(meteorobs) Asymmetric Geminids
Bruce McCurdy
bmccurdy at telusplanet.net
Thu Dec 15 19:54:16 EST 2005
[Part Two: Radio Science]
In his seminal work "Meteor Showers: A Descriptive Catalog", Gary Kronk
cites a paper by Williams and Hughes: "At the present time the Geminid
shower slowly builds to a maximum rate and then drops away from maximum
relatively sharply." I can attest to this not just from observing anecdotes,
but more quantitatively from another series of excellent results I obtained
on my home radio meteor observatory.
The Northern Claw (a.k.a. "Zubie") is a three-element Yagi antenna, a
digital radio tuned to FM 92.1, and Jim Sky's program Radio SkyPipe. I run
the thing 24/7, and have been fortunate to have very consistent reception
throughout the past week. I haven't yet had time to do a visual count of the
files, but the pro version of SkyPipe has a very handy feature called an
"event counter". This quick-n-dirty method counts the number of times the
volume exceeds a certain level in each data file, which I have set up to be
saved in three-hour blocks.
I set the threshold a little bit high to avoid multiple hits due to
interference patterns on the same overdense meteor, so the below numbers are
actually low to the true number of hits, however they are reasonably
self-consistent and seem to jive with the visual appearance of the charts. I
post below the results from 04-13 h UT each night, which corresponds to 9
p.m. to 6 a.m. MST. The radiant crosses the meridian near the centre of this
window.
Date/UT 04-07 07-10 10-13
Dec 10 null 51 85
Dec 11 67 66 75
Dec 12 74 94 57
Dec 13 288 158 187
Dec 14 254 222 197
Dec 15 71 41 35
It's like the Geminids slowly climbed to the peak and then fell off a
cliff. And this isn't the first time. In 2003 I had quite similar results;
the numbers are higher due to my using the more painstaking visual count,
but again they are self-consistent under good reception conditions:
Date/UT 04-07 07-10 10-13
Dec 11 94 82 71
Dec 12 92 118 109
Dec 13 177 231 116*
Dec 14 300 313 283
Dec 15 90* 66 60
(* only two hours of data in each case)
So in both cases the number of hits were reduced by a factor of about
five from the 14th to the 15th, and were lower on the 15th than they had
been on the 11th.
My understanding of the physics of this situation is, shall we say,
"incomplete", but I understand the debris trail of 3200 Phaethon is
asymmetric for physical reasons and there are fewer, but larger, particles
after the peak. (Would this be analogous to the fall ellipse of a fragmented
meteorite? A few big chunks at one end, a bunch of smaller ones nearer the
other.)
I will happily defer any further analysis to the individual and
collective expertise of other subscribers to this forum. My methodology is a
little crude, but the actual charts don't lie. I will try to get these up on
the SkyScan website soon; however, anybody who is interested can contact me
off-list for JPG pictures.
Thus my conclusion: "if you're going to observe Geminids on an off-peak
night, try to make sure it's *before* the peak."
Bruce
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