3:45 PM - 4:00 PM
[MZZ40-08] Constraints on Tunguska events from pressure fluctuations observed at distant locations : Contribution of Pekeris waves
Keywords:Tunguska event, atmospheric Lamb waves, Pekeris waves, atmospheric cratering
Introduction
After the Tunguska event of 1908, a celestial collision, significant pressure fluctuations were observed around the world (Whipple, 1930, 1934). Based on the time difference between the occurrence of the event and the pressure fluctuations observed in various regions, these pressure fluctuations were presumed to be Lamb waves.
Nakajima (2023, JpGU MZZ45-09) noted that the sign of the Lamb wave was negative and discussed the partitioning of the energy of the celestial collision based on analytical methods, concluding that more than 80% of the energy was used to blow up the atmosphere from a height of several to several tens of kilometers. However, subsequent hydrodynamic numerical calculations revealed that the framework of Nakajima's (2023) discussion needs partial modification, and the main points of the modification will be presented in this presentation.
Origin of the Negative Lamb Wave
A striking feature of the Lamb waveforms 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 Lamb waves associated with volcanic eruptions, etc. Considering that Lamb waves are weakly dispersive, and that the waveforms at distant sites also reflect the nature of the wave source, this suggests that the explosion of a meteorite in the lower troposphere caused the overall negative pressure anomaly. 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, the meteorite and a substantial mass of the lower atmosphere rise through this wake to form a plume that reaches the upper atmosphere before dispersing and descending over a range of several thousand kilometers (e.g. Artemieva et al, 2019, ICARUS). The overall process from the formation of the plume to its dispersal would lead to the existence of a concentrated negative mass source in the lower atmosphere (a few 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 The horizontal propagation of the negative anomaly as a Lamb wave can be understood as the far-field pressure fluctuation of the negative anomaly observed at a distant location.
Excitation of Lamb and Pekeris Waves
Hydrodynamic calculations of linear waves in compressible atmospheres show that not only Lamb but also Pekeris waves are excited at relatively high altitudes of various wave sources caused by celestial collisions. Pekeris waves are internal gravity waves that extends in the atmosphere up to an altitude of about 100 km and can propagate to a distance with a velocity of about 220 m/s, which is slower than Lamb waves. Recent examples are observed globally after the 2022 eruption of the Tonga volcano. The sign of the Pekeris wave, which is excited by the negative mass source associated with plume formation, is positive, and the absolute amplitude of the Lamb wave increases in association with the excitation of the Pekeris wave, as will be shown in the presentation. Therefore, to consider the collision process from the observed Lamb wave amplitudes, the increase in Lamb wave amplitude must be taken into account.
Effects on energy partitioning to plume formation
Without considering the contribution of Pekeris waves, the energy used for plume formation is estimated to be about 85% of the total, using plausible parameters. Details will be given in the presentation, but if the Pekeris wave contribution is taken into account, this could be as low as 65% of the total. Nevertheless, the conclusion that a significant portion of the collision energy would have been allocated to plume formation is not significantly affected. A closer examination of the barometric records from 1908 (the part reflecting the Pekeris wave is not included in Whipple's paper) is needed to verify this in more detail.
Acknowledgments
This research was supported by JSPS Grant-in-Aid JP22K18872.
After the Tunguska event of 1908, a celestial collision, significant pressure fluctuations were observed around the world (Whipple, 1930, 1934). Based on the time difference between the occurrence of the event and the pressure fluctuations observed in various regions, these pressure fluctuations were presumed to be Lamb waves.
Nakajima (2023, JpGU MZZ45-09) noted that the sign of the Lamb wave was negative and discussed the partitioning of the energy of the celestial collision based on analytical methods, concluding that more than 80% of the energy was used to blow up the atmosphere from a height of several to several tens of kilometers. However, subsequent hydrodynamic numerical calculations revealed that the framework of Nakajima's (2023) discussion needs partial modification, and the main points of the modification will be presented in this presentation.
Origin of the Negative Lamb Wave
A striking feature of the Lamb waveforms 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 Lamb waves associated with volcanic eruptions, etc. Considering that Lamb waves are weakly dispersive, and that the waveforms at distant sites also reflect the nature of the wave source, this suggests that the explosion of a meteorite in the lower troposphere caused the overall negative pressure anomaly. 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, the meteorite and a substantial mass of the lower atmosphere rise through this wake to form a plume that reaches the upper atmosphere before dispersing and descending over a range of several thousand kilometers (e.g. Artemieva et al, 2019, ICARUS). The overall process from the formation of the plume to its dispersal would lead to the existence of a concentrated negative mass source in the lower atmosphere (a few 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 The horizontal propagation of the negative anomaly as a Lamb wave can be understood as the far-field pressure fluctuation of the negative anomaly observed at a distant location.
Excitation of Lamb and Pekeris Waves
Hydrodynamic calculations of linear waves in compressible atmospheres show that not only Lamb but also Pekeris waves are excited at relatively high altitudes of various wave sources caused by celestial collisions. Pekeris waves are internal gravity waves that extends in the atmosphere up to an altitude of about 100 km and can propagate to a distance with a velocity of about 220 m/s, which is slower than Lamb waves. Recent examples are observed globally after the 2022 eruption of the Tonga volcano. The sign of the Pekeris wave, which is excited by the negative mass source associated with plume formation, is positive, and the absolute amplitude of the Lamb wave increases in association with the excitation of the Pekeris wave, as will be shown in the presentation. Therefore, to consider the collision process from the observed Lamb wave amplitudes, the increase in Lamb wave amplitude must be taken into account.
Effects on energy partitioning to plume formation
Without considering the contribution of Pekeris waves, the energy used for plume formation is estimated to be about 85% of the total, using plausible parameters. Details will be given in the presentation, but if the Pekeris wave contribution is taken into account, this could be as low as 65% of the total. Nevertheless, the conclusion that a significant portion of the collision energy would have been allocated to plume formation is not significantly affected. A closer examination of the barometric records from 1908 (the part reflecting the Pekeris wave is not included in Whipple's paper) is needed to verify this in more detail.
Acknowledgments
This research was supported by JSPS Grant-in-Aid JP22K18872.