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[MZZ45-09] Constraints on Tunguska event from atmospheric pressure variations observed at distant locations
Keywords:Tunguska event, atmospheric Lamb waves, atmospheric cratering
The undersea volcanic explosion in Tonga in January 2022 produced a pronounced pressure pulse that was trapped at the surface and circled the earth several times as an atmospheric lamb wave propagating at nearly the speed of sound. Such large-amplitude lamb waves have occurred in the past in the "Tunguska event" in 1908, when a meteorite or comet (hereinafter simply called "meteorite") fell over eastern Siberia and exploded in the lower troposphere, in addition to large-scale atmospheric nuclear tests around 1960 and the 1883 eruption of Krakatau volcano. In this presentation, we interpret the atmospheric pressure fluctuations observed in the UK associated with the Tunguska event based on the nature of Lamb waves.
A striking feature of the Lamb waveform observed in the UK associated with the Tunguska event (Whipple, 1930, Q.J.R. Meteorol. Soc.) is that the sign of the main pressure anomaly is negative. This contrasts sharply with the generally positive pressure anomalies of ram waves associated with the eruptions of Tonga and Krakatau volcanoes and nuclear tests. Considering that the Lamb waves are almost non-dispersive and that the waveforms at faraway locations also reflect the nature of the wave source, this suggests that the meteorite explosion in the lower troposphere caused the overall negative pressure anomaly. However, it is well known that trees fell outward in the vicinity of the meteorite explosion (e.g., Jenniskens et al, 2019, ICARUS), so it is natural to assume that the ground pressure anomaly immediately after the explosion was positive. How is it consistent that the pressure deviation would be positive from direct evidence in the vicinity and negative from distant observations?
Meteorite explosions differ significantly from volcanic and nuclear tests in that the region of the atmosphere through which the meteorite passed prior to the explosion becomes a very hot, low-density wake, and after the explosion a substantial mass of the meteorite and lower atmosphere rises through this wake to form a plume that reaches the upper atmosphere before dispersing and falling over a range of several thousand km (e.g. Artemieva et al, 2019, ICARUS). The formation of such a plume was observed during the 1994 impact of Comet Shoemaker-Levy 9 on Jupiter, and can also be supported by records of the Tunguska event, which "brightened like evening even though it was midnight" over a wide area of Europe shortly after (Whipple, ibid.) Taken together, the processes from formation to extinction of the plume would suggest the existence of a concentrated negative mass source in the lower atmosphere (several kilometers in altitude) near the meteorite's explosion point and a diffuse positive mass source in the upper atmosphere over a range of several thousand kilometers. The former of these is expected to settle down as a negative lower atmospheric pressure anomaly spread over a scale of tens of kilometers near the explosion point after the short time-scale dissipation and adjustment processes such as shock waves immediately after the explosion are over and the atmosphere returns to hydrostatic equilibrium (e.g., Bannon, 1995, J. Atmos. Sci.), and this propagated horizontally as a Lamb wave, resulting in negative anomalies in far-field pressure fluctuations observed at distant locations. However, meteorite explosions are also estimated to heat the atmosphere, and this heating must also be considered to create positive pressure anomalies, as in volcanic and nuclear tests.
In this presentation, we consider that the energy of the meteorite fall is distributed between plume formation and heating, and quantitatively estimate the amplitude (and sign) of the excited Lamb wave. As a result, we found that the observed pressure fluctuations can be explained by assuming that about 80% of the energy of the meteorite fall was used for plume formation. Since the observed waveforms include information on waves other than Lamb waves, further detailed analysis may contribute to the reconstruction of the Tunguska event and to the elucidation of meteorite impact events in general.
Acknowledgement
This work was supported by JSPS KAKENHI Grant Number JP22K18872.
A striking feature of the Lamb waveform observed in the UK associated with the Tunguska event (Whipple, 1930, Q.J.R. Meteorol. Soc.) is that the sign of the main pressure anomaly is negative. This contrasts sharply with the generally positive pressure anomalies of ram waves associated with the eruptions of Tonga and Krakatau volcanoes and nuclear tests. Considering that the Lamb waves are almost non-dispersive and that the waveforms at faraway locations also reflect the nature of the wave source, this suggests that the meteorite explosion in the lower troposphere caused the overall negative pressure anomaly. However, it is well known that trees fell outward in the vicinity of the meteorite explosion (e.g., Jenniskens et al, 2019, ICARUS), so it is natural to assume that the ground pressure anomaly immediately after the explosion was positive. How is it consistent that the pressure deviation would be positive from direct evidence in the vicinity and negative from distant observations?
Meteorite explosions differ significantly from volcanic and nuclear tests in that the region of the atmosphere through which the meteorite passed prior to the explosion becomes a very hot, low-density wake, and after the explosion a substantial mass of the meteorite and lower atmosphere rises through this wake to form a plume that reaches the upper atmosphere before dispersing and falling over a range of several thousand km (e.g. Artemieva et al, 2019, ICARUS). The formation of such a plume was observed during the 1994 impact of Comet Shoemaker-Levy 9 on Jupiter, and can also be supported by records of the Tunguska event, which "brightened like evening even though it was midnight" over a wide area of Europe shortly after (Whipple, ibid.) Taken together, the processes from formation to extinction of the plume would suggest the existence of a concentrated negative mass source in the lower atmosphere (several kilometers in altitude) near the meteorite's explosion point and a diffuse positive mass source in the upper atmosphere over a range of several thousand kilometers. The former of these is expected to settle down as a negative lower atmospheric pressure anomaly spread over a scale of tens of kilometers near the explosion point after the short time-scale dissipation and adjustment processes such as shock waves immediately after the explosion are over and the atmosphere returns to hydrostatic equilibrium (e.g., Bannon, 1995, J. Atmos. Sci.), and this propagated horizontally as a Lamb wave, resulting in negative anomalies in far-field pressure fluctuations observed at distant locations. However, meteorite explosions are also estimated to heat the atmosphere, and this heating must also be considered to create positive pressure anomalies, as in volcanic and nuclear tests.
In this presentation, we consider that the energy of the meteorite fall is distributed between plume formation and heating, and quantitatively estimate the amplitude (and sign) of the excited Lamb wave. As a result, we found that the observed pressure fluctuations can be explained by assuming that about 80% of the energy of the meteorite fall was used for plume formation. Since the observed waveforms include information on waves other than Lamb waves, further detailed analysis may contribute to the reconstruction of the Tunguska event and to the elucidation of meteorite impact events in general.
Acknowledgement
This work was supported by JSPS KAKENHI Grant Number JP22K18872.