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Overdense meteors (electron line density 1016 ≤ q ≤ 1019 electrons m-1 and diameters dm ≥ 4 mm up to small fireballs) are in the size regime of the meteoroids capable of generating shockwaves (Fig. 1) during the lower transitional flow regimes and prior to their terminal stage in the MLT (Mesosphere-Lower Thermosphere) region of the atmosphere, at altitudes between 75 km and 100 km [1].
However, small scale physico-chemical processes that accompany the early evolution of the high T meteor train remain poorly understood.
Primarily it is of interest to understand the mechanism behind subsequent rapid and intense electron removal from the postadiabatically expanding meteor train within the first 0.1 s after its formation (Fig. 2). A comprehensive background on the topic can be found in [2].
We examine subsequent hyperthermal chemistry occurring on the early diffusing boundary of the high temperature postadiabatically formed meteor train and the shock modified ambient atmosphere in the MLT region.
This study has been motivated by the recent observational evidence [3] that suggest slower thermalization times of the postadiabatically formed meteor trains which is conducive for hyperthermal chemistry.
1. INTRODUCTION
A theoretical approach was applied to approximate the temperature of the ambient atmosphere near the meteor train, which is heated by the passage of the overdense meteor cylindrical shock wave. This was accomplished by considering the meteor velocity and energy deposition, and evaluating the pressure ratios between the ablation amplified shock front and the ambient atmosphere (see Fig. 3 and [2] for further details about this treatment). We have modeled hypersonic meteor flow in the MLT region using a simplified model without ablation, incorporated into the computational fluid dynamics (CFD) software package ANSYS Fluent (Figs. 4, 5). The computation was performed using O2 and N2 as the only major species, at an altitude of 80 km. A spherically shaped meteoroid is assumed to have velocity of 35 km/s (M80km = 124.6). Two meteoroid sizes were modeled, dm = 2.5 cm and dm = 10 cm. Further details about the model are given in [2].
2. THEORETICAL & MODELING APPROACH
3. RESULTS
4. DISCUSSION AND CONCLUSION
The cylindrical shock waves produced by overdense meteors are strong enough to heat the ambient atmosphere to temperatures of ~6,000 K in the near field and subsequently dissociate oxygen and minor species such as O3, but insufficient to dissociate N2. This substantially alters the considerations of the chemical processes taking place at the meteor train boundary. We demonstrate that ambient O2
which survives the cylindrical shockwave, along with small quantities of O2 that originates from the shock dissociation of O3, participate primarily in high temperature oxidation of meteoric metal ions, forming metal ion oxide. For the case of overdense meteor trains, the subsequently formed meteoric metal oxide ions are predominantly responsible for the initial intense and short lasting electron removal from the boundary of the expanding meteor train, through a process of fast temperature independent dissociative recombination. This altitude dependent process is typically completed within 0.1 - 0.3 s, which in good agreement with the results suggesting substantially slower cooling of meteor wakes [3]. The rate of this process is also strongly dependent on the second Damköhler number. The potential implications of results presented here, toward the behavior of strong underdense radio meteors, should be further investigated. The full scope of implications of this work is presented in [2].
5. REFERENCES
[1] Bronshten V.A. (1983) Physics of meteoric phenomena. Dordrecht, D. Reidel. [2] Silber E.A. et al. (2017), MNRAS, 469(2), 1869-1882 (and references therein). [3] Jenniskens P. et al. (2004) AsBio, 4(81). [4] Baggaley W. and Fisher G. (1980) PSS, 28, 575 [5] Hocking W.K. et al (2016) AnGeo, 34, 1119, [6] Jones W. and Jones J. (1990) JSTP, 52, 185.
ACKNOWLEDGEMENTS
EAS gratefully acknowledges the Natural Sciences and
Engineering Research Council of Canada (NSERC) Postdoctoral
Fellowship programme for partly funding this project. WKH
acknowledges a Discovery Grant from the NSERC. MG
acknowledges support from the ERC, the Russian Foundation for
Basic Research and the Russian Science Foundation. RES
thanks NSERC Collaborative Research and Training Experience
(CREATE) Program for Integrating Atmospheric Chemistry and
Physics from Earth to Space (IACPES), and a Northern Scientific
Training Program grant.
E. A. Silber1, W. K. Hocking2, M. L. Niculescu3, M. Gritsevich4,5, R. E. Silber6
ON THE MECHANISM OF EARLY RAPID REMOVAL OF ELECRONS FROM POSTADIABATICALLY EXPANDING OVERDENSE METEOR TRAINS
Figure 2: Schematic depiction of an overdense meteor's early evolution, in which three distinct stages can be recognized. In the first
stage, the ablating meteoroid with the shock front in front sweeps the cylindrical volume of ambient atmosphere (depicted by the small
gray circle), ionizing and dissociating atmospheric gasses. This stage also coincides with the cylindrical shock wave expanding radially
outward, perpendicular to the meteor axis of propagation, with enough energy deposited within R0 to dissociate O2 and O3 in the
ambient atmosphere, but not enough for N2 dissociation (see [2] for the extended discussion). In stage two, the adiabatically formed
meteor train (which can be approximated as quasineutral plasma with the Gaussian radial electron distribution), begins to expand
under ambipolar diffusion and thermalizes. This stage coincides with formation of metal ion oxides which takes place and is
appreciable between approximately (3,000 – 1,500 K) at the boundary region of the diffusing trail. In this reaction, an ablated meteoric
metal ion will react in a thermally driven reaction with the shock-dissociated product of ozone (O2 in ground and excited states). In the
stage three, in the almost thermalized train, the newly formed metal ion oxide will consume electrons rapidly by temperature
independent dissociative recombination (see [2] for discussion).
Figure 1: Schematics of the meteor shock wave(s), flow fields and near wake.
The meteoroid is considered as a blunt body (with the spherical shape)
propagating at hypersonic velocity. (1) Bow (cylindrical) shock wave front; (2)
The “ballistic” shock front; (3) Sonic region; (4) Boundary layer; (5) Stagnation
point; (6) Turbulent region (in some older literature, this is referred as the dead
water region); (7) Meteoroid; (8) The neck and recompression region; (9) The
‘free’ shear layer; (10) The recompression vapour (or a true cylindrical) shock
wave front; (11) The region of turbulent vapour flow and adiabatic expansion.
Note that small circles with positive and negative signs indicate regions affected
by the presence of ions and electrons respectively. The diagram is only for the
illustrative purpose and is not to scale.
Figure 5: The temperature distribution around the (a) 2.5 cm and (b) 10 cm
meteoroid. Note that (a) and (b) have different axes scaling.
Figure 4: The mass fraction of O2 as a function of radial distance
from the propagation axis of the (a) 2.5 cm and (b) 10 cm meteoroid.
The top boundary (“white space” in the plot) represents a numerical
boundary condition without any physical significance (it is set up to
be far enough away from the body (meteoroid), such that the
influence of the body (meteoroid) no longer has any effect. Note that
(a) and (b) have different axes scaling. The colour scheme is
represented in log scale.
Figure 3: (a) Plotted are the initial radius (r0) of a typical bright overdense meteor (from [4]) and the radius of the meteor (rm) trail after t
= 0.3 s. These are compared to R0 as a function of constant energy deposition (see eq (1) in [2]) of 100 J/m and 1000 J/m for altitudes
from 80 km to 100 km. For rm at 0.3 s, we applied the geometrically averaged hot plasma and ambient atmosphere ambipolar diffusion
coefficients as per [5]. (b) The initial radius r0, plotted along rm at t = 0.3 s [6]. Here, shown is the comparison between rm as calculated
in panel (a), and rm as calculated using the Massey’s formula for the theoretical diffusion coefficient.
3. RESULTS – CONT’D
1Department of Earth, Environmental and Planetary Science, Brown University, Providence, RI, USA (e-mail: [email protected]), 2Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada 3INCAS - National Institute for Aerospace Research "Elie Carafoli", Flow Physics Department, Numerical Simulation Unit, Bucharest, Romania, 4Department of Physics, University of Helsinki, Helsinki, Finland (email: [email protected]),
5Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia, 6Department of Earth Sciences, The University of Western Ontario, London, Ontario, Canada (e-mail: [email protected])