The theory of radio meteor echoes.
Also called Meteor Scatter by amateurs.

 

Summary :

 

Astronomical definition of the meteors :

Meteor called the luminous phenomenon accompanying the entry into the atmosphere of an alien body (meteor). Bright drag is caused by the vaporization and ionization of the body of air in its path, due mainly to the phenomena of compression of the air before supersonic body (and not to the friction). A meteor that reaches the ground is called a meteorite and more generally the body can create such phenomena are called meteoroids.

The phenomenon occurs between 120 and 80 km altitude at high speed between 11 and 72 km / s. This is the greater part of the time at least at the speed of land release or the sum of the velocity of the object and the speed of the earth in its orbit.

Their size is very small, between 1 gram and 1 picogram beyond generally talking about meteor meteorite see which usually reaches the ground.

 

In contact with the Earth's atmosphere kinetic energy causes heating and ionization (plasma) in the atmosphere which causes emisson light. The properties of the plasma varies depending on its density and capacity conditions reflecting an incident radio wave. It is estimated that less than 1014 electrons per meter (critical density), the reflected little or no ionized trail the incident wave. In this case we name trail sub-dense (underdense). Above this threshold may be total reflection and we talking about trail dense (overdense). Logically the two phenomena are observed in the case of a strong "echoe" radio as illustrated by this schematic:


The theoretical aspect of the ionized trails of meteors long been studied among others by: McKinley DWR, `` Meteor science and engineering,'' McGraw-Hill, 1961. See more récement: NASA document. (Attention 44Mo !)

The difference between underdense and overdense is obvious when observing the time dependence with a recording of a radio echo meteor.


The sub-dense signal (underdense) results in a very pronounced peak and a logarithmic decay. The "length" of this decrease is a function of the duration of ioniosation.

The majority of the tiny body cause a simple vertical peak without decay signal.
There is also often a "previous-impact" before the main peak.

 

 

    © Frederick A. Ringwald 
© www.imo.net

 

 


©Astronomical Institute,
Ondrejov Observatory		 (overdense) signal results in large fluctuations, sometimes with a pronounced peak.
The "length" of this echo is also a function of the duration of ioniosation, but many distortion, disappearance and reappearance see may formed according to the wind on the high atmospheric layers geometrically changes the "channel" that sometimes persists ionized several minutes after the passage of the meteor.
Same phenomenon is observed in the visible because the "light" trail also persists and sometimes finds its deformation
© www.imo.net

©Pekka Savolainen
Fresnel Oscillatory mechanism also explains the phenomenon of oscillation of overdense echoes.
This is due at the fast velocity of the meteor and the delay induced on the wave at the reception.

© www.imo.net

 

The Daily modulation of the number of echoes   :

The Earth's rotation causes a modulation of the number of daily echoes.
If one accepts the quasi-isotropic presence of fine particles in the solar system first Earth orbit must be observed modulation of numbers daily echoes as a Gaussian with a maximum when the observer turned toward the advancement of the earth on its orbit motion.

This is indeed what is observed in periods absence of meteor shower that have them generally an origin
Below are three examples of Graphics observed with Colorgramme.
The graph represents the number of echoes per hour in abscissa and ordinate the hours of the days in the month.
The number of meteors counted and represented by a scale color from blue to red for maximum black for no data.
Dave Swan is an observer located in Europe England, we can see that the maximum "average" is around 8H UT, which is when the earth is facing the direction of the motion in its orbit.
This value itself is modulated by the position of Meteor Earth orbit and the directivity of the radio antenna used.
The maximum is located around 6am which confirms the theory
 Jeff Brower is an observer located in the USA
The maximum is located around 13am which confirms the theory.		  
Bruce Young is an observer located in Australia which an even more pronounced towards the end of the day offset.

  
These three Colorgramme graph show activity meteor of March, which is particularly quiet for the meteor shower.
This allows to observe the meteor background without major alteration due to the presence of an active meteor shower.
All data come from www.rmob.org compiles the data of an observer global network.

One can imagine a "super" annual modulation this time caused by the revolution of the earth around the sun.
It is quite difficult to show because meteor shower affect the calculation significantly.
Some publish graphs that suggest a maximum in summer especially in August, but often the source of visual observations are original and not radio reliability.

 

Meteors shower, observational impact :

Meteor shower is a groups of dense particles "collide" with Earth relatively well-defined periods.
In general their cometary origin.
It is well established for the Leonids in particular.
We can also note that meteor shower with the same Original give little more "rain" meteor periods different.
Indeed, a comet loses matter irregularly depending on its distance from the sun and the "strength" of the wind Solar eroding that surface. The comet then disseminates "burst" material throughout its orbit creating meteor shower.
Obviously in order to observe the earth must crosses at one point the orbit of the comet and meets "puffs" of material left by the comet.
Some meteor shower are due to cometary particles which have gradually deflected by gravity of the planets and sun and are not necessarily in the "orbit" of the comet.
This explains why we sometimes have difficulty explaining the origin of some meteor shower.

You will find a correct list of active meteor shower on the site www.imo.net
 Here is a fine example of observation of the famous Leonid meteor shower in 2002.
Two peaks were observed. 		  
 In my case an sensitivity "reduced" allows "absorb" the number of echoes and see all the Once the meteorite background and two peaks at 3h UT and 10 UT, imprecision Colorgramme lies in the fact that it displays only Hourly totals.
Predicted by the calculation were: 4H04 UT et  10h44 UT 
See this document :
WGN, Journal of the International Meteor Organization, vol. 30, no. 5, p. 144-148
  
 In the case of the observatory of Ghent in Belgium We also see two peaks at the same hours, but the "power" of the meteor shower is disappearing meteoritic background.
The difference in amplitude of the two peaks is very strong.		  
 Robert Savard in Canada we note that the first peak is absent because of time differences, the meteor shower is too low on the horizon at the first moment.
the second is clearly visible and corresponds to the observed time in Europe.
Again the magnitude of the problem is such that the bottom of meteoritic months disappears.		  

These observations confirm a theoretical model predicted by Jérémie Vaubaillon from l'Institut de mécanique céleste et de calcul des éphémérides (IMCCE) of Paris.
See more on this page :
Campagne des Léonides 2002 IMCCE

Jérémie issue in WGN in PDF :
Download here !
© IMCCE

 

 

 Some formula related of the meteor observations : 
 For those like this some exercises here and formulas reference :
 The maximum reflection is obtained when the meteor path is contained in a plane which is tangent to ellipse having the transmitter (B) and the receiver (A) as homes.
Obviously it is symmetrical in the transmitter can be (A) and the receiver (B).		  
 Maximum power received P (0) via underdense is approximately given by :
 where PT is the transmitter power, GT and GR respectively the gains of the transmitter and receiver antennas in the direction of the reflection point, RT and RR the distances of the transmitter and the receiver to the reflection point, λ the radio wavelength used, re the classical electron radius, q the line density of the meteor trail at the reflection point, γ the angle between the incident electric field vector and the direction of the receiver (as seen from the reflection point), φ the half forward scatter angle, i.e., the half of the angle between transmitter and receiver, also as seen from the reflection point, and β the angle between the trail and the propagation plane. r0 is the initial radius of the trail, a quantity that is discussed in more detail in [26]. Most of the geometrical parameters are shown in Figure 6.
  
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Conclusion : useful numbers !

The impact of the meteor velocity and mass condition the ionization rate. More speed and large mass are more reflection at high frequencies is possible.

However we can deduce some interesting information about this formula.
I will mention two that determine the "right choice" for observe the radio meteor echoes with passive installation, what interests us here.

1°) this is the wavelength used (or its equivalent in frequency), which determines the duration and keeping the received power in the other parameter identical.
Clearly the duration of the signal is inversely proportional to the square of the frequency.
For information Example: An echo is observed on 144.2 MHz which lasts 1 second, the same echo FM 90.1 MHz last 2.6 s and 7s on 55.25 MHz (channel 2) and finally 25 s at 28.5 MHz.


There is also a "break" frequency which is at around 200MHz in "normal" conditions.
The echo observations at higher frequencies have made but are extremely dependent on the sensitivity of equipment used and in general we observe that echoes the most powerful.
The signal amplitude depends on several factors such as transmitter power, antenna, and the relative position of the meteor. Strictly physical point of view the signal amplitude varies as the inverse of the frequency to the power 3/2. Thus an echo 144.2 MHz with an amplitude of 1 uV overlooks 90.1 MHz 2?V on 55.25 MHz, 4.2 uV and 28.5 MHz 11.4 microvolts.
 

So that it is useless or rather inefficient tells us, to try to capture the meteor echoes beyond 200MHz.
Typically observations are made between 28MHz and 145Mhz with a preponderance between 48Mhz and 65MHz spectrum due Electromagnetic human activity.
Below 20 MHz the ionosphere reflected the wave toward the ground and to extreme DIFFICULTIES distinguish echoes meteoritic of human activity.
This is discussed in more detail in the "Equipment".

2°) Finally another frequentely asked question is: Where to turn my antenna direction on the sky ? The calculation shows that there is an "ideal intersection" which guarantee a maximum area for receiving echoes.
It is located at around 45 ° elevation in the plane of the meridian to the south.

 Last just for fun :

Baggaley W.J., Bennett R.G.T., Steel D.I., Taylor A.D., ``The advanced meteor orbit radar facility: AMOR,'' Q. J. R. astron. Soc. 35, pp. 293-320, 1994.
gives an attempted relationship between the size of the meteor its speed and altitude:

Log10 ro = 0,019h - 1,92 + Log10 (V/40)  ro in meter, h altitude in km and V velocity in km/s.