WHAT ARE METEORS
Meteors are better known as "shooting stars": startling streaks of light that suddenly appear in the sky when a dust particle from outer space evaporates high in the Earth's atmosphere. We call the light phenomenon in the atmosphere a "meteor", while the dust particle is called a "meteoroid".
The meteoroid that caused the bright meteor in this photograph was only 1 centimeter in diameter. A Leonid meteor of magnitude +5, which is barely visible with the naked eye in a dark sky, is caused by a meteoroid that weights only 0.00006 gram and is only 0.5mm in diameter. Most visible Leonids are between 1 mm and 1 cm in diameter.
That tiny particle can cause a light so bright that it can be seen over distances of hundreds of kilometers. The reason is the astronomical speed of the meteoroids. Just before they enter the Earth's atmosphere, Leonid meteoroids travel at 71 kilometers per second, or some 2,663 times as fast as a fast pitch in baseball, or, if you want, around the Earth in 3.8 minutes!
When meteoroids enter the Earth's atmosphere, they collide with numerous air molecules. Those collisions sputter away the outer layers of the particle. In this process, electrons are stripped off the individual atoms. When these electrons are recaptured by the atoms, light is emitted. This is the same process as in gas discharge lamps.
The color of many Leonids is like the color of our sodium discharge lamps. For the same reason: meteoroids contain traces of sodium. The color of a meteor is an indication of its composition and the excitation temperature: sodium atoms give an orange-yellow light, iron atoms a yellow light, magnesium a blue-green light, calcium atoms may add a violet hue and silicon atoms give a red light.
Meteors do not normally cause audible sounds. Hence, they will pass by unnoticed if not seen. But watch out for hissing sounds that have been reported for very bright meteors. These sounds are thought to be due to low frequency radio waves interacting with the local environment. A sonic boom is sometimes heard for very bright meteors, the fireballs. If the particle is larger than the mean free path of the air molecules, a high Mach number shock wave forms in front of the meteoroid. Very rarely, this shock wave penetrates deep enough in the atmosphere that it can be heard. It sounds like the sonic boom of an airplane, but as a distant rumble.
Meteors sometimes leave a persistent train in their path. The brief glow behind the meteor head is called the wake. The wake is mainly the green light of neutral oxygen atoms. Wakes last 1-10 seconds. After a rapid decay in intensity, bright Leonids often leave long enduring trains that last for 1-30 minutes (typically 4-6 minutes) at an apparent brightness of +4 to +5 magnitude. The light of these long enduring trains is from hot air (0+, N+, O++) and from metal ions from the meteoroid (Na, S+, Ca, Fe).
Some days of the year show a larger number of meteors than usual that all have a common origin. This is a "shower of meteors", also called a "meteor stream". The Leonids are such a meteor stream.
Meteor streams are caused by the debris of comets. Leonid meteoroids move in the orbit of comet P/Tempel-Tuttle. The stuff of comets comes from interstellar space where the materials are assembled in the atmospheres of stars and in the dense molecular clouds of gas and dust between the stars. The comets are build of that material and were formed in the outer parts of the solar system, in regions beyond Saturn's orbit, at the time of the birth of our solar system.
Comets are mountains of ice and dust. When comets approach the Sun, the ices evaporate and the dust particles are ejected into orbit in geyser like fountains.
The large particles remain close to the comet and form a dust coma. The smallest particles (less than 0.1 mm in size) are ejected from the dust coma by solar radiation forces and form the dust tail of a comet. The remaining material stays close to the orbit of the comet. However, small ejection velocities cause large differences in orbital period of these particles. Hence, in the next return the slow particles will lag and the fast particles will proceed the comet. The result is a trail like structure in the orbit of the comet. We see a meteor storm when the Earth crosses that trail of dust.
A cloud of dust particles in the path of a comet is called a "meteoroid stream". When the dust is still freshly ejected from the comet, it is usually in the form of a "dust trail". The dust trail evolves into a meteoroid stream as a result of perturbations by the major planets. It is not yet known exactly how this process happens. When the Earth crosses these dust components, various forms of meteor stream activity result.
Every year the Earth travels through the debris of many comets. That debris has moved far enough away from the comet orbit to collide with the Earth. The resulting meteor showers are refered to as "the annual meteor streams". In most years, the Leonids are a rather insignificant annual meteor stream. Rates peak at 13 per hour on November 17. The main activity is between November 13 and 20, but Leonids occur annually at a rate larger than 1 per hour in the period between October 31 and November 30 (in good dark skies early in the morning).
Only when the Earth travels through relatively fresh cometary matter will rates go up significantly above the normal level of annual activity. Then, and only then, can we see a meteor outburst. The Leonid storms are such meteor outbursts. There were no Leonid outbursts reported between 1970 and 1993. The first Leonid outburst of a new series associated with the upcoming return of P/Tempel-Tuttle was seen in 1994.
Those meteor outbursts that happen when the comet approaches perihelion are called "near-comet type" outbursts. The Leonid outbursts are of near-comet type. There are a series of outbursts in the years before and after passage of the comet. These outbursts are caused by dust that is still relatively close to the position of the comet in its orbit. That dust is distributed in a sheet-like structure that emanates from the dust trail. Near-comet type outbursts are typically from comets with orbital periods less than a few hundred years.
Meteor outbursts that occur when the comet is far from perihelion are called "far-comet type". The Lyrid stream, for example, is a stream with occasional far-comet type outbursts.These outbursts are caused by the debris of long period comets. Their trail of dust is occasionally brought in collision with the Earth by planetary perturbations. The years that these outbursts occur can not be predicted from the position of the comet in its orbit, but can be predicted from the planet positions and the Sun's motion around the barycenter of the solar system.
A meteor storm is just an intense meteor outburst. We talk about meteor storms when rates increase to 1 per second (or Zenit Hourly Rate increases to 1000 per hour). The best annual streams peak at around ZHR =100 meteors per hour.
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