manuscript, which was edited and abridged by V.E.Shnitke and then published in the collection The Problem of the Tunguska Meteorite in 1967 [ 77 ]. It remains unclear why Suslov had not had it published before. Equally unknown have remained the initial records on which the paper was based. No original notes on the topic were discovered in Suslov’s papers after the man’s-death. The impression is that the paper was written by himself on the basis of either his personal memories or some notes now missing.

I.M.Suslov’s paper is a detailed presentation of a report of the Evenks of the Shenyagir kin who were at the moment of the Tunguska explosion in the middle part of the Avarkitta river. The scope of the present paper is not sufficient to completely present this report, but its essence can be summarized as follows.

There were five explosions, the second seeming to have been the most powerful. Light flashes followed at an interval of a few seconds and were seen at different spots of the sky. The last, fifth explosion took place far in the north, somewhere near the Taymura river. Trees began to fall and the fire began after the first explosion, while the Evenks were in their "chums" (tents of skin or bark), the latter being thrown down. There were traumatized people.

The data communicated by I.M.Suslov are quite detailed and enable the whole phenomenon to be estimated as lasting no less than 20-25 seconds.

Another report was conveyed to the TSB investigators by V.G.Konenkin, a local inhabitant and school teacher of physics in Vanavara, who had questioned old residents of settlements of the upper reaches of the Nizhnyaya Tunguska and the Tunguska-Chunya Region of Evenkiya for several years. Among those questioned was an Ivan lvanovich Aksenov, an elderly Evenk man, who was said to have been a shaman hiding for many years in taiga from the authorities. The entire record of I.I.Aksenov’s account is presented in Ref. [4]. At the moment of the catastrophe the eye-witness was on the Chamba river, hunting near the mouth of a tributary of the Chamba, that is some 40 km to the south from the catastrophe epicenter. A particular feature of Aksenov’s account (agreeing otherwise with the early evidence of Vanavara residents that Kulik had heard as far back as the 20s), is the assertion of the eye-witness that after the explosion he had seen an object flying down the Chamba, i.e. generally north to south. He called the object a "devil".

"As I came to myself, he told Konenkin, I saw it was all falling around me, burning. You don’t think, Viktor Grigoryevich [V.G.Konenkin], that was god flying, it was really devil flying. I lift up my head - and see - devil’s flying. The devil itself was like a billet, light color, two eyes in front, fire behind. I was frightened, covered myself with some duds, prayed (not to the heathen god, I prayed to Jesus Christ and Virgin Mary). After some time of prayer I recovered: everything was clear. I went back to the mouth of the Yakukta where the nomad camp was. It was in the afternoon that I came there..."

Afterwards I.I.Aksenov repeated his account in the presence of V.G.Konenkin and V.M.Kuvshinnikov, an active participant of the Tunguska expeditions. In this case, however, he said that he had seen the "flying devil" not during hunting but in the afternoon, when already in the camp near the mouth of the Yakukta, also a tributary of the Chamba. The devil was going flying southward along the Chamba. It was going faster than airplanes now do. While flying, the "devil" was saying "troo-troo" (which were not at all loud).

Later, during repeated questioning in Vanavara, he did not insisted on having seen the "devil", repeating nevertheless the other evidence. When evaluating Aksenov’s account it should be borne in mind that the eye-witness regarded the expedition people with distrust, considering them representatives of the "authorities", and thus the contact with him was not at all easy. On the contrary, his relations with V.G.Konenkin were fairly confidential, the latter being a local resident and a half-caste. Therefore, in our opinion, the first version appears to be more authentic, because Aksenov does not seem to have had reasons to lie to Konenkin.

What is the true meaning of this queer story and how trustworthy is it, it is now hard to say. Without overrating the significance of individual eye-witnesses’ evidence, note nevertheless that at least two reports provided by those (very few) eyewitnesses who were close to the epicenter of the Tunguska explosion are really peculiar.

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3. On some specific features of destruction of the forest at the Tunguska explosion epicenter

It has been ascertained that the main cause of forest destruction in the area of the Tunguska catastrophe was powerful energy release that took place at an altitude of 5.5-8.0 km. It must have been thus a huge explosion in the air which gave rise to a spherical shock wave, with the front at the epicenter generally parallel to the earth surface and inclined to it away from the epicenter. Thus, at the latter the vertical component of the shock wave was mainly active, while away from it the horizontal component was ever increasingly dominant, its action most pronounced in the area of interference of the incident and reflected waves [6].

As a first approximation, this is indeed so. Around the catastrophe epicenter there is a vast (about 8 km across) area of what is called "telegraphnik" (that is, looking like a forest of telegraph poles) - the dead forest, scorched and devoid of branches, but trees standing upright. This is the zone of action of the blast wave vertical component. Outside this area forest is put down radially, to distances from 12 to 40 km or more in various directions (see image below). This is, in its turn, the area of action of the blast wave horizontal component.

If the above model is fully correct, then, first, at the explosion epicenter there must be no radial forest falling, and second, the destruction pattern should be generally uniform.

The real situation is, however, essentially different. Firstly, even at the epicenter forest was not destroyed completely. Within 5-7 km from the epicenter, many small groups of trees survived [ 78 ], mainly of larches. Topography of such groups is not quite definite, though some of them are in shallow valleys between hills or along rivers and brooks. These groups of trees have attracted attention of investigators more than once [ 1; 4; 79] since as early as the times of Kulik; however attempts to account for them based on the relief features have not yielded unambiguous results.

The highest altitude above sea level of the whole area is 593 m, which, in the case of explosion at an altitude no less than 5.5 km, enables treating the whole area as a plane. At the same time, the very fact of existence of these groups and their patchy arrangement seem to suggest high non uniformity of the action of the kinetic factors responsible for destruction of forest at the catastrophe epicenter. This assumption is also supported by some other facts. According to calculations [71] and observations in the field [80], the thermal pulse at the epicenter must have been 15 to 55 cal cm^-2. This is certainly sufficient to singe cedar branches which are highly sensitive to thermal influences. Meanwhile, there is a group of cedars that survived the Tunguska catastrophe on the bank of the Southern Swamp, no more than 2.5 km from the projection of the center of the light flash of the Tunguska explosion [81], and right in the marsh there grow fir trees which also survived the catastrophe. B.I.Vronski, in the year 1960, found on the surface of the Southern Swamp, practically at the center of the projection of the light flash of the Tunguska explosion, a lively larch, aged over 60 years. The location of the tree made improbable its being screening at the moment of the explosion.

The structure of the forest fall area in the immediate vicinity of the epicenter also proved strange.

Firstly, the assumption of the absence of radial tree falling here is not true. Surface observations [82] evidence that there are some leveled trees in this area as well, and the general radial character of the forest falling is seen up to a "special point", viz. the geometric center of the fallen forest area, as calculated by V.G.Fast [17 ].

Secondly, Kulik’s interpretation of the fallen forest area on the basis of the large-scale aerophotography of 1938 not only corroborated the complex vector structure of the epicentral area, but also enabled assumption of the existence there of at least two or three subepicenters [ 83 ].

Thirdly, the vector structures of the forest falling on hill-sides facing the epicenter and the opposite ones are essentially different, which is in poor agreement with the assumption of the center of generation of the blast wave located high above the earth.

Thus, the conclusion suggests itself that along with great energy release 5.5-8 km above the earth, there were a number of low-altitude (maybe even right above the surface) explosions that contributed to the total picture of destruction. This seems to be sustained by other data concerning in particular the configuration of the zones of dead trees ("poles") [84] in the central part of the area of the catastrophe and deposition of aerosols immediately after the explosion.

Extraordinary variety of effects is also suggested by analysis of radiant damages of trees [85]. Literally beside trees carrying distinct signs of thermal effects, there are many undamaged trees, the cause being not always clear.

Thus, the features of destructions at the epicenter suggest in homogeneity of the physical parameters of the Tunguska explosion field and complexity of the physical processes underlying it.

It should be emphasized that though the patchiness of the effects associated with the Tunguska catastrophe has been noted in literature more than once, its origin still as a rule has not been discussed. This sees to be due to serious difficulties of its interpretation in terms of the existing TSB models.

Note that the range of unusual features of local effects caused by the shock wave and the thermal factors of the Tunguska explosion is not restricted to what has been said, as suggested in particular by the difficulties arising in interpretation of fiery damages of trees injured during the explosion-induced forest fire [86; 87].

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4. The energy balance of the Tunguska explosion

The TNT equivalent of the Tunguska explosion was 10 to 40 megatons, comparable to the equivalent of the largest thermonuclear explosions. Most energy was consumed by formation of the shock wave, no less however than 10% released as the flash [88; 89]. It was formerly assumed that the high yield of luminous energy was a typical feature of nuclear explosions and could attest to the nuclear nature of the Tunguska explosion [89]. Later however it was shown that the same effects could attend also other types of explosions, destruction of large meteoric bodies in particular [71].

The energy of the Tunguska meteorite has been independently estimated by analyzing seismo- and barograms [7; 8] and using mathematical modelling. However, as has been suggested by detailed works of V.P. Korobeynikov et al. [70; 90], the models of the Tunguska catastrophe based on the assumption of transition of the kinetic energy into the energy of the explosion do not provide adequate explanation to the observed pattern of destructions and require, for the energy balance, a certain "addition" from the TSB internal energy. This aspect of the TSB problem has not been studied more thoroughly. Thus, the question of the energy source of the Tunguska explosion still remains open.

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5. On the geophysical effects of the Tunguska catastrophe

One of the most striking geophysical effects associated with the Tunguska catastrophe is the local geomagnetic disturbance detected, shortly after the explosion, in Irkutsk, though not recorded by any other geophysical observatory of the world existing at that time [13; 14; 15]. This disturbance was similar to some effects following middle - and high -altitude nuclear explosions in the atmosphere [59; 91], but unlike the latter, it exhibited a kind of lag, i.e. it occurred some time after the explosion. It has provided the main argument to account for the geomagnetic effect of the Tunguska "meteorite" as due to entering of the shock wave the ionosphere: the lag was in good accordance with the period of time required for covering by the wave the distance from the explosion site to the lower ionosphere boundary.

Later, however, I.P.Pasechnik [92] has corrected the moment of the Tunguska explosion on the basis of direct experimental measurements of the velocity of the seismic wave between Vanavara and Irkutsk. It has been found that the "lag" was actually longer than 5.9 min. This fact, thoroughly analyzed in Ref.[56], is radically important to the problem, inasmuch as the ensuing velocity of a shock wave in the atmosphere is too low. Hence any mechanism accounting for this effect as a consequence of arrival of the shock wave in the ionosphere appears dubious. The question thus remains open, and again, as in 1960, we are faced with the problem, without explanation.

Neither have been found adequate explanations to the changes of the polarimetric properties of the twilight sky that appeared as deviations from the normal travel of the Arago and Babinet neutral points [22; 93]. This effect was noted on July 1^st, 1908 in Arnsberg (Germany) and disappeared by July 20^th. Its characteristics differ from the polarimetric disturbances observed after other cases of atmospheric dusting. It is not improbable that such effects are related to global development of mesospheric (silvery) clouds [94]; there has been however no experimental verification of this assumption.

The explanation of the "light nights" of the late June and early July, 1908, with recourse to dispersal of particles of a comet’s tail in the upper atmosphere of the Earth [95] is not at all convincing.

Indeed, according to this assumption, particles of the comet’s tail were to be decelerated at the altitude of 200 km or more, whereas most light anomalies formed at altitudes of 80 km (the zone of formation of mesospheric clouds), 50-60 km (diffraction effects that caused dawn and afterglow anomalies) and below that (atmospheric halos) [22 ]. Besides, in this situation the tail of the "Tunguska comet" should have been stretched over Canada [96 ], but this was not so. Recently, there has been an attempt [97] to ascribe the "light nights" of the summer of 1908 to transport of material from the site of the explosion by stratospheric winds. This assumption however is faced with two contradictions.

Firstly, at least at 10 places of Eurasia there were anomalous light effects on the night of June 29-30, 1908. i.e. practically simultaneously with (and even somewhat before) the Tunguska explosion, which makes it impossible lo explain the optical effects of June 30, 1908 as due to mechanical transport of space aerosols from the site of the Tunguska event.

Secondly, in discord with this explanation is also the sharp exponential decrease in the intensity of the atmospheric anomalies after the 1^st of July which well conforms to the assumption of the dominating contribution of photochemical reactions to formation of these. If, alternatively, the main contribution were given by refraction and scattering by aerosol particles, it would be more reasonable to expect gradual decrease in the effects, as in the case of volcano-induced optical anomalies [22].

Thus, explanation of the geophysical effects of the Tunguska "meteorite" has been faced with essential problems, the main being lack of a satisfactory explanation of the geomagnetic effect.

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