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This page was last modified on November 4, 1999 by Casper ter Kuile |
Leonids '99: discussions
Joe Rao, Rob McNaught, Marco Langbroek, ...
Joe Rao Those who have read my Leonid paper in WGN 27:3/4 will have noted how through orbital simulations I had integrated material from the great Leonid storm in 1833 forward to 1866. What I discovered was that the orbital separation between the Earth's orbit and this 1833 material had widened to almost 0.007 a.u. by 1866. Also, I integrated material from the 1966 forward to 1999 and found a separation between Earth's orbit and this material of nearly 0.003 a.u.
My initial supposition was that such a difference between 1866 and 1999 would result in 1999 seeing much higher hourly rates compared to the 6000 to 10,000 rates of 1866 (as noted in Icarus 138, 287-308 by Peter Brown).
Last week however, I had a chance to e-mail David Asher and inquire why he left out the meteor trail map for 1866 from the Armagh Observatory internet site. He kindly -- and very quickly -- responded by adding the map to the site, which indicated that a meteor trail shed by 55P/Tempel-Tuttle in 1733 was primarily responsible for the 1866 display. Over this past weekend, I performed an integration for 1733 myself and found that this material passed ~0.001 to 0.002 a.u. from Earth's orbit in 1866, pretty much confirming what David had indicated.
As a result of this, I am toning-down my original "guesstimate" of Leonid activity to 2,000 to 6,000 per hour. In WGN 27:2, David and Rob McNaught suggest a peak of 1,500 per hour -- a rate that is quite close to the lower-end of my revised range.
I should, however, point out something that is not-often cited in predicting Leonid rates and that is the propensity of meteor activity during a storm apparently coming in "waves" or "surges." The oft-quoted "40 meteors per second" from Dennis Milon's team at Kitt Peak Observatory in 1966 was arrived at after a consensus, but was probably the ABSOLUTE UPPERMOST LIMIT of observed activity. Indeed, there were others who claimed to have seen similar, if not even higher rates (James Young at Table Mountain Observatory suggested 50 per second!) but again, this may have only been an extreme upper limit. As I comment in a footnote in my WGN paper, many other witnesses of the 1966 storm noted lower rates of "only" 10 to 30 per second. A very telling description came from Dana K. Bailey of Boulder, Colorado, who commented in the January 1967 Sky & Telescope that " . . . no fewer than 10 new meteors were appearing each second, for many minutes, yet sometimes the rate was double or triple that . . . "
Another suggestion of meteor activity apparently coming in surges, comes from a description of the 1866 Leonids as reported in The London Times:
"The spectator had soon counted half a dozen; then he felt sure he had seen thirty; then six or seven in a minute . . . Then there came two or three together; then not less than a dozen of all kinds."
In essence . . . if indeed, a maximum of 1,500 per hour occurs this year, that's 25 per minute or one meteor every two or three seconds. It would not be surprising however, if -- based on the above description -- there come brief intervals when the rates may actually reach several times this rate. You've heard of major storms that, for example, produced " . . . sustained winds of 50 miles per hour, but with occasional gusts to 80." Well . . . in 1999, it could very well be that we'll hear of overall sustained single-observer rates of say, 2,000 per hour with occasional bursts ("gusts") to 7,000!
Joe Rao, Meteorobs, 1999 - november 1
Rob McNaught I agree that there is possible evidence of "gusts" (at the moment to over 50 knots outside, so I closed the dome!) in historical data, but it is common for people to assume that a random distribution should be uniform, and that non uniformity is non randomness. For this reason, unless the data is tested and shown to be non random, I would tend to think this says more of human psychology than the distribution of dust. Could there be micro-structure in the dust trail caused by resonances? I doubt it. Could jets from 55P/Tempel-Tuttle cause detectable localised "veins" within the dust trail? Possibly. The velocity distribution at ejection will extend the jet throughout the dust trail along it's length, the width being determined by how collimated the jet is.
Regarding the ZHR of 1,500 for 1999 in the WGN article by David and I, this should probably have been rounded to 1,000 (using ZHR0=1,200). I rounded it to 1,500 because the center of the dust trail is expected to be beyond the nominal center of the calculated dust trail and this would give higher rates. The value of 1,200 was close to the boundary between rounding up or down.
A more recent fit to the historical data, using slightly revised values for da0 and re-rd (refer to WGN article) makes a slightly messier fit (20% overall error to Peter Brown's historical ZHRs), but this is mostly now due to a discrepancy between 1833 (predicted high) and 1966 (predicted low). The new ZHR prediction for 1999 was somewhat lower than we got before. Assuming the 1966 value by Peter Brown is reasonable, I would increase the new 1999 prediction and again come out at around a ZHR of 1,000. For some future years, the situation has changed slightly, but it makes no sense to pay much attention to those when we will have another, and presumably the best so far, data point in just over two weeks. So close! 33 years has gone by already!
Because of the uncertainty in historical rates, I think the dust trail density model is restricted largely by the poor data. Also with improved data over the next few years and a better theoretical profile in da0 and re-rd than the assumed Gaussians, I would hope that the dust trail density model could be confirmed.
What is needed is a good ZHR profile of the 1999 activity. That is in the hands of all you visual observers!
Rob McNaught, Meteorobs, 1999 - november 1
PS the origin of the 1866 activity is stated in our WGN article and by Kondrat'eva et al much earlier.
The Leonid Meteor Shower
by Rob McNaught (ASTRONOMICAL SOCIETY OF AUSTRALIA)
Marco Langbroek I have a suggestion regarding the 'gusts' of Leonid meteors mentioned in the earlier communications. But first; good to see that the idea of flurries of meteors during the historic meteor storm of 1966 being responsible for the highest ZHR figures has been picked up. This is essentially what I have been arguing for some time now as a partial explanation of the discrepancies, e.g. in that notorious WGN exchange of a few years ago.
Regarding these 'gusts' of Leonids; in answer to Rob questioning their authenticity and thinking of an origin in the observer's minds, I cannot help but think of the video 'gust' that the Japanese recorded from Hawaii in 1997 -those 150 Leonids that suddenly poured down in a nice little fan in just one second or so. The most likely explanation for that remarkable video footage, which cannot be denied unlike visual reports of such pehnomenon, still is the breakup of a meteoroid in the pre-luminous part of its trajectory. If such occasions are typical for 'fresh', perhaps extremely fragile dustgrains (or 'dustbals' cemented by volatiles -volatile sodium according to Jiri Borovicka's recent research results, see ACM conference) in the stream, they might be a general feature of storm peak activity.
Marco Langbroek (Dutch Meteor Society), Meteorobs, 1999 - november 1
Rob McNaught My feeling is that the Japanese video is not in any way representative of storm activity. Surely the arguments about the rates in 1966 are about only a factor of 2 or 3, although some might argue there was a factor of ten or more between the extreme high and extreme low estimates. (Remember that the Milon "consensus" representes a EZHR of around 110,000 or so, as I've previously reported, not 144,000). Given the heterogenous nature of the reports, this doesn't seem like much of a disagreement to me. It is a well established fact that people report non randomness in a truely random distribution. Of course, this does not *prove* the inverse, that reports of non randomness are explained by psychology and I'm not trying to say that. Only that such reports, without the backup of a statistical study, need have no basis. That meteoroids break up in the near-Earth environment and beyond, isn't an issue for me, only the relative contribution they make and whether they could be clearly differentiated by the observer. Had the Japanese video event happened in a small region of the sky during the 1966 Leonid maximum, I feel sure it would have been noted as extraordinary, especially as there were probably many more than 150 meteors in the small region down to the visual limit. I doubt such an event would have been repoorted as a "flurry". Anyway, detailed analyses of the new data over the next few years should answer this, I hope.
James Young's comment of 50 meteors visible in one second seems to be from recollection many years later. If you look at what he wrote immediately after the event, the numbers are much lower. I have previously quoted this value of 50 meteors in one second (which does not equate to a *rate* of 180,000 meteors/hour as I've mentioned before) myself. I think both high and low values by Young appear in Littman's excellent book.
Joe: You commented that you learned of the age of the 1866 storm "last week" after contacting David, hence my comment on the value being already published by the Russians and David and I. It is important that Leonid researchers be aware of the priority in Leonid and Draconid storm predictions by the Russians (Kondrat'eva, Reznikov and Murav'eva). Their work has been overlooked for so long.
Rob McNaught, Meteorobs, 1999 - november 2
The Leonid Meteor Shower
by Rob McNaught (ASTRONOMICAL SOCIETY OF AUSTRALIA)
Marco Langbroek I agree with Rob that reports of non-randomness in meteor appearance should be regarded with caution. Small APPARENT groupings of meteors are just part of a poisson distribution and have no meaning in a physical sense (not necessary at least). They are also a very common phenomenon most active observers know from any meteor stream, especially those with the better rates. Then indeed, Rob is right, that the human mind can produce such things -a momentary lapse of attention, followed by a clear mind again, for example. It is unnattural to keep attention focussed for a long time, sooner or later, you have short periods of a wandering mind. That's because we are concious beings.
Yet, I do think that the 1997 Japanese video footage should merit some consideration, for they do rise some thoughts and it is better not to explain them away to quickly as something unique and thus not relevant. The footage itself is by all means unique indeed, yes, thus we have no way of estimating whether the intensity of this particular 'flurrie' is typical or atypical if we entertain for a moment the notion that such occurrences (breakup of dustballs/meteoroids in pre-luminous trajectory resulting in a flurry of simultanious meteors) could be typical of fresh dust such as in an outburst structure. With this, I want to say, that similar phenomenon might occur on a smaller scale, and need not be that intensive as the unique 1997 video phenomenon. If true, I am quite sure this will give the analysts of visual data a headache, for you can wonder whether an activity curve and flux profile of an activity occurence containing such events has the same 'meaning' as a curve/flux-profile from a 'normal' shower. Please note; I am not saying that it necessarily has to be this way. Only, that the 1997 video is in itself ground to entertain such a possibility, if alone to keep the mind sharp. We still know so little of these unique outburst phenomenon, that we are bound to be in for surprises (and headaches, and pittfalls). I still remember how we were set on the wrong foot initially by the unique (again!) brightness distribution of the alfa Monocerotids during the 1995 outburst (see our paper: P. Jenniskens, H. Betlem, M. de Lignie and M. Langbroek, Astrophysical Journal 479 (1997), 441-447)
Marco Langbroek (Dutch Meteor Society), Meteorobs, 1999 - november 2
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'Data does not speak for itself. I have been in rooms with data and
listened very carefully. The data never said a word.'Milford Wollpof (1975)
Rob McNaught Those are fair points. While small scale breakups could be part of the overall phenomenon, and potentially bias the rates, the Japanese object was almost certainly *not* of a "fresh" meteoroid (no young dust trails near enough the Earth in 1997). Certainly, small scale events like that could contaminate the results, but if it were more common in fresh meteoroids present in storms from young dust trails I still feel is would have been specifically commented as something other than a flurry. There are probably (at least) two ways the breakups could be noticed:
- a) breakup in the near-Earth environment. In this case, all the fragments would necessarily be very closely spaced in the sky and in time. For Leonids this would imply near simultaneity and within a few degrees, unless unrealistic high velocities of separation are assumed
- b) systematic breakup during their lifetime in orbit by whatever mechanism. In this case, only those that have a breakup shortly before (months?) entry into the Earth's atmosphere are likely to be recognisable (consciously and statistically) as non random.
Presumably this has all been studied before and the consequences explored. I could imaging aging effects to operate either way in increasing or decreasing the likelyhood of breakup depending on what mechanism is involved. It is an important issue that I would wish to know thw answer to. In assuming it is insignificant, this could bias the rates predictions based on the dust trail density model in two ways: the data used in the model may require correction for breakup, and the predictions likewise. It affects the dust trail density model through both the increased flux AND the changed mass index.
One point I've made informally for many years, and might be well known, is that in looking for non uniformity in occurance due to breakup, the important quantity to test, is not just the time gap between meteors, but their physical closeness in 3-D space. The observations are converted to a 3-D column through the stream and examined for clumping.
Robert H. McNaught, Meteorobs, 1999 - november 3
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This page was last modified on November 4, 1999 by Casper ter Kuile |