(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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