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LEONID DAILY NEWS: November 12, 2000

Y2K persistent train

Figure right: A remarkable video record of the Y2K persistent train emission.


Persistent trains were first described during the 1866 Leonid storm. Once bright fireball would have faded, a persistent glow was seen that sometimes persisted for several tens of minutes. Ever since, researchers have wondered why the glows persist so long and what is causing the visible emission.

Now, that secret has been partially unlocked. In an upcoming issue of Earth, Moon and Planets, Peter Jenniskens and Matt Lacey at NASA Ames Research Center and Beverley Allan, Daniel Self and John Plane of the University of East Anglia, UK, report on the first calibrated optical spectroscopy of persistent trains. The measurements were taken onboard the ARIA aircraft participating in the Leonid Multi-Instrument Aircraft Campaign and show that the visible emission of persistent trains is the result of chemistry that also causes the natural airglow.

Until now, only slit-less spectra of trains were obtained. Jiri Borovicka measured the rare train spectrum shown below left of the "Y2K" persistent train. The spectrum of the train is the fuzzy band to the left of the image of the train, seen in the picture above, and is created by using an objective grating in front of the camera. Features in the spectra smear out when the train diffuses and the wavelength calibration is always somewhat uncertain in this type of measurements. Hence, the origin of the band emission remained in doubt.

Trace from slit-less spectrum

This figure shows a trace of the slit-less spectrum.

Result from slit spectrograph

Figure shows the result using the slit spectrograph. Both spectra show a broad emission band.

The slit-spectroscopy, however, points a telescope at the glow, collects the light into a fibre and leads it into a spectrograph. Thus, a spectrum is obtained for which the wavelength scale can be calibrated. Comparisson with laboratory data identified the band as caused by iron-oxide (Full paper - PDF).

The spectroscopic observations confirm that most intense emission arrises from the sodium (Na) D-line and from the molecular emission band of iron-oxide (FeO). These emissions occur almost certainly through the Chapman airglow mechanism, where oxygen atoms and ozone molecules recombine to oxygen molecules in a process that is catalysed by the meteoric metals sodium (Na) and iron (Fe):
Na + O3 -> NaO + O2
NaO + O -> Na* + O2
The excited sodium then emits orange light. Similarly:
Fe + O3 -> FeO* + O2
FeO + O -> Fe + O2
Now it is the excited FeO that emits the orange light.

Model train spectrum Images of the Chippenham train show two parallel structures. It is now also suspected where the ozone and oxygen come from. Many trains turn out to have a tubular structure, with two bands of intense emission where the line of sight crosses the edge of the tube.

Chippenham train

The new train model.

Subsequent modeling by John Plane of the University of East Anglia showed that the tubular structure can be explained if the oxygen atoms are created by the fireball, while the ozone is that in the ambient atmosphere. Diffusion processes bring the catalytic metal atoms out to the edge where ambient ozone diffuses into the train environment, and are responsible for the gradual expansion of the trail shown by the model cross section at three different times in the figure above (Full paper - PDF).

Many fundamental questions remain. The model does not yet explain why the trains are so dark in the center and why they expand linearly in time. It is clear that persistent trains can help understand the physical processes that lead to the natural airglow in the upper atmosphere by setting constraints on diffusion timescales and relevant chemistry mechanisms.

Previous news items:
Nov. 12 - Train airglow chemistry
Nov. 11 - Hard bits and persisting glows
Nov. 10 - Meteoroid debris detected
Nov. 09 - New meteor picture
Nov. 08 - Spin city
Nov. 07 - Meteors affect atmospheric chemistry
Nov. 06 - Listen to this!
Nov. 04 - Fear of heights?
Nov. 03 - The pale (infra-red) dot
Nov. 02 - Twin showers
Nov. 01 - Leonids approaching Earth
Oct. 31 - Prospects for Moon Impact Studies
Oct. 30 - Comet dust crumbled less fine
Today's news

These and other results of Leonid storm research will appear in a special issue of the peer-reviewed journal "Earth, Moon and Planets", published by Kluwer Academic Publishers, the Netherlands.

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