A New Explanation of Airglows of the Tunguska Event B. R. German Institute of Physics of the Ukrainian Academy of Sciences, Donetsk, Ukraine (german@mail.fti.ac.donetsk.ua) Abstract A new hypothesis for explanation of airglows of the Tunguska 1908 explosion as ionospheric spread phenomena during tectonic events is presented. 1. Introduction On June 30, 1908 at 0:15 ± 0.05 min UT a powerful explosion occurred in the Kulik-caldera in Siberia. For the first two nights after that skies of Eurasia were exceptionally bright. At night on June 30 in such cities as Tashkent the solar depression was more than 26 o , that is, the atmosphere was directly lit by the rays of the sun at an altitude of 700 km. Nevertheless the sky was of such brightness that photographic exposures with an astrograph were not possible at all [3]. Geographical boundaries of airglows were limited by the Yenisei River in the east, the Atlantic shore in the west, along Tashkent– Stavropol–Sevastopol–Bordeaux line in the south, and at least along Aberdeen–Stockholm line in the north (the northern border merged with the area of 'white nights' usual in the summer). Only twilight emissions with a broad diffuse spectrum like the extended twilight which usually follow volcanic eruptions have been registered. Earlier it has been proved [12] that it is impossibly to explain 'bright skies' after the Tunguska explosion by any optically active cometary dust particles because they can not remain above 100 km for a period of days. 2. Ionospheric Spread Phenomena It was shown, days before earthquakes, even before rather weak ones, the turbulization of the plasma of ionosphere/exosphere changes [7]. There are im- ages that clearly showed patches of turbulence associated with spread bubbles drifting across the sky [11]. During spread phenomena both the pro- duced airglow structures, and the altitude covered with them can reach more than 1000 km [7]. For example, enhancements of light ion density have been observed in the inner plasmasphere at al- titudes of 2500 km above the seismically active zone prior to the Iranian earthquake on 20 June, 1990 [1], and ionospheric variations during the Wenchuan earthquake on 12 May, 2008 extended more, than 1500 km in a latitude and 4000 km in a longitude [14]. The above parameters fit the bill to observations airglows of the Tunguska event. Fur- thermore, we assume that increased ionospheric conductivity could contributes to the formation of so-called 'earthquake lights'. Ionospheric spread phenomena are caused by an enhanced activity of ULF infrasonic waves radiated by hypocentral zones closely to pre- and post- seismic periods. Prof. L. Weber reported about registrations of such daily regular oscillations with a period of 3 min at Kiel on 27-30 June, 1908 from 17:00 to 0:30 UT [13]. These pulsations were de- tected in the evening time only and ended on June 30 through 15 min after the explosion in Siberia. Our previous studies showed that these pulsations were caused by infrasound waves, extending from Siberia to Europe because of a preparation of earth- quake in the Kulik-caldera (these waves had a cor- relation with a daily nightly interval of radon emis- sion etc.; but a connection with the IMF is not ex- cluded as well) [5]. The velocity of infrasound wa- ves ~ 330 m/sec for a moving of seismic irregulari- ties well explains the difference in 15 min between the time of explosion in the Kulik-caldera and the ending of the pulsations at Kiel on 30 June, 1908. Thus we have a confirmation of the spread pheno- mena during the Tunguska event. In June 1908 changes of sky polarization were detected by F. Bush at Arnsberg [6]: for one day before the Tunguska explosion the polarization minima were displaced. It is known that usually polarization minima register when the sun is under horizon. It has been explained by the change in critical frequencies of ionospheric layers Е and F. In turn, change of frequencies is caused by the following: at twilight an intensity of pulsations of electric vector of the geofield directed in parallel to a plane of scattering of light, reaches a night maxi- EPSC Abstracts Vol. 5, EPSC2010-429, 2010 European Planetary Science Congress 2010 c  Author(s) 2010