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Re: (meteorobs) Fireballs & sonic booms
At 02:58 PM 1999-07-07 -0400, Lew wrote:
>By the same token, the radiometeorists on our list will correct me if I
>stray here, but there's also no (well-described?) correlation between
>a meteor's radio reflectivity or duration and its visual brightness.
I would think there'd be little correlation if only because it is known, or
at least thought, that the primary lightwave emissions from a meteor are
due to whatever elements are burning. The radiowave reflections are caused
by different phenomena; i.e., ionization of the atmosphere gasses as the
meteor passes. If I remember correctly, though, a reflective tail may be
caused by the residue of the burning meteor's passage and shedding of
material? Yes, I remember well that most explanations of such reflective
tails claim they are still correlated ions, all aligned such that they
still strongly reflect for seconds or even several minutes afterward.
Further, since it is thought that the vast majority of meteors are bits and
pieces of comets, which are also thought to be primarily ice with very
little, if any, "metallic" content, it would seem that meteors from comets
would, indeed, leave very little "residue" which could cause radiowave
reflection. So, I suppose that the usual explanations of gas ionization
would certainly seem more plausible.
But consider this: the brighter the meteor, the higher the energy release.
I would think that the higher the energy release, the higher the likelihood
that energy would extend over more than just the lightwaves; i.e., down
into the VLFs, perhaps. And, possibly, even into the microwave frequency
bands.
Radio amateurs have, through the years, thought that they have "heard" the
passage of particularly-reflective meteors (I've personally sometimes heard
"band noise" increase during a particularly strong meteor ping or burst);
but it's not clear whether any such noise increase in the VHF bands could
not have been simple reflection of noise sources from the earth, atmosphere
noise from storms, sun noise, or whatever. In other words, any such noise
increase could as easily be a reflection of a noise source as an emission
from the meteor itself.
Not being a physicist (isn't it nice to be able to qualify one's guesses
like that?!), I'm uncertain whether, if a meteor actually did emit
radio-frequency "noise", it would be broadband over the whole radio
spectrum, or whether it might be possible that the meteor's burning "head"
or even a reflective trail, could not be "resonant" so to speak, and thus
cause a more-efficient radiation of noise within a narrow band of radio
frequencies. That is, were one to be able to sample a broad band of radio
frequencies during a meteor burn, one might find a noise peak within some
narrow frequency range. I remember that several meteor texts claim that
meteors do not emit radio noise; but I have my doubts just how accurate
those statements really are, based upon what I've personally heard through
the years.
A tangent here for radio meteor observers: if you observe using an FM
radio, you will NOT hear any such noise increase because of the design of
the radio. The radio's design uses what are called limiters, which cause
the noise output of the radio to be constant no matter how strong the
antenna noise may be. This is how an FM radio provides such "crystal-clear"
reception of one station even when there are several others on the same
frequency: the strongest station "captures" the radio's limiter and
gain-control circuits.
What I would really love to be able to do is to have my own V/UHF amateur
station set up such that I could monitor meteors over several bands
available to we radio amateurs simultaneously, such as 50, 144, 222 and 432
MHz; both for simple radio-noise emission, and for reflections of signals.
Unfortunately, there don't seem to be very many other radio amateurs in
North America who share the same enthusiasm for such research :o(((
>So is it unreasonable to suggest there might also be little correlation
>between the production of ELF or VLF radio waves and visual magnitude?
Yes (not knowing any better), I suspect that in the vast majority of cases,
that would be true. But again, I think that the brighter the visual
magnitude of the meteor, the higher the energy release. But then again, if
you've ever watched a gas welder at work, you remember that when his torch
illuminated some small speck of certain metals, there could be a brilliant
flash of light. In other words, some combinations of elements that burn
within meteors may be able to generate considerable energy at certain light
wavelengths with little or no consequent energy release anywhere else in
the electromagnetic spectrum.
I used to work with a guy who was/is a high-energy physicist, and asked him
once to try to help me to better understand the mechanism of radio wave
reflections from meteors. My impression was that the subject was so mundane
and run-of-the-mill to him that he almost didn't want to get into it.
Perhaps it is; if so, it'd sure be nice for one of those guys to enlighten
we ignorant mere mortals!
Clear skies!
SteveH
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