Rodney W. Whitaker

A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors

Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave, but owing to lack of observations this is uncertain. Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×1012 joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques. This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.

Radiation-driven implosions in molecular clouds

We present two-dimensional radiation-hydrodynamic calculations of the interaction of ionization and shock fronts with a geometrical inhomogeneity in a molecular cloud. These regions consist of low density clumps of masses < or =2 M/sub sun/, for which self-gravity is negligible. The radiation transport is both angle and frequency dependent. Ionizing stellar radiation and density variations in molecular cloud models are shown to produce a convergent shock that creates mass concentrations with densities significantly above those expected from one-dimensional compressions. The masses and time scale for concentrations to increase depend on the geometry of the initial density variations and the intensity of ionizing stellar radiation. The radiation of OB stellar associations is shown to be effective in forming dense mass globules from preexisting low density inhomogeneneities near the edges of H II regions. Our calculations suggest that the cloud environment surrounding the cluster may influence subsequent star formation within the cloud. We include an analytic treatment of isothermal interacting shocks. The possible relationship between the observed concentrations and ionization shock compression is discussed.

Star formation in colliding gas flows

Les ecoulements gazeux entrant en collision avec les nuages moleculaires produisent des regions de gaz comprime et de poussiere qui sont hydrodynamiquement instables

Some Atmospheric Effects on Infrasound Signal Amplitudes

In this chapter two types of infrasound propagation modes or paths through the atmosphere are introduced and their interaction with the atmosphere is discussed. A method is presented for an inversion process to obtain atmospheric properties from infrasonic signals. Two of the most important modes for atmospheric wave propagation, stratospheric region returns (S signals) and thermospheric region returns (T signals), are discussed and contrasted. The emphasis is on signals from chemical, or high explosive (HE) explosions and atmospheric nuclear explosions from past tests and earthquakes. The role of atmospheric winds in controlling S signals and the relative independence of T signals are explained. An empirical method for the prediction of wind effects, or normalization of those effects, on S signal amplitudes is presented. Three methods are presented for the inversion of amplitude observations to obtain atmospheric wind parameterization.

Automatic infrasonic signal detection using the Hough transform

[1] The Hough transform is a mathematical device that allows the retrieval of parametric curve information from binary-pixelated data in the presence of noise. This slope-intercept transform maps each point in the image space S into a straight line in parameter space P and has the very useful property that all points in S that lie along the same straight-line map to the same number of straight lines in P with a common intersection point. Thus with a suitable counting procedure, the problem of extended straight-line detection in noisy pixelated data becomes one of local peak finding, a problem that may be substantially more tractable. In this study, an algorithm that utilizes the Hough transform for the detection of signals in International Monitoring System style infrasonic array data by seeking periods of constant backazimuth that are associated with coherent acoustic signals is described. A system of synthetic signal implants is used to assess the performance of the detection algorithm by generating a set of pseudo Receiver Operator Characteristic curves. A feature of the detection algorithm is the ability to accommodate full three-dimensional array geometry.

KELVIN-HELMHOLTZ AND THERMAL-DYNAMIC INSTABILITIES WITH SELF-GRAVITY: A NEW GRAVITATIONAL INTERFACE INSTABILITY

In the vortex sheet limit, we generalize our previous work on compressible, anisentropic Kelvin-Helmholtz, and related instabilities by including self-gravity in the calculations. In addition to significantly modifying the Kelvin-Helmholtz modes, if the background media are of unequal density, self-gravity gives rise to a new instability that persists in the static limit. If the media have significant density contrast (ρ1/ρ2=½, say), the growth rate of this new gravitational interface instability is of the order of the free-fall time in the denser medium, and, unlike a Jeans instability, it depends only weakly on the perturbation wavelength. Such instabilities may initiate star formation near the boundaries of molecular clouds in the ISM on timescales of ~ 106-107 yr.

Star formation within OB subgroups: Implosion by multiple sources

We present the results of new detailed two-dimensional radiation-hydrodynamical calculations of the effects of radiation-driven shock waves from two O stars on inhomogeneities embedded in molecular clouds. The calculations indicate the neutral primordial clumps of gas with 84 M/sub sun/ can be highly compressed in 3 x 10/sup 4/ yr with density enhancements greater than 170 over ambient densities and 40 M/sub sun/ remaining. Inhomogeneities that are compressed in this manner by stars in the range O7--B0 survive ionization evaporation and may rapidly form new stars. Low-mass objects would not survive, and there would be a natural cutoff of low-mass and high-mass stars. We present a scenario for hierarchical radiation-driven implosion as a potential, new highly efficient mechanismfor star formation that may explain aspects of recent observations of new star formation in ultracompact H II regions.

Infrasound from the El Paso superbolide of October 9, 1997

During the noon hour on October 9, 1997 an extremely bright fireball (approximately -21.5 in stellar magnitude putting it into the class of a super-bolide) was observed over western Texas with visual sightings from as far away as Arizona to northern Mexico and even in northern New Mexico over 300 miles away. This event produced tremendously loud sonic boom reports in the El Paso area. It was also detected locally by 4 seismometers which are part of a network of 5 seismic stations operated by the University of Texas at El Paso (UTEP). Subsequent investigations of the data from the six infrasound arrays used by LANL (Los Alamos National Laboratory) and operated for the DOE (Department of Energy) as a part of the CTB (Comprehensive Test Ban) Research and Development program for the IMS (International Monitoring System) showed the presence of an infrasonic signal from the proper direction at the correct time for this super-bolide from two of our six arrays. Both the seismic and infrasound recordings indicated that an explosion occurred in the atmosphere at source heights from 28 - 30 km, having its epicenter slightly to the northeast of Horizon City, Texas. The signal characteristics, analyzed from approximately 0.1 to 5.0 Hz, include a total duration of approximately 4 min (at Los Alamos, LA) to greater than approximately 5 min at Lajitas, Texas, TXAR, another CTB IMS array operated by E. Herrin at Southern Methodist University (SMU) for a source directed from LA toward approximately 171 - 180 deg and from TXAR of approximately 321 - 4 deg respectively from true north. The observed signal trace velocities (for the part of the recording with the highest cross-correlation) at LA ranged from 300 - 360 m/sec with a signal velocity of 0.30 plus or minus 0.03 km/sec, implying a Stratospheric (S Type) ducted path. The dominant signal frequency at LA was from 0.20 to 0.80 Hz, with a peak near 0.3 Hz. These highly correlated signals at LA had a very large, peak to peak, maximum amplitude of 21.0 microbars (2.1 Pa). Our analysis, using several methods that incorporate various observed signal characteristics, total distance traveled, etc., indicates that the super-bolide probably had a source energy in the range between 10 - 100 tons (TNT equivlaent). This is somewhat smaller than the source energy estimate made using U.S. DoD satellite data (USAF news release, June 8, 1998).

Resonance-fluorescence in barium ion clouds

Abstract The problem of resonant-fluorescent scattering of sunlight by a high altitude, plane-parallel, barium ion cloud is solved numerically. Line strengths and profiles are computed using our modified version of the computer program LINEAR (Auer, Heasley and Milkey, 1972). Hyperfine structure of the spectral lines becomes important for very thick layers and is taken into account. Comparisons are made between coherent and completely noncoherent scattering results and finally the influence of collisions on the radiation field is estimated.

Infrasonic observations of bolides on October 4, 1996

During the evening of October 3, 1996, at least six bright fireballs were observed over the western United States with reports from California to Louisiana. The event over California produced tremendous sonic boom reports in the Los Angeles area. This event was also detected locally by 31 seismometers which are part of a network of seismic stations operated by the California Institute of Technology. Subsequent investigations of the data from the four infrasound arrays used by LANL (Los Alamo National Laboratory) and operated for the DOE (Department of Energy) as part of the CTBT Program (Comprehensive Test Ban Treaty) Research and Development program showed the presence of an infrasonic signal from the proper direction at the correct time for this bolide from two of our four arrays (Nevada Test Site; NTS and Pinedale, WY; PDL). Both the seismic and infrasound recordings indicated that an explosion occurred in the atmosphere, having its epicenter near Little Lake, Calif. for possible sources heights from 40 - 60 km. The infrasonic arrays are each composed of four elements, i.e., low frequency pressure sensors that are in near-continuous operation. The nominal spacing between elements is 150 - 200 m depending on the specific site. The basic sensor is a Globe Universal Sciences Model 100C microphone whose amplitude response is flat from 0.1 to 300 Hz. Each sensor is connected to 12 porous hoses which act to reduce wind noise. The signal characteristics, analyzed from 0.1 to 5.0 Hz, includes a total duration of 5 (NTS) to 20 minutes (PDL) for a source directed toward 230 - 240 degrees from true North. The signal trace velocities ranged from 300 - 360 m/sec with a signal velocity of 0.30 plus or minus 0.03 km/sec, implying a stratospheric (S type) ducted path (with a reflection altitude of from 40 - 60 km). The dominant signal frequency is from 0.20 to 0.80 Hz, with a peak near 0.2 to 0.25 Hz. These highly correlated signals had a maximum amplitude of 1.0 microbars (0.1 Pa) at PDL and 4.0 microbars (0.4 Pa) at NTS. Our analysis indicates that the bolide had a probable, maximum source energy in the range from 150 - 390 tons (TNT equivalent).