[meteorite-list] Subaru Astronomers Measure Meteoroid Tunnels in Earth's Atmosphere

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 11 Sep 2007 17:29:30 -0700 (PDT)
Message-ID: <200709120029.RAA15751_at_zagami.jpl.nasa.gov>

http://subarutelescope.org/Pressrelease/2007/09/10/index.html

FOR IMMEDIATE RELEASE September 10, 2007 9 am HST

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Subaru Astronomers Measure Meteoroid Tunnels in Earth's Atmosphere

FOR MORE INFORMATION CONTACT:

Principal Investigator
    Professor Masanori Iye, National Astronomical Observatory of Japan
    email: iye at optik.mtk.nao.ac.jp
    phone +81-422-34-3520, fax +81-422-34-3527 (JAPAN, GMT+9 hours)

Media Contact
    Dr. Tetsuharu Fuse, Subaru Telescope, National Astronomical
Observatory of Japan
    email: tetsu at subarutelescope.org
    phone +1-808-934-5922, fax +1-808-934-5984 (USA, GMT-10 hours)

Subaru Astronomers Measure Meteoroid Tunnels in Earth's Atmosphere

When meteoroids flash through Earth's atmosphere, they bore tunnels
through the air, leaving behind narrow meteor tracks that are heated
by the collision of the fast-moving incoming object with atoms of
highly diluted atmospheric gases. Most meteoroids are bits of space
debris the size of a grain of sand. The width of the tracks they make
has long been known to be narrower than a meter, but until recently,
more precise measurements have been impossible to make.

Researchers from the National Astronomical Observatory of Japan, the
University of Tokyo, the Japan Aerospace Exploration Agency, the
University of Electro-Communication, the RIKEN research institute,
and Nagano National College of Technology have evaluated the
diameters of the heated tunnels left behind as typical sporadic
meteors as penetrated the upper atmosphere, scattering atmospheric
atoms and releasing photons of light. The team compared the number of
special photons produced as a meteoroid collided with the atmospheric
atoms and found a typical column width as narrow as a few millimeters
across. This is the first time the width of a meteor track column has
been precisely measured using a physical analysis of the light
emitted during the event.

The study was the result of an observation run at Subaru on the
nights of 12-15 August, 2004. During that time, observers imaging the
Andromeda galaxy using Subaru's Suprime-Cam noticed a number of
meteoroid tracks traversing the field of view of the camera. As M31
is fairly close to the radiant of the Perseid meteor shower (which
peaked just before the start of the observation) observers took a
detailed look at the tracks.

Since Subaru Telescope focuses at infinity, meteors shining at 75
miles above Earth's surface are considerably out of focus. Artificial
satellites orbiting at altitudes 300 to 12,000 miles) are also out of
focus, but not as much. Figure 1 shows typical tracks of a meteor and
a satellite. The angular size distribution of all the measured tracks
during the observation indicates a distinct separation of meteors
from satellites is feasible just from their track widths. Satellite
tracks often show periodic luminosity variation since the rotation of
their solar panel produces the change in their reflected light. Some
of the meteors show sudden outbursts while penetrating the atmosphere
as shown in Figure 3.

During the 19 hour-long CCD exposures, 55 tracks were recorded. Among
them were 13 meteor tracks. Only one was from the radiant of the
Perseid meteor shower. Another was associated with the Aquarid meteor
shower. Most of the remaining meteor tracks were from sporadic
meteoroids. (See Note 1). The actual size of meteoroids studied in
the current observation was estimated to be between 0.1 and 1
millimeters (derived from their luminosity).

The physical analysis of the tracks was carried out by team member
Professor Masanori Iye, who took a close look at "forbidden line"
photons of neutral oxygen atoms radiating at 558 nanometers (nm).
These special photons are generated when a high-speed meteoroid (or
atoms hit and accelerated by the meteoroid) collide directly with the
neutral oxygen atoms. The collision "excites" the oxygen atoms (in
other words, the state of the electron orbiting around the oxygen
nucleus is elevated to a higher energy orbit). At 0.7 seconds after
the collision, on average, the atoms drop back down to their normal
state. In this process, they release the special 558-nm "forbidden
line" photons (Figure 4).

Typical meteoroid spectra show that these special "forbidden line"
photons make up about 10% of the total photons measured through the
yellow V-band filter on Suprime-Cam. Therefore, by measuring the
number of total photons recorded in the CCD images of meteor tracks,
one can calculate the total number of forbidden photons. This
requires the same number of collisions of neutral oxygen atoms. Since
the volume density of the neutral oxygen atoms at 75 miles is known,
and the speed of meteoroids can be estimated, it is possible to
calculate the cross section of the column to produce the same number
of collisions. Calculations for four meteor events observed in V-band
yielded the column diameter of a few millimeters.

Interestingly, the 0.7-second time span(?) at which the neutral
oxygen recover their ordinary state by releasing the "forbidden line"
photons is an extremely long time for atomic processes, and the
excited oxygen atoms hover about 300 meter away from the collision
column during that time. Therefore, the width of the "forbidden line"
trail ([OI] wake) is much wider than the main body width of the
meteors as derived in the present study.
By focusing the Subaru Telescope to the altitude of meteors, one can
make highly sensitive imaging observation of faint meteors and
further study the population of micro-meteoroids.

The results of this study were published in the August 25, 2007 issue
of the Publications of the Astronomical Society of Japan.

Title and the authors of the paper
SuprimeCam Observation of Sporadic Meteors during Perseids 2004
Iye,M., Tanaka,M., Yanagisawa,M., Ebizuka,N., Ohnishi,K., Hirose,C.,
Asami,N., Komiyama, Y., and Furusawa,H.
2007, Publ. Astron. Soc. Japan , Vol. 59, No.4

(Note 1)
Separate 21-hour-long Suprime-Cam imaging observations at the Subaru
Deep Field during the time when no known meteor showers appear gave
similar meteor event rates. This supports the interpretation that the
meteors evaluated in this research were mostly sporadic ones and not
Perseids.

Figure 1
Typical images of a meteor (left) and an artificial satellite (right)
as recorded by Subaru Telescope focused to infinity. Meteors at 110
kilometers (74 miles) altitude defocused much more than satellites at
500 - 20,000 kilometers (310-12,427 miles) altitude appear as
distinctly diffuse tracks. Artificial satellites with their rotating
solar panel often show periodic luminosity variation as shown in the
right panel. Images are shown in negative prints.

Figure 2
Meteors shining at 75 miles high appear as distinctly defocused
tracks wider than 10 arcseconds while artificial satellites orbiting
300-12,000 miles in height are less defocused and their track width
are narrower than 6 arcseconds as shown in this figure.


Figure 3
This image shows two meteors. The brighter one shows luminosity
variation. The concentric shadows in the left are due to the optical
ghost images of a bright star outside of this field of view. Another
ghost image in the upper right shows the cross shadow of the spider
that supports the secondary mirror of the telescope and obstructs the
telescope aperture.

Figure 4
A model of a meteoroid shows it entering the upper atmosphere at 75
miles at high speed and colliding with nitrogen atoms (in blue) and
oxygen atoms (in green) and scattering them. (There are other atoms
and molecules but they play similar roles for the forbidden line
emission and are not shown here for simplicity). Neutral oxygen atoms
directly hit by the meteoroid or by accelerated nitrogen or oxygen
atoms are "collisionally excited" (in orange). Such excited neutral
atoms return to ordinary state on average 0.7 sec after the collision
by emitting a 558-nm "forbidden line" photon. By counting the number
of these special forbidden line photons, astronomers were able to
derive for the first time the number of associated collisions and
evaluated the width of the collision tunnel bored by the meteoroid.
Received on Tue 11 Sep 2007 08:29:30 PM PDT