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Re: (meteorobs) meteor? meteoroid? how many?



At 08:53 PM 10/28/99 -0700, Jim Wray wrote:
>Hi Steve  and All
>
>General question:  What is the meaning of the term "head echo" and the term
>"trail" in the context of your observational system?  

Radio scatter observers mostly hear radio signals reflected from a cylinder
of ionized gas left behind by the energetic passage of a meteor.
Ocasionally they will hear a head echo, described by Jim Richardson as:

"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."

Jim's "Audio Gallery of Radiometeor Events"
(http://www.amsmeteors.org/audio/index.html) has descriptions of the
various types of events as well as audio files that let you listen to them.

Radar observations, in contrast to radio scatter observations, will almost
always detect the head echo because of the geometry of the co-located
transmitter and receiver.

Sigrid's picking up head echoes but not trails might be due to the very
small events being observed.  The ionized trails generated might be below
the detection threshold of the instrument, a sort of under-underdense
event.  I don't have any of Dr. Meisel's papers from his work using the
Arecibo dish as a radar to refer to, but that would be a good reference to
compare.

Jim B.

Jim Bedient
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