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RE: (meteorobs) where art thou Orionids?

Geoff,

As I mentioned yeaterday, the answer to not hearing and seeing simulataneously is essentially a mathematics (or optics) problem.

First, when you are visually observing a meteor, you are dealing with an object which is emitting light. As long as you (the observer) are positioned so there is an unobstructed straight line path between the meteor and you (i.e. the meteor is within your line of sight), you will see the meteor. This means the meteor can be seen over a very large area (relatively speaking).

Second, visual observation is a simpler (than radio observation) system because it is a two-point system -- the meteor and you.

The radio detection of meteors is more complicated because (i) the meteor is not the source, it is merely a reflector; and (ii) it involves a three-point system -- the FM broadcast transmitter, the meteor and the receiver. (This is a bistatic system as opposed to a 'normal' radar [monostatic] system where the transmitter and receiver are located at the same site.) The geometry of getting a signal from the transmiiter to the meteor and then to a specific receiving site is more restrictive.

Given the unity of radiation and therefore the premise that the many forms of radiation -- light, heat, radio, television, radar, etc. -- differ from one another in frequency but not in kind, we know that these different forms of radiation are generally understood in terms of the same theory. We can therefore assume that the FM broadcast signal will be reflected by a meteor in the same way that a beam of light is reflected by an optical mirror (i.e. the angle of incidence equals the angle of reflection).

It is easy to see therefore that there may be many meteors in the sky, but only a small number which will be correctly positioned to reflect the FM signal between two specific sites. In fact, the meteor must be positioned so it is tangent to an ellipse which has the transmitter site as one focus and the receiver site as the second focus. Actually, if you are familiar with mathematics or satellite communications (and optics), you'll know that parabolic radio antennas can be designed as paraxial reflectors (the 'normal' type of reflector) or as off-axis reflectors (the off-axis reflectors eliminate the detector (at the focus) and the structural supports from blocking part of incoming signals. Similarly, meteors are temporary, off-axis, refelctive elements of what can be considered a concave elliptical 'mirror.'

This is further complicated because you can detect meteors produced by smaller meteoroids by radio methods than can be detected visually. Particles must be larger than 10^-2 g or 0.008 cm to be detected visually; particles down to 10^-7 or 0.002 cm can be detected by radio. As a result, you will generally hear more meteors than you will see.

There is yet another factor. The light signal and the refelcted radio signal can be as much as a few seconds apart. This is due to the formation and positioning of the ionized trail. So there are times when you might conceivably be seeing and hearing the same meteor, but it appears as though they are two different meteors.

The bottom line is that statistically, the probability of a meteor being in the correct position to reflect a distance FM signal is far lower than it is for seeing a meteor. Combine this with the fact that you will hear meteors than you cannot possibly see and it becomes clear that the chance of hearing and seeing the same meteor is very low.

Phil

Geoff Wolfe <gef@ski.com.au> wrote:

>Actually steve, I was monitoring 88.3 Riverina (the one which Bruce
>gets in Brisbane) which is only 200km from my location here at Cooma.I
>too can hear this one via meteor scatter. 
>
>It was while I monitored this frequency, that I saw one of the orange
>meteors with a trail in that direction.
>
>Is it possible that the meteors that reflect radio are too high up in
>the atmosphere to start glowing, hence we don't see them? Interesting
>to see fellow observers opinion on this.
>
>Cheers Geoff <Cooma NSW Australia>
>
>ps. As I type this a good train is being heard on 93.7 - Poss. an
>Orionid!
>
>
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Geoff Wolfe <gef@ski.com.au> wrote:

>Hi all
>Have been monitoring the FM since the 14th & conditions have been
>pretty well dead. Disappointing Orionids this year!
>
>I actually did some visual meteors obs (something I rarely do) this
>morning 22/10. I even had my car radio running with a speaker hanging
>out the window (reminded me of a drive-in!). Was a fabulous morning
>with excellent clarity & cold temps (1.5 C)meant that the skies were
>very still.
>
>One thing I found obvious was that meteors detected on the car radio
>were not seen, whereas those that I did see did not reflect any FM
>signal.
>
>Activity was very poor with just 2 orange meteors seen with trails
>which lasted about 5 secs.
>Everything else was ordinary. Sorry guys, I don't understand anything
>about astro bearings etc etc to give a proper report. Pity they
>couldn't have used standard compass bearings etc!
>
>Cheers
>Geoff
>
>
>__________________________________________________________________
>Get your free Australian email account at http://www.ski.com.au/mail
>
>The archive and Web site for our list is at http://www.meteorobs.org
>If you are interested in complete links on the upcoming LEONIDS, see:
>http://www.meteorobs.org/storms.html
>To stop getting email from the 'meteorobs' list, use the Web form at:
>http://www.meteorobs.org/subscribe.html
>
The archive and Web site for our list is at http://www.meteorobs.org
If you are interested in complete links on the upcoming LEONIDS, see:
http://www.meteorobs.org/storms.html
To stop getting email from the 'meteorobs' list, use the Web form at:
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