Meteor Streams

Meteor Streams, Stream Observing and Stream Evolution:

Nearly all meteors that are observed are produced by particles which originate in comets. These particles are released form the comet at the time of the comets closest approach to the Sun. A fair amount of material is released form the nucleus of comets. for example the nucleus of P/Halley at its time of closest approach in 1986 was releasing approximately 3.1 tonnes of material every second. Gas jets emerge from below the comets surface and erupt carrying away fragments of the comets crust and other dusty material into the coma surrounding the nucleus.

After the particles are released, solar radiation pressure forces lighter particles into orbits which are different to that of the comets nucleus. The heavier particles in the order of 0.1 g in mass released from the nucleus end up pursuing independent orbits around the Sun as orbits of a meteor stream. At the start the dust that will form a meteor stream will stay fairly close to the nucleus of the parent comet returning to perihelion at roughly the same time as the comet ( a little before and a little after ). This is called a meteor swarm. This type of meteor stream is one which is in its youngest stages and activity is high in the years when the parent comet returns to perihelion. For example, the comet p/Temple-Tuttle produces the Leonid meteor stream, and at the times of its closest approach to the Sun, which is about 32-33 years, produces increased, often intense periods of activity called a meteor storm. When the comet is at aphelion, activity is low.

After time, several effects combine to spread debris along the orbit to form a closed loop of relatively even meteor activity. The main reason for the particles spreading out along the orbit is gravitational perturbations from the Sun. Low mass bodies are much more subjectable to gravitational perturbations than is the heavier cometary nucleus from which they were ejected. Therefore, debris ejected ahead of and behind the comet will arrive at perihelion progressively earlier and later and farther from the parent cometary body. Eventually, the particles ahead and behind the comet meet up and form a closed loop.
During the Earths orbit about the Sun it encounters these loops of debris. When this happens we see a Meteor Stream. These streams take time to develop, but once the debris is evenly distributed around the orbit of the comet, we see stable streams at the midpoint of there development. A few examples are the Perseid meteor shower in August, the Eta Aquariids and Orionids shower produced by comet P/Halley.

The effect of a meteor radiant in the sky is a result of the Earth encountering a meteor stream. The debris in a particular stream have parallel orbits about the Sun, therefore they also have parallel trajectories into the Earths atmosphere. This causes meteors from a particular stream to emanate from a point in the sky called a radiant (See Below). The radiant effect is the result of perspective. Imagine a road with parallel sides moving off into the distance. Its sides appear to converge at the horizon. With meteors seen over a portion of the sky look as if they are following paths diverging from the same point in the sky.

Occasionally a stable stream will produce higher than usual activity. It is believed that this happens when the Earth passes through a denser filament of debris along the orbit of the meteor stream. For instance there is evidence that within the Orionid stream there are filaments released from P/Halley's nucleus at different epochs even when the comets orbit was at a slightly different inclination than it is at now. The result is that there is several different sub-peaks in Orionid activity.
With the use of advanced computers which have become available recently, studies have been done into meteor streams and their evolution. In one study the results suggest that under the gravitational influence of the planets, for example one of Jupiter's family of short period comets with a period of around 7 years wound take about 300 years to become filled with a continuous stream of meteoroids. On a cosmic time scale this is a very short time. The gravitational influence of the planets also do much to smear out meteor streams. The effect is particularly pronounced at aphelion where the Suns gravitation is weakest meteoroids can be pulled into modified orbits of lost from the streams orbit altogether. Over time, this effect can turn a relatively compact stream like the Perseids into a diffuse stream.
Other effects can produce a loss of meteoroids from a stream. The most pronounced of these is the Poynting-robertson effect. Essentially the Poynting-Robertson effect is when a meteoroid absorbs and re-radiates solar energy and in re-radiating the energy it looses orbital energy. The result of it loosing orbital energy is that the particle spirals in towards the Sun.

The effect favours small particles which have 1. Less orbital energy and loose it quicker and 2. Have the highest ratio of surface area to volume. As a result they absorb more solar energy per volume than does a larger particle. Over the long term this effect causes meteor streams to become deficient in smaller particles as they age.
In summary the older a stream, the brighter the average meteor is. The best example of this is the Taurid stream. Being older, it produces meteors of a higher brightness an average and it is more diffuse than a younger stream therefore it has a longer period with a broad maximum.

The ultimate fate of a meteor stream is for its rates to become so low that it no longer produces a noticeable shower. At this point in a streams evolution its activity will have merged with that of the sporadic background produced by dust which fills the inner Solar System.

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