by James Richardson
American Meteor Society
Radiometeor Project Coordinator
The Poplar Springs Radiometeor Station is located in the rolling rural farmland of northwest Florida, near to the point where Florida, Alabama, and Georgia all meet. This amateur built station continuously collects empirical data on forward-scatter radiometeor events for the American Meteor Society (AMS) Radiometeor Project. The station utilizes distant Channel 2 television transmitters as its forward-scatter signal source, monitoring for meteor reflected echoes of the stations' video carrier signals. With the antenna beam for the system pointed towards the northeast, most meteor echoes are returned from three primary transmitters, located in Charleston, South Carolina; Sneedville, Tenessee; and Baltimore, Maryland. This Web page contains audio examples of the five types of meteor events monitored at Poplar Springs, recorded on August 20-22, 1997.
With no meteor events present the listener will hear normal background static, with one or more continuous audible tones superimposed on this background. These tones are the tropospheric scatter signals from each of the transmitters closest to the receiver, with the receiver BFO adjusted for the most comfortable tone pitches. It will be noted that when more than one tropospheric scatter signal is present, the tones will usually be on different notes. This is because the tolerance for transmitters assigned to a particular frequency, while quite narrow, will still permit deviations of up to a few hundred hertz, which is easily detected as different pitches by the human ear. The transmitters being monitored here are assigned to the plus offset frequency for the Channel 2 video carrier signal (55.260 MHz). In the background, the listener will also be able to hear the video modulation "buzz" from nearby stations on the zero offset (Atlanta, Georgia - 55.250 MHz), and minus offset (Mongomery, Alabama - 55.240 MHz). This buzz is usually present as a continuous background artifact, due to the use of a video signal as the monitored source.Below each sample is a portion of an audio frequency analyzer output depicting the most interesting part of each recording.
Above the background noise the listener will be able to hear occasional tonal "pings," similar to the sound made by striking a tuning fork. These are the underdense meteor trail echoes, which generally last less than 1-2 seconds. The term underdense refers to the fact that the meteor trail free electron line density is below a critical factor, usually taken to be near Q = 10^14 electrons per meter of meteor trail length. Below this density, the electrons in the meteor trail scatter the radio wave independently, creating a received signal which rises very sharply when the meteor trail forms, but then rapidly decays exponentially as the trail diffuses. Most meteors forming underdense trails will be just at, or below, the magnitude level seen by the naked eye. In these examples, the events represent meteors of about 5-8th magnitude.
Underdense Event, 11.50 sec, 248 kB
This recording contains 4 faint underdense meteor events: Three very short events followed by a 1 second duration event.
Less frequent than the underdense trail "pings" will be the louder "fooms" of the overdense trails. These trails are above the critical electron density, and reflect the radio wave as a unit. generally, this causes a relatively slower signal strength rise (compared to the underdense trail) up to some sustained peak level which usually last for only a few seconds but can stretch on to several minutes. Following this, the signal level will decay back to the noise level in the same gradual fashion. Overdense trails are generally caused by meteors which can be easily seen with the naked eye (greater than about magnitude 5).
Overdense Event, 12.29 sec, 265 kB
This recording contains a 4 second overdense event, with a short underdense event appearing quickly on its tail. Plotted on a strip-chart recorder, the overdense trail has a smooth, rounded appearance while the underdense event has a sharp, spiked appearance.
The third type of event which can be heard is the oscillating overdense trail. As soon as it is formed, a meteor trail undergoes twisting and scattering by upper atmospheric winds. An overdense trail may be broken apart to form separate "glints," each capable of reflecting radio waves. These separately moving patches wil create an oscillating diffraction pattern at the receiver, with the received signal strength fluctuating in a pulsating pattern during the duration of the event. Because Sporadic E (Es) and afternoon D-layer scatter can also create such oscillating events, distinguishing true meteor trails of this type can sometimes be difficult. During times of light sporadic E and D-layer scatter, the listener will often hear "waves" of such oscillating events in the receiver background noise.
Oscillating Event, 12.75 sec, 297 kB
Preceeded by an underdense trail, this recording contains a short, 3 second oscillating overdense event. The event undergoes 4 oscillations in signal strength prior to decaying completely. A very short underdense event also occurs on its tail.
Another form of bright meteor echo is the occasional "bong" of a transition event, as a sort of hybrid between the underdense and overdense types. These are actually overdense events, based upon magnitude and electron line density, but whose echoes begin with the sudden rapid signal rise characteristic of an underdense event. This is due to the low reflection incident angles present in forward-scatter. This low angle allows the outer, less dense portions of the trail to also cause a reflection, in addition to the inner, overdense core.
Transition Event, 24.30 sec, 524 kB
This recording contains a 14 second duration transition event. Note the sharp, strong initial rise, followed by the more gradual, "fluttery" decay.
Very infrequently, the rapidly descending tonal pitch "whistle" of a meteor head echo can be heard. The sound is reminiscent of bombs falling in old war movies. This is caused by the compressed and rapidly expanding ionization around the meteor head itself reflecting doppler shifted radio waves as the meteor descends through the atmosphere. Often, the meteor head echo will be accompanied by a loud underdense "ping" or Transition "bong" as the meteor head reaches the first fresnel zone and causes a specular reflection as well as a meteor head reflection. The first fresnel zone is, roughly speaking, the primary point at which the meteor trail meets the requirements for forward-scatter geometry for a particular system.
Meteor head echoes occur most frequently when the meteor has a very low inclination angle to the earth's atmosphere. This causes the meteor path length to increase, and the meteor to remain in the zone where radio wave reflections occur for a longer period of time. For meteor shower members, this type of event occurs most frequently when the radiant point is very low in the sky (see the Leonid recording, below).
Meteor Head Echo , 14.72 sec, 318 kB
This recording of a sporadic meteor event begins with three brief underdense events. Then comes the meteor head echo, with a strong 2 second underdense event close on its heels.
Leonid Head Echo , 31.8 sec, 247 kB
Lastly, a very nice 0.5 second Leonid meteor head echo, followed by a 25 second transition type trail reflection, which was recorded during radiant rise on November 17, 1997. Near the end of this recording, the chirping sound of the computer floppy disk drive can be heard, as data about this event is automatically recorded to diskette.
AMS Radiometeor Project Coordinator
Davies, K., (1965). Ionospheric Radio Propagation. NBS Monograph 80. (Reprinted by Dover Pub., 180 Varick St.)
McKinley, D.W.R., (1961). "Meteor Science and Engineering." McGraw-Hill Book Co.
Richardson, J.E. & Meisel, D.D., (1997). "The American Meteor Society Radiometeor Project," AMS Bulletin No. 203 (revised). The American Meteor Society, Ltd., Geneseo, New York.
Richardson, J.E. & Meisel, D.D. (1997). "An Amateur Radiometeor Network and its Scientific Results." Presented at the 190th meeting of the American Astronomical Society (BAAS-33.02).
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This page last updated on July 15, 2000.