09Spectra of Meteors
Small dust grains that happen to fall into the Earth's atmosphere from space are accelerated to high speed and become high temperature due to frictional heating. Ingredients melted from the dust grains are mixed with components of the atmosphere. The mixture becomes plasma with a temperature of thousands Kelvin and shines. We call such phenomena ‘meteors’. A very bright meteor that falls as a meteorite on the Earth is called a ‘fireball’ (or a bolide).
When we observe satellites reentering the atmosphere or a large fireball that finally becomes a meteorite, we sometimes detect a continuum of the heated-up main body. When we make spectroscopic observations of meteors, we usually obtain a collection of emission lines. However, main features are emission lines of calcium, magnesium, sodium, and iron originating in meteoroids (the main bodies of meteors) and those of nitrogen, and oxygen, i.e. main components of the atmosphere. We often see "colors" of meteors. These visible colors are combinations of several kinds of emission lines.
The strengths of the emission lines and the intensity ratios depend on ground speed and entry angle as well as physical and chemical conditions of the atmosphere. In addition, individual differences in each meteoroid change spectral features. Recent research has revealed that the contents of volatile elements such as sodium are different even for meteors in the same meteor shower.
In relation to a meteor's mother comet, spectra of a meteor are studied using a sample of refractory elements. When a meteor has passed by, a meteoric trail that appears like smoke along the trajectory is sometimes left. A short meteoric trail disappears within seconds, while a persistent one stays visible for minutes, and sometimes for nearly one hour. The latter is thought to be related to the nitrogen molecular band. However, the mechanism that keeps the trail shining is still a matter of debate.
： “Difficulty in Meteor Observations”
Except for space observatories such as “HAYABUSA”, difficulties in meteor observation can be summarized in the following two points. Firstly, we cannot predict when, where, and how bright meteors appear. Secondly, the emission time of each meteor is really short. Thus, preparing observational instruments beforehand, observers usually wait for a meteor shower when lots of meteors are expected. At that time, a spectrograph is set up as its dispersion direction becomes orthogonal to the trajectory of the meteors. High time-resolution data is recorded by appropriate devices such as a video camera.
During the Leonids' active phase, lots of new spectral data were taken all over the world including in Japan. Our research team was able to detect OH emission lines at that time. Taking advantage of our forecast of the Bootes meteor shower's activity, we were successful in obtaining spectra of meteors. This was the first success in obtaining spectra for a small meteor shower.