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An interesting development that only spectroscopy can reveal an insight into…
(Or possibly throw a red herring!)
I received a tweet from another observer (JS) enquiring as to the possibility of this particular fireball being a Southern Taurid.
Initially I said no because the observations by the NEMETODE members had indicated a sporadic but the elements had some similarities. I remembered that last year around the same time I had caught another fireball that I was confident was a STA but didn’t have any orbital data.
I dug out the image and plotted the spectrum, then overlaid the spectrum of this years “sporadic”. Apart from some minor lines that may or may not be real, the bulk spectrum signatures are essentially identical! I found this to be both an impressive correlation and an intruiging result!
So what is the truth?, difficult to say but in the first instance the evidence point to the two meteoroids being made of the same stuff. JS had caught a still image with the fireball of this year and it certainly tracks back to the general region of the STA radiant despite not matching exactly (by UFO). Perhaps this one is a little bit of Comet Encke that’s been perturbed gravitationally (by Jupiter) or has been ejected with such a velocity that it has evolved into a slightly different orbit.
No matter, through spectroscopy, the evidence of the compositional similarity is very good.
Cheers,
Bill.
…and some more number crunching that I’m not very sure about.
If the absolute magnitude and velocity as derived by UFO capture are roughly correct then this yields a mass of around 20g for the meteorid. This is with some of those assumptions I mentioned but it’s the correct “order of magnitude” as they say… 😉
Hi,
With the power of modern software it is possible to calculate all sorts of things. Thanks to the other observers of the NEMETODE group the orbit of this particular body has been determined. Using a couple of the orbital elements the meteoroids velocity can be calulated not only at the impact point with Earth but also at any point. However it’s interesting to compare the velocity at perihelion, ~64.1km/s and at aphelion, ~ 5.5km/s to the Earths own ~30km/s. From cruising out near Jupiter to whizzing along as it rounded the sun before it’s fateful encounter….
Bill.
Hi Robin,
Thanks for your thoughts.
What you’ve described is pretty much what it is I think. The issue is with some of the remenant lines in the flare (the cooling section if I understand you) that are not atmospheric.
This isn’t a particulalry good graphic but if you look closely at the strong line at 777.4 nm, immediately to the left of it is another line. This can be seen if you examine the composite closely. (Purple is the flare section, blue is the weaker “gap” sepctrum and red is the fragment.)
This MAY be a triply ionised silicon line. In the only other case that I’ve recorded this type of behaviour, a Perseid fireball in 2013, this and other features were attributed by Jiri Borovicka as being deep blue lines overlapping from the next order. Jiri kindly provided a fully annotated graphic that I included in the Perseid short paper mentioned. Due to the quality of the Persied spectrum I couldn’t really argue (and of course it may very well have been completely different) but in this case I don’t think there is any doubt. Looking at the blue lines they suffer rapid de-focus and the same effect would be seen from the next order up. In this case the line is quite sharp so I believe it is a geniue emission at that wavelength. Also the other peaks in the purple line may be from silicon. Absolute identification is impossible, though due to the low resolution and that is the big problem!
There must be a short interval of time, of the order of a few milliseconds when the crushing pressure on the meteorid is a max and so probably is the temperature. This may momentarliy cause what might be called a “flash boiling” of the surface. The calculations are fraught with so many assupmtions that it’s impossible to say for sure what any given case is though. Clearly there is a fragment that exists for a few more milliseconds so it mast have had some material integrity. So what I think is being seen is emission from the cloud of meteoroid vapour generated at the max P and Max T if the meteoroid is tough enough to survive the whack it gets at this point. However if these lines are indeed silicon from the breakdown of SiO2 (silica/quartz) one of the peaks in the flare spectrum should (perhaps could ?) be O at 615.7nm and be of meteoroidal origin and not atmospheric.
I can’t lay my hands on it as I type but Jiri Borovika also wrote a paper in 1994 about a “twin temperature” model that could be used to explain particular emissions. I think these were in the region of 3000K and 10000K if memory serves. I’m afraid I just don’t know enough about the chemistry/physics/quantum mechanics to fully understand how it actually works. The best I can do is to read it as some “part” of the meteor is at one temp, maybe this is the hotter flash cloud. The another “part”, the surviving fragments, are at the lower temp due to whatever particular bulk thermal properties the meteoroid has.
We need more examples and better resolution!
Cheers,
Bill.
Hi,
Here’s a spectrum from the image taken just before the brightest flare.
It’s almost the same as the “fragment” spectrum. Perhaps un-surprising, what goes in ends up coming out!. But it begs the question what happens during the flare to stimulate the other emissions. More digging required….
Anyway, it looks very nice…
cheers,
Bill.
Hi,
Andy, There was a lot of work done by the pros during the Leonid storms. There were several examples of this kind of behaviour caught but on CCD, not on video as far as I know. The experts like Borovicka and Jenniskens have written many scientific papers about this sort of thing. There are a lot of theoretical issues that need to be considered and I don’t fully understand all of it. I just don’t have the experience of “modelling” they do!
Steve, No it’s not a star analyser but still a grating just with more grooves in it. The one used in this case has 600 grooves/mm and is 50mm square (I think the star analyser has 100 or 200 grooves/mm). If you’ve got one and you’re interested in trying meteor spectroscopy then a star analyser can be used but it can only produce low resolution spectra.
cheers,
Bill.
Hi Andy,
I hope you have a go sometime. The arrangement for meteor spectroscopy couldn’t really be any simpler. I use an empty filter holder of the appropriate size (so the square transmission gratings fits into the circular aperture) secure it with the mounting ring (sometimes a piece of black card and some tape are needed to make up a holder to ensure a secure fit) then screw the the thing onto the lens.
There are some schools of thought about tilting the grating, since we’re using transmission gratings, to optimise performance in a given order. I don’t think its really worth the extra mechanical akwardness. Mind you I’ve yet to see any spectra from a tilted grating system to do a comparison with, so maybe it is…. Achieving good focus across as wide a range of wavelengths as possible is difficult (this is where tilting the grating might make a difference as well) It’s definitely a trade off between utility and absolute performance. Optical aberations, quality of lens design and the way it’s been corrected come into play as well. Is it worth tilting the grating and all that entails to do it only to have the quality dimished by lens limitations? Defocus in the near UV/Blue is rapid and dramatic in most CCTV lenses as they’re not really designed for astronomical spectroscopy! If they were they’d be made of fused silica/quartz and cost a LOT! (however such lenses are available if you REALLY want to get into it….)
Yes, I’ve been looking at ISIS as well. Some of the results Christian gets are simply remarkable. I had a look at his page recently and theres stuff about detecting startspots. Who’d have thought that amateurs would ever have the tools to do that!
You’re right about the learning curve but I think that’s a consequence of whats he’s trying to do, process a lot of stellar spectra in as automated and efficient a manner as possible. Like most things once you figure it out, it all seems quite straighforward 😉
Personally I think it’s rather wonderful he makes his software available for free!
Anyway, I’m preparing to set off to the International Meteor Conference in Austria and looking forward to it immensely….
cheers,
Bill.
Hi Andy,
No worries, an assumption on my part that the meaning was understood. This is the start if the process, Then the supressing lines etc etc etc.
It is interesting you say you crop the dodgy stuff, that’s part of the problem with meteors. If you want to get a real feel for what’s going on you can’t! I suppose thats the premise of much of the discussion here.
Maybe I missed it but what software do you use? One of my fellow meteor spectroscopists from Switzerland has been doing some really interetsing stuff with the ISIS package, also by Christian Buil, and orthographic corrections to properly linearise (is that a word?) meteor spectra. I am hoping, if nothing else, that a few people will set up a camera with a grating and stick it out for a year.
If half a dozen people do that then at one fell swoop we’ll have more spectra that at any time in the past. I got 105 meteor spectra in 714 hours of observing and I live in probably the cloudiest and wettest part of the UK. That was two cameras and occasionally a third during showers.
The EDMOND database which I believe both Nemetode and UKMON contribute to, now has 3 MILLION light curves (not orbits as I intially thought). The number of spectra from all the amatuers (Basically me so far in the uk on an ongoing basis as far as I can tell!) and several professional projects across Europe might have produced a few hundred or a thousand spectra. To do any decent statistical work, of course, more would be nice!
I hope you’ll maybe have a go. Any prior experience with spectroscopy makes it even easier and there are programs like RSpec which are quite user friendly. Some of the basic geometric transformations needed are available as little slider bars.
Its easy(ish) 😉 all one needs is a little patience.
Cheers,
Bill.
Hi,
Sorry Andy, I forgot to add the response to your question, which is I’m not sure. This is at the hub of the “temperature problem”.
The curve should be the emission spectrum superimposed on a black body curve dependant on the physical temperature in the atmosphere. However in physics speak it is a “far from equlibrium process” This I believe is what leads to the catch 22 with the analysis based of line ratios. The get the correct line intensities you need to know the temperaure but to get the temperature you need to use the corrected line ratios. Somewhere an assumption has to be used to break the log jam. That’s how I read it anyhow. I can’t put my hand on the copy I have right now but there is a good book from the 60’s that give a good outline of whats going on in reasonably understandable terms. I’ll put the details on here shortly in case you want to get into it a bit more.
cheers,
Bill.
Hi Gents,
Thanks for the comments. It’s great to hear alternative perspectives and different experiences!
Robin, do you mean this….
Perhaps not worded too cleverly on my part. The “oscillations” being the converging H lines. However using this method on meteors is not quite the same as using it on stars. For example when you divide a meteor spectrum through by a stellar spectrum there is still the issue of where exactly to select points to derive a continuum to act as the response.
I tended to end up with curves that were rather wavy. Below is an a quick plot I’ve made using an actual repsonse from an early test. Maybe its the methodology (as you describe in the examples on your web site) but for meteor spectrum correction I just don’t think it’s quite right.
If one uses ones imagination it approximates the nominal silicon photodiode curve as it surely must. (Unless there is evidence that a silicon based detector was not used in the visible of course). Thats what put it into my mind to invesitgate further.
Here is a nominal silicon photodiode response curve I produced from date taken from a large photodiode manufacturers website.
Dividing thorugh by this will give a form to the meteor spectrum then the other calibrations and corrections can be done as you discuss. However the problem with this approach is that although the form of the curve remains the same the peak moves slightly depending on how each photodiode site is biased by the electronics. THAT is a dark secret known only to the manufacturer!
From my experience at work I’d say the peak was closer to 820-850nm in real photodoides but I think that can be corrected itself by the appropriate normalisation point before division.
But then again maybe we’re right back to where we started…. It’s great fun!
There are a couple of papers by the various experts such as Borovicka and Jenniskens deriving absolute fluxes which is what we’d like. They seem to rely on various measurements made on the ratio of particular groups of lines (such iron lines in the blue) to derive an effective temperature. I’m not sure I understand the maths behind it but in some cases, but as promoted by Borovicka, there are different temperature regimes that complicate the matter further. These temperatures are generated by the particular properties of the meteoroid and its encounter with Earth, size, composition, velocity angle of entry etc. So getting what would be in effect an inverse black body curve for the appropriate physical temperature to do further calibration with is very difficult because we don’t know this in advance!
It is rather ironic that the very atmosphere that generates the meteor itself scatters efficiently in the blue. Right where the interesting lines are that we need to measure in the first place. Who’s say Pacha Mama doesn’t have a sense of humour….
I’m beginning to think “tricky” just doesn’t cover it ;-).
cheers,
Bill.
Hi Andy,
Thanks for the comments. Using a A0 star was exactly what I did to start with. By removing the hydrogen lines this left a nice continuum. But what I found was that at the blue end it produced some odd results because of the shape of the continuum in this regime. In the basic library with Visual Spec the spectrum of Vega, for example has a sharp cut off then some oscillating components. Smoothing this out leads to dips in the continuum which I believe is not reflective of whats going on with a meteor. I could be plain wrong about this of course but I’ve made the assumption that even though a meteor may not have a genuine continuum the spectrum would be superimposed on a hypothetical blackbody continuum of the meteor temperture.
So it might very well be a two step process in the end. Divide through by the si response then do another division with the appropriate black body curve. The plan is to sort out this years Perseid results then go back and try some comparisons with normalised spectra from the ones I’ve collected using both the stellar method and si curve to see how they look.
However this issue isn’t of importance for basic line ID and wavelength calibrations but it’s a necessary step to get a good handle on it for proper flux calibration and measurement. The scientific papers I seen on this seem to me to be VERY complex and need complicated modelling of various parameters. So it’s possible to do but it might take someone with more computing and maths skills than me to crack!
Mmmm, tricky, very tricky but a fascinating challenge! 😉
cheers,
Bill.
Hi,
Much to my surprise the night of the Perseid peak was clear all night. From22.00ut to 03.20ut I recorded 377 perseids and 51 others/sporadics. Out of these I captured 28 spectra, 8 of which were reasonably good.
One issue that has been problematic is correct “instrument correction” of the spectra. A flux corrected stellar spectrum can be used but what class of star to use? The temperatures that exist with the meteor head are high, several thousand kelvin but using a corresponding temperture of star/black body equivalent means that the shorter wavelength won’t be right. Similalry the reverse is true, correct for short wavelength then the long wavelengths go awry. This is especially a problem as silicon based detectors are hugely more sensitive in the red/near IR than the blue.
I thought it might be interesting to do a instrument correction by dividing through by a nominal silicon photodiode response.
I got a curve from a large photodiode manufacturer and normalised the curve at ~700nm, about halfway between the lowest responsivity and the peak at 820nm. The actual curve varies slightly depending on how the juntion is biased but this mostly affects the longer wavelengths which are being lowered by the division so the errors are small over all (I think 😉 )
Anyway here are a couple of graphs from a spectrum of a Perseid from this years peak. The difference is huge!, they don’t even look like the same spectrum the red has been lowered by that much. This actually shows how INsensitive silicon devices are to the far blue. (film was much better, who’d have thought….)
Uncorrected “raw” spectrum.
and the “corrected” spectrum
The red end may be a little over done as the original shows a rise in the graph caused by smoke wafting across the fov raising the backgroud level. However the blue end was reasonably dark and is fairly representative. This was the only spectrum of the night that had some blue lines to test the idea on. Its not a perferct example but demonstrates the point.
Having said all that acutal flux calibration is still subject to much uncertainty and is quite difficult to do in an easy manner for us amateurs.
Food for thought….
cheers,
Bill.
Hi,
Whilst I was a diligent physics student many moons ago I’m afraid my magneto-hydrodynamics at an event horizon is a bit ropy….
However, it is always interesting to see an analogue that demonstrates the physics more clearly than any equation!
Take a look at this story on the BBC News website: http://www.bbc.co.uk/news/world-us-canada-33282343
A great big vortex in a lake being drained to control flooding. What is REALLY interesting is the spontaneous generation of instabilities that propogate UPWARDS out of the vortex throat and against the flow. #4 right at the end is the best! Stretch the plughole analogy to curved space (that is imaging the ring of foamy water being a sphere) and what we might have at V404 is a smilar event. The “flooding” is accrection of new material and the flaring is instabilities of material flashing around the event horizon. I wonder how long before this will appear in a paper somewhere. What do you know!, it happens right here on Earth, just not so violently…… 😉
cheers,
Bill.
….And another….
A really well dispersed spectrum of a sporadic meteor captured on the night of the Lyrid shower peak.
The instrument corrected graph.
And of course the celestial art which just looks better….
cheers,
Bill.
Hi,
Another interesting spectrum to consider. I caught an usual spectrum on the morning of the 13th. Unusually it only had one bright line in the green. I initially assumed this would be magnesium. WRONG! The alarm was raised when the atmospheric lines (to the right hand side of the spectra) didn’t fit. After trying a few lines from the NIST reference library the best candidate was from iron. Re calibrating using the atmospheric oxygen line, the green line did seem to fit with an iron emission. So did the blue iron and calcium lines. So I’m confident this is a reasonable fit.
I’ve compared it to the spectro-orbital capture from the 4th to show the differences. The one from the 4th looks like a stony/stony iron with the rich Fe, Mg and Na lines. What this one is I don’t know.
Yet another one for the zoo….
cheers,
Bill.
Hi,
After a bit of messing around I managed to produce this sky track for the meteor in UFO Analyser. William will be doing the orbital analysis of myself and Dave’s data (as well as hopefully Dennis’s)
It seems that this might well be a Lyrid. However once the orbit has been determined we should have a better idea of the certainty of that.
(To his credit he came up with the highly imaginative name/acronym for my spectroscopy programme. Much better than what I had originally thought :-)))
Looking good.
Cheers,
Bill.
Hi,
A first (as far as I know) for UK meteor observing early this morning. Working with some of the Nemetode group I finally caught a meteor that was also captured with the Nemetode observers’ camera (David, just outside Girvan). This is a great step forward as it means we can now have orbital AND compositional information for meteors.
Here is my capture frame:
And here is Davids image:
Doing the usual spectrum processing reveals very strong Magnesium and Sodium emmisions but there are also bands of iron in the spectrum. So it looks like this was a stony iron meteor, thus:
and the usual colour treatment:
There is the possibilty that it was caught from another location which would give a much better orbital fit. Hopefully Dennis will have re positioned his camera.
It’s all good stuff :-))
cheers.
Bill.
Hi,
I think this is quite an interesting little spectrum. 0508UT 15/3/15. Not very bright and very few lines visble. However it has exceptionally bright OI emission from the “forbidden” transision at 557.7nm. Looking at any other of my spectra here there is no comparable emission in any of them!
Due to the quantum mechanics involved this is a meta stable state, the O atoms sit excited for 0.74 seconds before emitting the photon. So it only occurs with meteors that are very swift and start to or completely abalate above ~100-110km. Below this the atmosphere quenches the transition through collisional loss.
The usual strong Mg and Na so looks like a stony job but it must have been coming straight at us for a very fast effective impact speed. A fascinating addition to the diagnostic info available.
cheers,
Bill.
