Meteor

A meteor is the visible path of a meteoroid that enters the Earth's (or another body's) atmosphere, commonly called a shooting star or falling star. The visibility is due to the heat produced by the ram pressure (not friction, as is commonly assumed) of atmospheric entry. A very bright meteor, brighter than the apparent magnitude of Venus, may be called a fireball or bolide.

Ionization trails

During the entry of a meteoroid into the upper atmosphere, an ionization trail is created, where the molecules in the upper atmosphere are ionized by the passage of the meteor. Such ionization trails can persist up to a minute at a time. Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in a given region, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced of these trails, it is called meteor scatter communication.

Related Topics:
Upper atmosphere - Ionized - Sand-grain

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Meteor scatter has been used for experimental secure military battlefield communications systems. The basic idea of such a system is that such an ion trail will act as a mirror for radio waves, which can be bounced off the trail. Security arises from the fact that as a mirror, only receivers in the correct position will hear the transmitter, much as with a real mirror, what is seen in reflection depends on one's position with respect to the mirror. Because the sporadic nature of meteor entry, such systems are limited to low data rates, typically 9600 baud.

Related Topics:
Receiver - Transmitter - Baud

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Amateur radio operators sometimes use meteor scatter communication on VHF bands. Snowpack information from the Sierra Nevada mountains in California is transmitted from remote sites via meteor scatter. Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail.

Related Topics:
Amateur radio - VHF - Sierra Nevada - Meteor radar - Decay rate - Doppler shift

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Large meteoroids can leave behind very large ionization trails, which then interact with the Earth's magnetic field. As the trail dissipates, megawatts of electromagnetic energy can be released, with a peak in the power spectrum at audio frequencies. Curiously, although the waves are electromagnetic, they can be heard: they are powerful enough to make grasses, plants, eyeglass frames, frizzy hair and other materials vibrate. See for example, Listening to Leonids (NASA, 2001) for details and references on the auditory phenomenon.

Related Topics:
Earth's magnetic field - Megawatt - Power spectrum - Audio frequencies

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