Some science behind the scenes

Bolides

 

The word bolide comes from the Greek βολίς (bolis) which can mean a missile or to flash. The IAU has no official definition of "bolide", and generally considers the term synonymous with a large meteor or  "fireball". The term generally applies to fireballs reaching magnitude -14 or brighter. Astronomers tend to use "bolide" to identify an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).  If the magnitude of a bolide reaches -17 or brighter it is known as a superbolide.  The fact that they produce infrasound is well documented, for example…..

1. Introduction

Infrasonic sound (0.01 to ~20 Hz) produced by large meteoroids (0.1–1 m
diameters) entering the Earth’s atmosphere at hypersonic velocities has been
a subject of interest for many researchers since the occurrence of the 1908
Tunguska blast in Siberia (Ben-Menahem, 1975). Produced either by the
shock front of the entering meteoroid or by subsequent fragmentation of the
object at altitude, several authors have reported the recording of meteor
infrasound by microbarometer arrays in the past (e.g. ReVelle, 1976; Brown
et al., 2002a; Pichon et al., 2002). However, until recently the number of
microbarometer arrays has been very small. This has changed over the past
several years with the construction of the international monitoring network.

Source: BOLIDE ENERGY ESTIMATES FROM INFRASONIC MEASUREMENTS, see reference

Thus as we can see from the quote, there is a well-established ground monitoring network set up to monitor these events.  Furthermore with the release of new data from satellites, the nature of these meteors or fireballs can be far better studied ……………….

With the public release of DoD and DoE satellite data, it is now possible
to study empirically the bolide infrasound phenomena in a way that was
never possible before, by cross-calibrating airwave measurements against
satellite optical energy estimates, which will provide a means to ground-truth
previous bolide infrasound theory.

Source: BOLIDE ENERGY ESTIMATES FROM INFRASONIC MEASUREMENTS, see reference

One of the perhaps most exciting developments over the past few years has been the construction of a database of these events with data correlated from a number of sites and sources.

The database of bolide infrasound has been collected over the past decade
using fireball information gleaned from various optical camera networks,
published literature and DoD and DoE press releases of satellite observed
bolides; a total of ~50 separate bolide events with observed infrasound has been compiled. For many events, only a single station has detected an infrasonic signal, though a significant fraction (~20%) have been observed by two
or more arrays. In addition, for some events, supplemental information is
available from independent sources in the form of velocities, optical energy
measurements and trajectories/location information. For a few events, meteorites have been recovered on the ground providing the ultimate ground-truth
(c.f. Spurny´ et al., 2003 for an example related to the Neushwanstein meteorite fall) and yet another means to calibrate pre-atmospheric size through radionuclide measurements.

To study the relationship that exists between bolide energy and observed
airwave signals, a subset of the bolide infrasound dataset was selected with
the goal of removing any possible biases and/or dissimilarities that may exist
within the database due to observational range. In particular, only signals
observed from a range of >250 km have been included in the subset to isolate
only stratospheric arrivals, that is only those waves that have refracted back
to the ground at least once from the stratosphere. Similarly, only airwave
signals having a minimum average signal velocity >0.28 km/s were included,

Source: BOLIDE ENERGY ESTIMATES FROM INFRASONIC MEASUREMENTS, see reference

Figure 1. Wind corrected maximum amplitude of the signal envelope for bolide infrasound.
r 2= 0.584. Source: see reference

 

 The graph above summarises, and the quote below describes, their findings….

Now that it has become possible to examine bolide infrasound in a
statistical sense, a comparison to similar man-made impulsive sources of
infrasound in the atmosphere may provide insight into aspects of bolide
infrasound that may be unique to this type of natural source. If the regression
fit to the bolide peak to peak amplitude is compared alongside similar data
from nuclear and high explosive data for standard 1 kt and 2500 lbs
(1134 kg) yields, respectively (Reed, 1977), it is found that the bolide curve is
significantly steeper and lies beneath both curves in the region where most
bolide data are available (Figure 5). Thus it appears that bolide infrasound is
more effectively attenuated then man-made explosive infrasound and is
commonly observed at lower amplitudes then might otherwise be expected.
One possible explanation of this discrepancy between nuclear-high
explosive and bolide infrasound may be the generally higher altitudes from
which bolide infrasound is generated. Nuclear and high explosive data have
commonly had sources at or near ground level, while in contrast typical
bolides have terminal points at a range of altitudes from 15 to 40 km. Higher
source altitudes for infrasound requires that as the waves conserve energy in
propagating from altitude to the surface through an increasingly dense
atmosphere, the signal amplitudes should decrease. Thus when detected at
the surface the amplitude is smaller then would be expected for an equivalent

Source: BOLIDE ENERGY ESTIMATES FROM INFRASONIC MEASUREMENTS, see reference

So the infrasound from bolides is of a lower frequency than that obtained from man-made sources such as nuclear explosions or the use of high explosives for blasting.  It is also of lower intensity.

Bolide events are not uncommon events.  There was a bolide event on April 23, 2001 in the North Pacific that produced infrasonic waves that were observed at six separate infrasonic arrays located in Hawaii, Alaska and the continental US. . The average value of the cross-correlation from six microphone pairs was  - 0.10Hz to 5.0 Hz.

There was an earlier bolide event on August 25, 2000 from which an infrasonic signal had been detected in Fairbanks by the DOE array.  Infrasound was also observed from the August 25th event by three other infrasonic arrays in South America and in Hawaii.  This signal was about 0.3Hz  to 0.8 Hz.

In the figure below we can see a chart showing a bolide infrasonic signal that was observed at the Windless Bight, Antarctica on September 3, 2004.

 

Chart showing a bolide infrasonic signal observed at
Windless Bight, Antarctica on September 3, 2004

The following are also reported to be similar events all in 2009

Mason Dixon - July 06
Italy - July 17
Russia - July 21
Belgium - Aug 15
Namibia - Sept. 19
Southern Ontario - Sept. 25
Netherlands- Oct. 13

In 1994 the US Department of Defence placed its records on energetic bolide-type asteroids, over a time span of about twenty years, in the public domain. This data indicated that, from 1975 to 1992, there were 136 airbursts of energy greater than 1 kiloton, but the real number was probably at least 10 times higher, because the satellite system does not cover the entire surface of the Earth.

Don Yeomans, Paul Chodas, Steve Chesley - NASA/JPL Near-Earth Object Program Office, October 23, 2009

On October 8, 2009 about 03:00 Greenwich time, an atmospheric fireball blast was observed and recorded over an island region of Indonesia. The blast is thought to be due to the atmospheric entry of a small asteroid about 10 meters in diameter that, due to atmospheric pressure, detonated in the atmosphere with an energy of about 50 kilotons (the equivalent of 110 million pounds of TNT explosives).

The blast was recorded visually and reported upon by local media representatives. See the YouTube video at:

Atmospheric Fireball Blast Indonesia 8 October 2009

A report from Elizabeth Silber and Peter Brown at the University of Western Ontario indicates that several international very-long wavelength infrasound detectors recorded the blast and fixed the position near the coastal city of Bone in South Sulawesi, island of Sulawesi. They note that the blast was in the 10 to 50 kT range with the higher end of this range being more likely.

Assuming an estimated size of about 5-10 meters in diameter, we would expect a fireball event of this magnitude about once every 2 to 12 years on average. As a rule, the most common types of stony asteroids would not be expected to cause ground damage unless their diameters were about 25 meters in diameter or larger.

…..Based on these initial reports, a detailed examination was made of all International Monitoring System (IMS) infrasound stations of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). From this initial examination, a total of 11 stations showed probable signals from a large explosion ….consistent with the media reports. This signal was notable for having been ….. confined to very low frequencies.

References

BOLIDE ENERGY ESTIMATES FROM INFRASONIC
MEASUREMENTS

WAYNE N. EDWARDS
Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7,
Canada

PETER G. BROWN
Canada Research Chair in Meteor Science, Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada

DOUGLAS O. REVELLE
Atmospheric, Climate and Environmental Dynamics, Meteorological Modeling Team, P.O. Box1663, MS D401, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA