Production of nitric oxide by a fragmenting bolide: An exploratory numerical study

By Mihai L. Niculescu, Elizabeth A. Silber, Reynold E. Silber

Abstract: A meteoroid’s hypersonic passage through the Earth’s atmosphere results in ablational and fragmentational mass loss. Potential shock waves associated with a parent object as well as its
fragments can modify the surrounding atmosphere and produce a range of physico-chemical effects. Some of the thermally driven chemical and physical processes induced by meteoroidfragment generated shock waves, such as nitric oxide (NO) production, are less understood. Any estimates of meteoric NO production depend not only on a quantifiable meteoroid population and a rate of fragmentation, with a size capable of producing high temperature flows, but also on understanding the physical properties of the meteor flows along with their thermal history. We performed an exploratory pilot numerical study using ANSYS Fluent, the CFD code, to investigate the production of NO in the upper atmosphere by small meteoroids (or fragments of meteoroids after they undergo a disruption episode) in the size range from 10^-2 m to 1 m. Our model uses the simulation of a spherical body in the continuum flow at 70 and 80 km altitude to approximate the behaviour of a small meteoroid capable of producing NO. The results presented in this exploratory study are in good agreement with previous studies.

Read more: (30 pages).

Verification of the flow regimes based on high fidelity observations of bright meteors

By Manuel Moreno-Ibáñez, Elizabeth A. Silber, Maria Gritsevich, Josep M. TrigoRodríguez

Abstract: Infrasound monitoring has proved to be effective in detection of the meteor generated shock waves. When combined with optical observations of meteors, this technique is also reliable for  detecting centimeter-sized meteoroids that usually ablate at high altitudes, thus offering relevant clues that open the exploration of the meteoroid flight regimes. Since a shock wave is formed as a result of a passage of the meteoroid through the atmosphere, the knowledge of the physical parameters of the surrounding gas around the meteoroid surface can be used to determine the meteor flow regime. This study analyses the flow regimes of a data set of twenty-four centimeter-sized meteoroids for which well constrained infrasound and photometric information is available. This is the first time that the flow regimes for meteoroids in this size range are validated from observations. From our approach, the Knudsen and Reynolds numbers are calculated, and two different flow regime evaluation approaches are compared in order to validate the theoretical formulation. The results demonstrate that a combination of fluid dynamic dimensionless parameters is needed to allow a better inclusion of the local physical processes of the phenomena.

Read more: (41 pages).

The threat of Centaurs for terrestrial planets and their orbital evolution as impactors

By M.A. Galiazzo, E. A. Silber, R. Dvorak

Abstract: Centaurs are solar system objects with orbits situated among the orbits of Jupiter and Neptune. Centaurs represent one of the sources of Near-Earth Objects. Thus, it is crucial to  understand their orbital evolution which in some cases might end in collision with terrestrial planets and produce catastrophic events. We study the orbital evolution of the Centaurs toward the inner solar system, and estimate the number of close encounters and impacts with the terrestrial planets after the Late Heavy Bombardment assuming a steady state population of Centaurs. We also estimate the possible crater sizes. We compute the approximate amount of water released: on the Earth, which is about 10^−5 the total water present now. We also found sub-regions of the Centaurs where the possible impactors originate from. While crater sizes could extend up to hundreds of kilometers in diameter given the presently known population of Centaurs the majority of the craters would be less than 10 km. For all the planets and an average impactor ∼size of 12 km in diameter, the average impact frequency since the Late Heavy Bombardment ∼is one every 1.9 Gyr for the Earth and 2.1 Gyr for Venus. For smaller bodies (e.g. > 1 km), ∼the impact frequency is one every 14.4 Myr for the Earth, 13.1 Myr for Venus and, 46.3 for Mars, in the recent solar system. Only 53% of the Centaurs can enter into the terrestrial planet region and 7% can interact with terrestrial planets.

Read more: (32 pages).

Nitric Oxide Production by Centimeter-Sized Meteoroids and the Role of Linear and Nonlinear Processes in the Shock Bound Flow Fields

By Silber E.A., Niculescu M.L., Butka P., Silber R.E. 

Abstract: Nitric oxide (NO) is a critical indicator of energy deposition in the lower thermosphere because of its formational pathways. Thus, it is important to constrain sources of NO, such as meteoroid generated hypersonic flows below 95 km altitude. This paper aims to examine the process of and place the upper estimate on NO production in high temperature flow fields of strongly ablating meteoroids. For centimeter-sized meteoroids, the production of NO is bound within the dynamically stable volume of bright meteor plasma trains in the region of 80–95 km. Our estimate of the upper limit of the cumulative mass of NO produced annually by centimeter-sized meteoroids is significantly lower than that reported in previous early studies. In the context of shock waves, we explored the reasons why centimeter-sized meteoroids are the most efficient producers of NO. Effects of nonlinear processes on meteoric NO production are discussed.

Read more: (21 pages).

Physics of Meteor Generated Shock Waves in the Earth’s Atmosphere – A Review

By Elizabeth A. Silber, Mark Boslough, Wayne K. Hocking, Maria Gritsevich, Rodney W. Whitaker

Abstract: Shock waves and the associated phenomena generated by strongly ablating meteoroids with sizes greater than a few millimeters in the lower transitional flow regime of the Earth’s  atmosphere are the least explored aspect of meteor science. In this paper, we present a comprehensive review of literature covering meteor generated shock wave phenomena, from the aspect of both meteor science and hypersonic gas dynamics. The primary emphasis of this review is placed on the mechanisms and dynamics of the meteor shock waves. We discuss key aspects of both shock generation and propagation, including the great importance of the hydrodynamic shielding that develops around the meteoroid. In addition to this in-depth review, the discussion is extended to an overview of meteoroid fragmentation, followed by airburst type events associated with large, deep penetrating meteoroids. This class of objects has a significant potential to cause extensive material damage and even human casualties on the ground, and as such is of great interest to the planetary defense community. To date, no comprehensive model exists that accurately describes the flow field and shock wave formation of a strongly ablating meteoroid in the non-continuum flow regime. Thus, we briefly present the current state of numerical models that describe the comparatively slower flow of air over non-ablating bodies in the rarefied regime. In respect to the elusive nature of meteor generated shock wave detection, we also discuss relevant aspects and applications of meteor radar and infrasound studies as tools that can be utilized to study meteor shock waves and related phenomena. In particular, infrasound data can provide energy release estimates of meteoroids entering the Earth’s atmosphere. We conclude with a summary of unresolved questions in the domain of meteor generated shock waves; topics which should be a focus of future investigations in the field.

Read more: (65 pages).

On Shock Waves and the Role of Hyperthermal Chemistry in the Early Diffusion of Overdense Meteor Trains

By Elizabeth A. Silber, Wayne K. Hocking, Mihai L. Niculescu, Maria Gritsevich, Reynold E. Silber

Abstract: Studies of meteor trails have until now been limited to relatively simple models, with the trail often being treated as a conducting cylinder, and the head (if considered at all) treated as a ball of ionized gas. In this article, we bring the experience gleaned in other fields to the domain of meteor studies, and adapt this prior knowledge to give a much clearer view of the microscale physics and chemistry involved in meteortrail formation, with particular emphasis on the first 100 or so milliseconds of the trail formation. We discuss and examine the combined physico-chemical effects of meteor-generated and ablationally amplified cylindrical shock waves which appear in the ambient atmosphere immediately surrounding the meteor train, as well as the associated  hyperthermal chemistry on the boundaries of the high temperature postadiabatically expanding meteor train. We demonstrate that the cylindrical shock waves produced by overdense meteors are sufficiently strong to dissociate molecules in the ambient atmosphere when it is heated to temperatures in the vicinity of 6,000 K, which substantially alters the considerations of the chemical processes in and around the meteor train. We demonstrate that some ambient O_2, along with O_2 that comes from the shock dissociation of O_3, survives the passage of the cylindrical shock wave, and these constituents react thermally with meteor metal ions, thereby subsequently removing electrons from the overdense meteor train boundary through fast, temperature independent, dissociative recombination governed by the second Damköhler number. Possible implications for trail diffusion and lifetimes are discussed.

Read more: (48 pages).

Optical Observations of Meteors Generating Infrasound – II: Weak Shock Theory and Validation

By Elizabeth A. Silber, Peter G. Brown and Zbigniew Krzeminski

Abstract: We have recorded a dataset of 24 centimeter-sized meteoroids detected simultaneously by video and infrasound to critically examine the ReVelle [1974] weak shock meteor infrasound model. We find that the effect of gravity wave perturbations to the wind field and updated absorption coefficients in the linear regime on the initial value of the blast radius (R0), which is the strongly non-linear zone of shock propagation near the body and corresponds to energy deposition per path length, is relatively small (<10%). Using optical photometry for ground-truth for energy deposition, we find that the ReVelle model accurately predicts blast radii from infrasound periods (τ), but systematically under-predicts R0 using pressure amplitude. If the weak-shock to linear propagation distortion distance is adjusted as part of the modelling process we are able to self-consistently fit a single blast radius value for amplitude and period. In this case, the distortion distance is always much less (usually just a few percent) than the value of 10% assumed in the ReVelle model. Our study shows that fragmentation is an important process even for centimeter-sized meteoroids, implying that R0, while a good measure of energy deposition by the meteoroid, is not a reliable means of obtaining the meteoroid mass. We derived an empirical period-blast radius relation of the form R0=15.4τ–0.5 (τ≤0.7s) and R0=29.1τ–11.6 (τ>0.7s) appropriate to cm-sized meteoroids. Our observations suggest that meteors having blast radii as small as 1m are detectable infrasonically at the ground, an order of magnitude smaller than previously considered.

Read more: (58 pages).

Optical Observations of Meteors Generating Infrasound – I: Acoustic Signal Identification and Phenomenology

By Elizabeth A. Silber, Peter G. Brown

Abstract: We analyze infrasound signals from 71 bright meteors/fireballs simultaneously detected by video to investigate the phenomenology and characteristics of meteor-generated near-field infrasound (< 300 km) and shock production. A taxonomy for meteor generated infrasound signal classification has been developed using the time-pressure signal of the infrasound arrivals. Based on the location along the meteor trail where the infrasound signal originates, we find most signals are associated with cylindrical shocks, with about a quarter of events evidencing spherical shocks associated with fragmentation events and optical flares. The video data indicate that all events with ray launch angles >117° from the trajectory heading are most likely generated by a spherical shock, while infrasound produced by the meteors with ray launch angles ≤117° can be attributed to both a cylindrical line source and a spherical shock. We find that meteors preferentially produce infrasound toward the end of their trails with a smaller number showing a preference for mid-trail production. Meteors producing multiple infrasound arrivals show a strong infrasound source height skewness to the end of trails and are much more likely to be associated with optical flares. We find that only about one percent of all our optically-detected meteors have associated detected infrasound and estimate that regional meteor infrasound events should occur of order once per week and dominate in numbers over infrasound associated with more energetic (but rarer) bolides. While a significant fraction of our meteors producing infrasound (~1/4 of single arrivals) are produced by fragmentation events, we find no instances where acoustic radiation is detectable more than about 60° beyond the ballistic regime at our meteoroid sizes (grams to tens of kilograms) emphasizing the strong anisotropy in acoustic radiation for meteors which are dominated by cylindrical line source geometry, even in the presence of fragmentation.

Read more: (85 pages).