Popular Science Monthly/Volume 79/August 1911/The Origin of Luminous Meteor Trains
| THE ORIGIN OF LUMINOUS METEOR TRAINS |
By DR. C. C. TROWBRIDGE
COLUMBIA UNIVEESITY
THE nature of the luminous cloud occasionally left glowing in the wake of large meteors and called the "persistent" streak or train has long been regarded as a mystery by astronomers. Many of these trains have been observed which have remained visible to the naked eye for quite as long as fifteen or thirty minutes after the disappearance of the burning metorite itself. In numerous instances trains have lasted for more than one hour, floating in the cold upper atmosphere, a luminous mist-like cloud projected against the dark night sky.
Meteors are usually visible but for a few seconds in their rapid flight through the upper regions of the atmosphere at a velocity of from twenty to thirty or more miles a second. Their track is almost always marked by a bright streak of fast-fading luminosity which also disappears from view in a second or two. Occasionally, however, the streak remains for many minutes brightly glowing, continually expanding in size, and drifting with the moving atmosphere. This is the phenomenon which has been called the "persistent train."
Meteors which leave these trains are a very small proportion of the total number that are seen, yet authentic and definite facts concerning the trains have been recorded at various astronomical observatories in all parts of the world, hence the chief characteristics of this remarkable phenomenon are known.
A serious study of the subject has been made only recently, but it is now recognized as being of considerable importance because it teaches important facts concerning the upper atmosphere.[1] Most of the observations of meteor trains which from time to time have been made by different astronomers have been incidental in the course of the usual investigations of the heavens. Perhaps the best way to show the extraordinary features of the self-luminous meteor train is by drawings of a bright and long-enduring train carefully observed in England many years ago. The drawing marked A in the first illustration is a sketch made by one of the observers of the train as it appeared only a few seconds after the meteor nucleus had disappeared. This train, which was seen by many observers in the vicinity of both Sidmouth and Cardiff in England at the time of the great meteor shower of 1866, was formed by a meteor belonging to the swarm known as the Leonids, and appeared at first, as is always the case, a narrow glowing streak. The train soon after its appearance was sixteen to eighteen miles in length and at an altitude above the earth of fifty-six miles as determined by triangulation from Sidmouth and Cardiff, which are fifty miles apart. It appeared lance-like for a few moments and then was seen to be bending like a long floating ribbon, slowly curving about as shown in sketch B. The train also gradually expanded in size. The enlargement of these luminous clouds is unquestionably due to the diffusion of the particles of which they are composed. A careful study of the observations of a large number of trains has proved conclusively that the distortions, such as those shown in the illustrations, are due to the different wind currents that are in the atmosphere at great heights, even as far distant from the earth as sixty-five miles. It is also evident that these currents may vary both in direction and in velocity at different levels at one time, quite similar in fact to the drifts of the atmosphere in the cloud region near the surface of the earth.
Meteor trains have been observed more frequently during the periodic meteor showers than at other times, and apparently the meteors which have produced the greatest number of trains are the so called Leonids and Perseids. The former, it may be remembered, appear every year about November 14, but produce what is known as a meteoric shower every thirty-three or thirty-four years, the actual period being
Fig. 2. Meteor Train seen at Aberdeen Observatory, England. Observed on November 14, 1866, described as a pale yellow band of light about half the diameter of the moon. A at 2:41 a. m., B at 2:43 a. m., C at 2:45 a. m. (The largest star in the cluster is Aldebaran.)
3314 years. The showers of 1833 and 1866–7–8 produced many persistent trains. The Leonid shower in 1901 was very meagre as compared with those of 1833 and 1866, yet a number of persistent trains were observed at that time, which was the date of the last meeting of the earth and that part of the orbit of the Leonid meteor swarm where the meteoric masses are clustered most thickly.
The Perseid meteors appearing about the eleventh or twelfth of August are far more evenly distributed along in the track of their orbit than the Leonid meteors and hence there is no recurrent period when they are particularly abundant. Both the Leonids and Perseids are very
Fig. 3. Meteor Train seen at the Leiden Observatory, Holland. Observed on November 13, 1865. From a picture by the observer, Dr. Van Hennekelen. A as it appeared at 12:45 a. m., B at 1:13 a. m., C at 1:24 a. m.
Fig. 4. Double Trains observed at the Palisades Observatory, N. Y. The trains are drawn as they appeared in a telescope; the double appearance is probably due to greater luminosity on the border of the train.
Among the astronomers who have made important observations of these trains are Mr. W. F. Denning, of Bristol, England, the acknowledged authority on meteors; the late Professor A. S. Herschel, who died about two years ago, the last of the three great Herschels; and a number of other well known astronomers, members of a committee especially organized by the British Astronomical Association for the observation and study of meteors. On the European continent Professors J. F. J. Schmidt and G. Von Niessl have contributed a great deal to the subject of meteoric astronomy. In the United States the late
Fig. 5. Comet-like Train seen by Professor E. E. Barnard, November 14, 1901. A as it appeared at 3:10 a. m., B at 3:40 a.m. The drawings are copies of sketches sent to the author by the observer.
Professor H. A. Newton, of Yale University, was a constant observer of meteors and published many observations, and in recent years Professor E. E. Barnard, of the Yerkes Observatory, has added some important records of long-enduring trains.
Meteor trains are by no means as rare as might be supposed, and it is safe to predict that if a plan were organized for their observation on nights of the year when meteoric showers occur, many trains would be observed. This would be a source of new records which would throw more light on the subject. That trains would be seen if systematically looked for is demonstrated by the fact that nine different trains were seen by an observer in England during one night with the aid of a small telescope. The watch was kept at the time of an ordinary August Perseid shower.
Fig. 6. Greenish Train observed at Jamaica Plain, Mass., November 14, 1901. Meteor fell at 5:09 a. m. A as the train appeared at 5:12 a. m., B at 5:17 a. m., at 5:25 a. m. Mr. Denning and Professor Barnard have pointed out that meteor trains visible to the naked eye for one or two minutes have been seen in the telescope for a quarter of an hour or more, and that by the use of a small low power telescope they can be studied to greater advantage than by the naked eye alone. If the track of every bright meteor could be examined with a field glass, it is probable that many persistent trains would be observed which would be invisible to the naked eye. Moreover, Mr. Denning has shown that a great many meteors can be seen with a telescope which are otherwise invisible, and he also cites instances where he has detected persistent trains by the telescope.
Fig. 7. Daylight Train of a Detonating Meteor. Seen from the Lick Observatory, 7:30 p. m., July 26, 1896, visible for one half hour, from a drawing published by the observatory. The enlarged part of the train is the point where the explosion occurred, twenty-eight miles above the surface of the earth.
Leo, "like the spokes of a wheel, three or four at a time"! The radiant point is the region in the heavens from which meteors appear to emanate. Many trains have been observed on the European continent from time to time. One of these is shown in one of the illustrations. It was seen at Leiden in Holland on the night of November 13, 1865.
Recent Remarkable Trains Seen in England
A train which was observed all over the south of England during the evening of August 12, 1894, was formed by a meteor at 10:20 p. m. and was very remarkable. Its drift across the country was noted by many observers. It was eight miles in length at first and drifted at the rate of 122 miles per hour at a height of 54 miles, and it was watched by observers in different cities for thirty minutes. It finally assumed a globular form, and at one time was calculated to be four miles in diameter, thus covering a space of at least ten or fifteen cubic miles!
A meteor leaving another long-enduring train appeared over the south of England at 7:30 p.m., February 22, 1909. Mr. W. F. Denning considers this meteor with its train the most remarkable one in modern times. The train gradually increased in brilliance and twisted about, assuming grotesque shapes. A part of it drifted to the northwest at a velocity of 80 miles per hour and remained plainly visible until 9:30 or 10 o'clock. Another portion of the phosphorescent cloud drifted at a very much greater speed; according to the calculations of Mr. Denning, the best authority on meteors, the rate of 300 miles per hour was observed! This velocity is more than double that of any other drifting train hitherto observed, and indeed shows an extraordinary movement of the atmosphere. The train may have been illuminated for a time by sunlight on account of its appearance so early in the evening. This is the longest instance of visibility of a train seen at night. The record duration of a train illuminated by reflected light of the sun is that of the smoke train left by the meteor which exploded over Madrid, Spain, on the morning of February 10, 1896, and which was visible for five and one half hours as a reddish cloud.
Meteor Trains Observed in the United States in 1901
Professor E. E. Barnard, of the Yerkes Observatory, who has made a number of interesting observations of meteor trains reported two that he observed early on the morning of November 15, 1901. The first was formed by a brilliant meteor which radiated from the constellation Leo. This train was greenish-white in color, about 5° long, or a distance equal to ten times the diameter of the moon. It remained visible from 2:54 to 3:40 a.m. Another, which is shown by an illustration, was first seen at 3:10 a.m. and was conspicuous until 3:40 a.m., when it was obscured by clouds. Its color was greenish-white also. This train gradually expanded until it covered 20 or 25 degrees of the sky, forming two tails and appearing much in the likeness of a comet. Another train shown by three sketches made by the observer was seen by Mr. E. M. Dole, November 14, 1901, about 5:09 a.m. at Jamaica Plain, Mass. The large size of the train is made evident by comparison with the familiar stars of Ursa Major, "The Big Dipper," which appear in the drawing. The train was greenish in color, gradually turning to white. It was also seen at the observatory of Brown University, Providence, R. I. This cosmic cloud, according to an estimation which cannot be far wrong, was about two miles in thickness and fifteen miles in length. About the same time several persistent trains were seen at Dunmore, Northwest Territory, Canada, one of which remained visible twenty minutes, and several others were reported from the Mount Lowe Observatory, California, by Professor F. L. Larkin.
The Meteor Train Zone
It is evident that meteor trains that are observed at night occur at a very definite altitude, and furthermore various facts indicate that the formation of the train is due rather to the state of the earth's atmosphere where the train is formed than to the constitution, size or condition of the meteor itself. Also that the train is a glow phenomenon of the phosphorescent type and probably of electrical origin.
When meteors penetrate into the atmosphere of the earth they blaze forth at various heights, some even at as great an altitude as 100 miles, but the average height at which they appear has been found to be about 80 miles. The usual height at disappearance is about 40 miles. These altitudes are based on many observations. There is strong evidence in favor of the hypothesis that there is a certain density of the air which is favorable for the formation of the persistent train, because the altitude above the surface of the earth at which meteor trains occur when seen at night is usually confined to definite limits between forty-five and sixty-five miles, and the height which appears to be most favorable for longest visible duration is about fifty-five miles. Thirteen trains carefully observed at two or more stations, the altitudes of which have been determined by triangulation by well known astronomers, give a height of fifty-four miles as the mean altitude of the middle portions of the trains. The region where these self-luminous streaks of meteors occur extending from a little above sixty to a little below fifty miles altitude may well be called the meteor train zone of the atmosphere.
Trains of meteors which fall in daylight or twilight are not infrequently seen. They appear as thin smoke trains illuminated by the light of the sun, and according to triangulation observations which are possible only when the train is seen from two stations, they occur as low as twenty-five miles altitude, but seldom above forty miles. They are thus as a rule at a much lower altitude than the trains seen at night, which are usually, if not always, above forty-five miles. This fact would seem to indicate that in the upper levels of the atmosphere the glow does not mainly arise from light reflected from fine meteoric dust, but is a luminosity of the gas in the meteor's track.
The colors of the meteor trains show a good deal of variation according to the records of observations of different trains. The colors of trains observed at night include orange-yellow, emerald-green, bluish-green, silver and also white. In numerous cases green trains changed gradually to white, and in one instance from greenish to a "dull reddish." Apparently the color of many of the self-luminous trains observed are recorded as various shades of green, gradually changing to white as the train fades, and the remainder either bluish, silver or white. The residue of the meteorite is evidently a cloud composed of very finely divided smoke-like particles arising from the volatilization, combined with gases liberated at the same time. This cloud, if seen in daylight or illuminated in twilight by the sun, is usually red, yellow, or silver colored, much like the ordinary clouds as they appear just after sunset. The greenish luminosity of trains seen at night may be due to continued brush-like electrical discharges near the border of the train, but it is probable that it is the phosphorescence of the rarefied atmospheric air in or around the train. This hypothesis is suggested by the following experiments.
Experiments on Phosphorescent Gases in the Laboratory
An attempt has been made to reproduce the phenomenon of the meteor train in the laboratory. One of the interesting effects produced under certain conditions in a vacuum tube by electrical discharges is the phosphorescence of the particles of gas which still remain in the tube. This after-glow has been the subject of careful study. This type of luminescence is, of course, not the same as the oxidation of the element phosphorus, and is apparently different from, though very similar to, the phosphorescent light given out by insect larvæ, like the glowworm, and insects such as the firefly. Phosphorescence is named for its likeness to the slow burning of phosphorus, but in the case of solids and gases, according to the best authorities, it is due to an unstable chemical condition, brought about by the excitation by light or by electrical discharges. The production of the phosphorescent light or afterglow is supposed to take place while the substance is returning to the former stable chemical state.
In one of the illustrations a photograph is shown of a vacuum tube containing a gas at low pressure, which is illuminated by gas phosphorescence. The gas was previously excited by an electrical discharge, but at the time the picture was taken all electrical discharges had been entirely cut off. Thus the photograph was taken by means of the phosphorescent light of the gas alone, no other light and no electrical discharge directly contributing to the formation of the picture. The gas which is glowing is atmospheric air with most of the oxygen extracted, and therefore is chiefly nitrogen. This gas phosphorescence has many points of resemblance to the meteor train, the chief of which are first, it is formed at a density of the air that, as near as can be determined, is the same as the computed density of the atmosphere at heights where meteor trains occur. Second, the comparison of observations of the spectra of meteor trains and those of gas phosphorescence apparently show that both are the same kind of spectra, although the location of 
Fig. 8. Vacuum Tube used to Produce Gas Phosphorescence and Photograph of Gas Phosphorescence taken by its own Light. Rapid oscillating electrical currents are sent through the coils about the vessel.
the spectrum lines of the former have not been determined. The general color of the phosphorescence in air is greenish-yellow which suggests the observed green color or meteor trains. Third, that recently by laboratory experiments the author of the present article has determined the law of the diminishing intensity of gas phosphorescence at successive intervals of time after the phosphorescence has been formed. The results of these experiments show that if the luminous meteor train is phosphorescent gas, the long duration of its visibility, often of half an hour, is readily explained from the law of the slow decaying or dying out of the luminosity shown by experiments on phosphorescent gases. This evidence and other facts point strongly to the hypothesis that the meteor train seen at night is due to phosphorescence of gas in the meteor's track.
How the Meteor Trains may be Formed
In the great Leonid meteor shower of November, 1866, and the showers following in the years 1867–68, it was noted at several observatories that the persistent luminous trains of meteors were not formed throughout the entire length of the visible path of the meteor. These observations are substantiated by the others more recently made. In Fig. 9, the tracks of seventeen trains are shown. The lengths of the paths with respect to altitude are drawn only. The paths of the meteors had different slants and hence differed in length very much more than shown in the drawing. The location of the persistent train is indicated with
Fig. 9. Chart giving Vertical Projections of the Paths of Seventeen Meteors, showing the altitudes of the trains. In a number of cases the mean altitude of the train has been measured only.
respect to the length of the entire track or trail, and it is evident from the chart that the production of the long enduring glow has something to do with the altitude of the meteor above the earth. As already stated, the upper and lower limits of the zone in the atmosphere where the trains are formed appear to be usually about sixty and fifty miles respectively from the earth's surface. If the glow is considered to be a phosphorescence of the rarefied air in the meteor's track, the conclusion that must be drawn, based on laboratory experiments, is that the barometric pressure at these heights is not far from two-tenths of a millimeter of mercury, or, in the neighborhood of from one two-thousandth to one four-thousandth of the pressure at the surface of the earth. Any information relating to the density of the atmosphere at great altitudes would be of value, making this part of the research an important one.
It has been shown by the work of physicists, and particularly by the recent researches of Professor Richardson, of Princeton University, that when a body is very hot an immense number of negatively charged corpuscles or ions are given forth from the body. Air containing free ions becomes a conductor of electricity, hence we have in a meteor rushing through the atmosphere a condition extremely like a very long electrical discharge tube containing a gas at low pressure. The passage of the burning meteor through the atmosphere must form a column of highly ionized air thirty or forty miles in length. Moreover, at a certain altitude, corresponding to a pressure near two-tenths of a millimeter of mercury pressure, or about from one two-thousandth to one four-thousandth of one atmosphere pressure, the conditions are precisely right for the formation of phosphorescence in the meteor track. If at different levels in the upper atmosphere the air is at different electrical potentials, discharges must certainly occur in the meteor's track, and the burning meteoric mass thus may readily play the part of an incandescent electrode in a very long discharge tube, the column of ionized air being a ready conductor of electricity. When the meteor nucleus has been consumed, all that remains visible in the dark sky is the body of phosphorescent gas in the part of the track where the gas pressure conditions were correct for the formation of the persistent glow.
Under the above circumstances, it is not surprising that luminous effects are produced in the meteor train zone of the upper atmosphere where the density of the air is apparently the same as that at which luminous effects can be produced in vacuum tubes in the laboratory by even very weak electrical discharges.
It is not certain that electrical discharges takes place in the meteor track, but they may not even be essential for the formation of the phosphorescence. It has already been pointed out that the flight of a meteor through the atmosphere at the rate of twenty to thirty miles a second produces an exceedingly high temperature immediately about the meteorite, probably a matter of many thousands of degrees. The air thus heated and highly ionized by the burning meteorite, a condition which is sure to occur, may readily suffer chemical or physical changes in its composition which on gradually reverting to its original state gives out a long-enduring phosphorescent glow, just as is apparently the case in the formation of gas phosphorescence. Thus it is not unlikely that the production of the phosphorescent light of the meteor train is connected directly with the highly ionized state of the air and that this condition is produced by the outpouring from the intensely heated meteor of electrons, those electrically charged minute particles discovered by Sir J. J. Thomson which are supposed to play the important rôle in all electrical phenomena.
- ↑ A grant has been made by the National Academy of Sciences from the J. Lawrence Smith Fund to the author of the present article to enable him to extend his researches on meteor train phenomena.