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Re: (meteorobs) Re: Question on the Apex thing
This is a repost from 9/25/99, , due to the recent questions on the topic:
One area which I did not address in the article on meteor speed and radiant
distributions (Meteor Trails No. 3, March 1999)is the additional affect of
the known broad, discrete
sporadic "radiant" areas. These are areas on the celestial sphere about 10
- 30 deg in diameter which are known to produce significantly higher
numbers of sporadic meteors than other areas of the sky. There are seven
such areas, 4 primary areas and 3 more nebulous ones. These are: apex,
antapex, helion, anthelion (major); north toroid, south toroid, and cyclid
(minor).
** The major areas are ecliptically centered (roughly), and remain at the
same locations with respect to the sun throughout the year. That is, they
drift eastward along the ecliptic at the same rate that the sun does.
* The Apex region is centered at the apex of the Earth's way, and is
composed of fast speed (about 50-72 km/sec) meteors having low-inclination,
retrograde heliocentric orbits. Due to apex attraction effects, this
region is theoretically the smallest in diameter of the four major regions.
This area culminates in the sky around sunrise.
* The antapex region is located opposite the apex region on the celestial
sphere, and is composed of very slow speed meteors (about 12-18 km/sec)
meteors, having low-inclination, direct orbits. Due to the radiant apex
attraction effect, this region is theoretically broader in diameter than
the other major regions. This area culminates in the sky around sunset.
* The Helion and anthelion regions are located in the region of the sky
near the sun (radio detected only) and opposite the sun in the sky
respectively. They are composed of medium to slow speed meteors (about
16-40 km/sec) meteors, having low-inclination, direct heliocentric orbits.
The anthelion region culminates in the sky at around midnight.
** The minor regions also follow the sun in that they complete one
revolution about the celestial sphere once a year, and maintain a
relatively constant daily position in the sky.
* The cyclid region is a very difficult to detect, and thus questionable
region, also centered at the apex of the Earth's way (along with the apex
region, mentioned above). However, this source is composed of *very slow*
meteors (12-16 km/sec) meteors in a low-inclination, direct heliocentric
orbit. These particles are thus ones which the Earth catches up to and
sweeps up in its orbit. These are certainly not what one thinks of when
considering apex region meteors (generally very swift), so observers should
keep an eye out for these very slow odd apex counterparts.
* The north toroid and south toroid regions are also rather odd and
nebulous in definition, but generally comprise regions about 60 or so
degrees above and below the helion (sun) region. These would be medium
speed meteors, originating from high-inclination, direct and retrograde
orbits. They have been seen fuzzily in the radar data, but are not well
defined at this stage (from what I currently know.
Note that all of the above regions are in addition to the normal sporadic
background, which is, in general, most highly concentrated about the
ecliptic and sparsest in the polar regions. Three (perhaps four) of the
above regions should be detectable in the visual observer data: the
antapex, anthelion, and apex (plus cyclid) regions. The antapex region
will be the most difficult, because it is best observed in the early
evening -- when most meteor observers are still napping. The anthelion
region is probably most easily detected, and will coincide with the medium
speed ecliptical shower complex which Bob Lunsford has already mentioned as
seeming to continue on through the year -- even with no active showers in
that region. The apex region has also been recently mentioned in Bob's
posts as being detectable from meteor plots -- which points out that
plotting would probably be essential to any positive detection of these
sporadic regions.
Best regards,
Jim
James Richardson
Department of Physics
Florida State University (FSU)
Operations Manager
American Meteor Society (AMS)
http://www.amsmeteors.org
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