(meteorobs) New Mexico Fireball - Composition? - Ice?

bob71741 bob71741 at yahoo.com
Thu Sep 20 13:37:49 EDT 2007


Meteor velocity, among many factors, play a role in forward scatter.
I copied the following from the AMS Bulletin 203. The next to last
paragraph is very apropos to this discussion. The complete bulletin is
available at : http://www.amsmeteors.org/radio/ams203.txt
I also recommend chapters 8 and 9 of D.W.R. McKinley's book, METEOR
SCIENCE AND ENGINEERING for a better understanding of the processes
involved.

Best Regards
Bob

A1.5  Meteor Velocity Considerations

     In order to cause a forward-scatter event, the meteor must
generate a specific density of free electrons within the trail.  At
altitudes of greater than about 105 km, the electron density
produced is too low and the diffusion rate too rapid to cause many
detectable events.  Below about 85 km, the density of the
atmosphere becomes high enough to cause rapid attachment of the
free electrons, also with a corresponding loss of meteor
detectibility.  The optimum altitude for forward-scatter detection
is centered at about 95 km, where the ionization and
diffusion/attachment effects 

are most equally balanced.  

     Meteor velocity, mass, and angle of entry into the atmosphere
all affect the ability of the meteor to cause a forward scatter
reflection.  Meteor velocity will affect the height of ionization
and the path length, with faster meteors ionizing higher in the
atmosphere and having longer path lengths.  Meteor mass will affect
the free electron line density of the trail and path length, with
higher mass meteors producing the highest densities (strongest
signals) and longest path lengths.  Meteors with a high inclination
will penetrate further into the
atmosphere  than those with low
inclinations, but high inclination meteors will have shorter path
lengths than those entering at low inclination.  These parameters
will have the following effect on the forward scatter system:

     Fast meteors have a tendency to ionize higher than the optimum
altitude band, causing the forward-scatter system to discriminate
against the lower magnitude, fast meteors.  Only the brighter fast
meteors will penetrate deep enough into the atmosphere to get below
the "height ceiling" and cause reflections.   

     On the other hand, slow meteors have a tendency to ionize
below the optimum altitude band, causing the system to discriminate
against the higher magnitude, slow meteors.  Only the fainter slow
meteors will ionize fully in the correct region to cause full
reflections.  Bright, slow meteors may begin their ionization in
the proper region, but then pass under the "height floor," causing
only a partial, low duration reflection.

     Along these same lines, very bright, slow fireballs which have
the potential for dropping meteorites will generally also not cause
noticeable reflections.  These fireballs often do not produce
visible light and ionization until they are below about 70 km,
making them rather "stealth" to radio detection.

     Meteors of medium velocities tend to have the best forward
scatter capability throughout their magnitude range.



--- In meteorobs at yahoogroups.com, Thomas Ashcraft <ashcraft at ...> wrote:
>
> 
> I am trying to figure out why the September 13, 2007 0920 UT fireball 
> over New Mexico made such an insubstantial forward scatter reception as 
> received by my system. I consider this an anomaly. This fireball made 
> only a weak signal at best.  You can hear it sort of "singing" lightly 
> in the movie if you play it loud enough. Radio frequency was 83.250
MHz, 
> tv channel 6 video carrier wave.
> 
> I am wondering if this particular fireball was merely made of ice or
was 
> possibly a little comet or comet core?
> 
> Chart and movie below:
> http://www.heliotown.com/Fireball_Sep_13_2007_Ashcraft.html
> 
> Am still analyzing.
> 
> Thomas Ashcraft
> 
> ---
> Mailing list meteorobs: meteorobs at ...
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