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.
