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(meteorobs) Re: Fireball Questions



I received some answers from Juergen Rendtel in regards to fireballs...the
same questions that I sent to Peter Brown and posted yesterday. Below is the
questions and answers from Juergen.
George
--------------
>>Hi George,
 
>GeoZay>> It is said in the Meteor Observers Handbook about Fireball rates
that they
> have preferred periods where most are seen....Namely around the Vernal
> Equinox and in the early evening for the Northern Hemisphere.  I was
> wondering if this is in reference to sporadics only...or is this a broad
> statement that includes the various major meteor showers such as the
> Perseids, Quadrantids, Leonids and the Geminids that don't occur near the
> Vernal Equinox? Or does this refer to only the meteorite dropping fireballs
> that are sporadics and asteroidal? It mentions that Rendtel and Knofel
proved
> that the rates for visual and photographic fireballs are about three times
> the rates seen around the autumnal equionox....again, is this in reference
to
> the slow moving asteroidal sporadic fireballs, or does this include the
> faster shower related cometary fireballs?

Juergen>>All these figures refer to (potential) meteorite droppers.
Particularly,
there are all (cometary) showers excluded (also the Geminids, to be
complete). Generally, the brightness strongly depends on the entry
velocity. According to investigations of photographs, the velocity
goes into the brightness with more than just an exponent 2, probably
the exponent is larger than 3. This means that the same particle
population colliding with the Earth head on, will lead to a larger
number of bright meteors (fireballs). However, the high entry velocity
will prevent these from dropping meteorites. Additionally, we have to
keep in mind, that also the sporadic complex is a summary of several
(rather diffuse, but separate) sources as described in the visual
handbook. So the particle size distribution will differ from one
source to the next. Probably the portion of cometary material is
larger in the apex component, while the helion source might be
richer in asteroidal (more solid) material.<<

GeoZay>> Another question...These slow moving asteroidal fireballs taht have
long
> durations...such as those lasting 15 to 60 seconds, can they travel thru
our
> skies upon entry in an assorted number of directions? That is, can they
> travel North to South, South to North, East to West, West to East and the
> sort? Is there anything that dictates they have to travel in certain
> directions only?

Juergen>>As I said, the sporadic sources are not concentrated like meteor
shower
radiants, but they are not distributed over the entire sky. From the
figure in the `sporadics' chapter you may roughly derive the centers
of the sources for different times of the night. Then you will find
that there is a slight preference of directions for meteors coming
from the various sources. The most obvious source is the apex region
towards the morning, which makes it sometimes difficult to distinguish
between meteors from weak showers and just the apex-sporadics, particularly
if the radiant and the apex line up (e.g. late-evening Orionids and
epsilon Geminids may be mixed with sporadics if the observer faces
southeast).<<

>>Hope I was able to make clear what I mean. If not, please let me know
and I will try again better ;-)
Best wishes,
Jurgen<<