Paper delivered at the International Workshop Tunguska -'96 Bologna, Italy
by Victor Zhuravlev
Geomagnetic effects produced by the Tunguska Event had no features in common with known disturbances caused by meteors. However, the disturbances strongly resembled local magnetic storms that occur after man made thermonuclear explosions in stratosphere. The similarity between the geomagnetic effects of the Tunguska Event and of the thermonuclear explosions just mentioned were generally dismissed by the scientific community because of its seeming unliklihood. As a result, the geomagnetic effects, as part of the accumulated Tunguska data, were forgotten, although the data were very significant and informative. An understanding of the significance and essence of those geomagnetic effects could well influence conceptual schemes in providing protection for our planet against potential asteroid and comet impacts.
Two phenomena in the history of the Tunguska research are of key importance: 1) The explosion of the cosmic object occurred in the air; and 2) Analysis of the geomagnetic effects of 30 June 1908 recorded by the Irkutsk Observatory. The first phenomenon is commonly known nowadays, but the second has been all but forgotten.
In spring of 1959, G.F. Plekhanov and N.B. Vassiliev found a brief report in a German scientific journal of 1908 that professor Weber of Kiel University observed unusual regular periodical deviations of the compass needle [Astronomische Nachrichten, 1908]. This effect was repeated each evening from 27 June through 30 June 1908 and lasted for 7 hours during each occurrence. After the Tunguska explosion this effect disappeared. This publication, in addition to information from later scientific journals about the geomagnetic effects of thermonuclear explosions, in 1958 prompted Plekhanov and Vassiliev to send inquiries to various observatories that were in operation in 1908. However, the magnetograms that were received in response to their inquiries revealed no anomalies.
However, in February 1960 the above investigators received a reply from the Irkutsk Observatory geophysicist Kim G. Ivanov informing them that he had discovered two unusual magnetograms dated 30 June 1908. Both almost certainly had recorded the Tunguska disturbance. Ivanov himself did not offer his own interpretation of the physical nature of this unusual, unexpected effect. The analysis by geophysicist Alexander F. Kovalevsky and other researchers from Siberia's university city of Tomsk led scholars there to conclude that the magnetic effects of the Tunguska explosion had nothing in common with disturbances usually caused by typical meteors bodies. The closest analogy of the recorded magnetic effects, strange for meteoritics, turned out to be regional geomagnetic storms. Such storms had been recorded by the magnetographs in the Pacific Ocean atolls in 1958 following the thermonuclear testing [Plekhanov et al., 1960]. Those new geomagnetic effects caused a sensation that year. But even more sensational was the realization that almost an identical geomagnetic effect had occurred on our planet 50 years earlier!
A detailed analysis of the magnetograms of a short term local geomagnetic storm recorded in Irkutsk (970 km away from the Tunguska explosion) was published in the 1960s [Plekhanov et al. 1960; Kovalevsky, 1963; Ivanov,1964; Zhuravlyov et al.,1967; Zolotov, 1969]. Independently, both Kovalevsky and Zolotov made thorough comparisons of the Tunguska geomagnetic effects and the effects of the high-altitude thermonuclear explosions in the Pacific. According to Zolotov, both the effects occur in four stages. The initial stage is caused by a magnetohydrodynamic wave. It's energy depends on the volume and temperature of the explosion plasma. The main phase is caused by the diffuse motion of secondary electrons held within a magnetic trap. Primary electrons and ions are generated by radioactive isotopes in the plasma of the fireball of a nuclear explosion. The reserve of the radioactive isotopes accounts for the duration of the geomagnetic storm, since only the recombination of electrons and ions would not permit such a storm to last more than an hour. For this reason, the duration of the magnetic disturbance of a chemical explosion could not last longer than 10 minutes. However, the geomagnetic storm caused by the Tunguska explosion lasted for 4 to 5 hours. This is the main difficulty in explaining the Tunguska explosion within a cometary hypotheses frame.
Attempts to explain the unusual Tunguska geomagnetic effect, based on hypotheses about the blast wave of the explosion affecting the ionosphere, were criticized in Zolotov's monograph and in a computational paper by Zhuravlev and Demin [Zolotov, 1969; Zhuravlyov et al., 1967]. In 1969, a paleomagnetic anomaly was discovered in the area of the Tunguska explosion [Boyarkina and Sidoras, 1974]. Computations made by the present author allowed suggest that this anomaly is the result of the first stage of the geomagnetic storm of 30 June of 1908. The main stage of the storm could have been caused by the shift of fast electrons along the magnetic force lines above Irkutsk (as happens during a nuclear geomagnetic storm). Analysis of the regional geomagnetic storm of 30 June 1908 is very important to an understanding of the Tunguska Event. The analysis of this unexpected effect leads to a conclusion of a paradoxical nature: the first nuclear explosion occurred not in New Mexico in 1945, but in 1908 in Tunguska! This "crazy idea" was published by Kazantsev in 1946. This type conclusion is a challenge to the scientific paradigm of the 20th century. In this author's opinion, it is the reason for the geomagnetic effect-a most significant and informative effect-being lost by the way and essentially forgotten, the effect that is most likely to lead to understanding of the Tunguska phenomenon.
However strange it may seem, all theoretical calculations made on the bases of the cometary hypothesis ignored this fundamental fact. The regional geomagnetic storm of 1908 is the evidence that either some comets contain an unknown source of plasma of a very high density, or that the Tunguska object was not a comet, but a cosmic object the composition and structure of which is unknown to astronomers and physicists. This conclusion may affect the conceptual approach in devising protective systems for Earth against asteroids and comets. Information regarding the geomagnetic storm of 30 June 1908 should be made available to engineers developing nuclear missiles for destruction of comets.