[Prev][Next][Index][Thread]
(meteorobs) Fwd CCNet SPECIAL: THE 1999 LEONID METEOR SHOWER
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
Sender: HUMBPEIS@livjm.acdot uk
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 10 November 1999
Date: Wed, 10 Nov 1999 09:41:55 -0500 (EST)
CCNet SPECIAL: THE 1999 LEONID METEOR SHOWER
10 November 1999
---------------------------------------------
(1) STRONG ACTIVITY OF LEONID METEORS EXPECTED
Marc Gyssens <gyssens@charlie.luc.acdot be>
(2) LEONID DUST TRAILS AND METEOR STORMS: UPDATE
David Asher <dja@star.arm.acdot uk>
(3) A REVIEW OF THEORIES FOR LEONID STORM PREDICTION
Rob McNaught <rmn@aaocbn.aaodot gov.au>
(4) THE AEROSPACE CORPORATION TO PROVIDE REPORTS DURING
LEONID STORM
Ron Baalke <baalke@ssd.jpl.nasadot gov>
(5) TELESCOPIC VIDEO OF LEONIDS & LINEARIDS SOON
Joan and David Dunham <dunham@erols.com>
=============
(1) STRONG ACTIVITY OF LEONID METEORS EXPECTED
>From Marc Gyssens <gyssens@charlie.luc.acdot be>
I N T E R N A T I O N A L M E T E O R O R G A N I Z A T I O N
Night of November 17-18: strong activity of Leonid meteors expected
>From most of Europe, the Mediterranean area, and northern Africa,
people may see a lot of meteors - "shooting stars" - between midnight
and dawn of the night of November 17 to 18, provided skies are clear.
These meteors belong to the so-called Leonid shower. The peak of this
shower is expected around 2 a.m. Greenwich Mean Time, which is 3
a.m. local time for most of continental western and central Europe and
mid-northern and west-central Africa, and 4 a.m. local time for eastern
Europe, Turkey, Israel, Jordan, and northeast Africa. At that time, an
observer will see at least 50 to 100 meteors per hour, but there is a
fair chance that a veritable meteor storm will materialize with 1000 or
more meteors per hour around the abovementioned time. The International
Meteor Organization, who collects meteor observations world-wide for
the purpose of analysis, wishes to point the attention of the public to
this spectacular natural phenomenon.
The Leonids are caused by a stream of predominantly very small
particles, less than 1 mm in size, which orbit the Sun with a period
of 33 years, together with their parent comet, Tempel-Tuttle. The
orbit of the Leonid particles happens to intersect the Earth's orbit.
Each year around November 17, when the Earth is at this intersection,
Leonid particles may enter the Earth's atmosphere and cause meteors,
popularly called "shooting stars." This year, around 2 a.m. Greenwich
Mean Time, in the morning hours of the night of November 17 to 18, the
Earth will pass through the outer regions of a dense dust trail of
Leonid particles ejected by Comet Tempel-Tuttle 100 years ago.
Comparison with similar events in the past results in an expected
activity of around 500 or 1000 meteors per hour around the
abovementioned time, but these numbers are only indicative: the real
frequency may be both higher or lower! However, even if the storm would
fail to materialize, a frequency of 50 to 100 meteors per hour is
guaranteed. Should Leonid meteor activity disappoint in 1999, it is
good to know that Leonid meteor storms are possible in 2000, 2001, and
2002, too!
Actually, Leonid meteors can be seen every year around November 17.
Along the larger part of Comet Tempel-Tuttle's orbit, however, Leonid
particles are scattered sparsely, so that, in most years, we see only a
few Leonid meteors per hour. Only in the vicinity of the Comet, the
density of Leonid particles is much higher. Therefore, we observe much
higher Leonid activity every 33 years during a couple of years, when
Comet Tempel-Tuttle revisits our region of the Solar System. In some
instances, we even see a real meteor storm!
Old chronicles contain references to past Leonid meteor storms back to
the 10th century A.D. The best-known Leonid meteor storms are those of
1833 and 1966, when tens of meteors per second darted across the skies
during the peak hour! The 1833 meteor storm was so spectacular that it
in fact launched meteor research as a branch of astronomy. Since the
1966 meteor storm, Comet Tempel-Tuttle has completed another revolution
around the Sun. The passage of the Comet through its closest point to
the Sun on February 28, 1998 marked the beginning of a five-year period
(1998-2002) during which strongly increased Leonid meteor activity is
again possible.
In 1998, a meteor storm did not materialize around the expected peak
time. The night before, however, saw an unexpected shower of very
bright meteors and fireballs. Astronomers managed to figure out what
had happened, and new computations match past Leonid meteor storms so
closely that there is good hope that the most recent predictions for
the period 1999-2002 are reliable.
The expected activity of Leonid meteors can in principle be seen from
any place in the abovementioned part of the world. Of course, the sky
must be clear and the selected observing site should preferentially be
free of light pollution; the less light, the more meteors will be seen!
Leonid meteors cannot be seen before around midnight. Hence, there is no
point in starting an observation earlier. Die-hards who do
not want to miss anything of the show should then continue to watch
until dawn. People who cannot afford to stay up that long should focus
on the period between 1:30 a.m. and 3 a.m. Greenwich Mean Time.
Mind that it can be very cold in mid-November: warm clothing adapted
to the local climate is essential! For comfortable observing, use a
reclining chair, and install yourself in a suitable sleeping bag or
under several blankets. While observing, do not fix a particular star,
but look relaxedly and patiently to a wide area of sky and wait
for shooting stars to appear.
- ------------------------------------------------------------------------
More information on the Leonids can be found in the International
Meteor Organization's bimonthly journal WGN and on the internet, at
http://www.imodot net.
For questions, contact Marc Gyssens at wgn@imodot net or +32-477-64 05 48.
Notice that the International Meteor Organization will send out a new
release with first results on the Leonids during the European early
morning hours of November 18, immediately after the event. All
recipients of the present release will automatically receive the new
release.
=====================
(2) LEONID DUST TRAILS AND METEOR STORMS: UPDATE
>From David Asher <dja@star.arm.acdot uk>
Note to readers: if you do not have access to reference [1], please
skip sections that are incomprehensible without it. Things such as the
Table 3 predictions should be clear by themselves.
Last modified 1999 Nov 9
LEONID DUST TRAILS AND METEOR STORMS: UPDATE
R.H. McNaught and D.J. Asher
- -----------------------------------------------------------------
In WGN 27, 85-102 (1999), details of the Earth's encounters with Leonid
dust trails were presented. Moderately close encounters will lead to
substantial meteor outbursts during the 1999 and perhaps the 2000
Leonids, while even closer encounters will produce storm level activity
in 2001 and 2002. Here we summarise the predictions of the dust trail
model for 1999 and the following few years, for the last time before
observations of the 1999 Leonids afford the first test of these
predictions. The updated analysis here is a little more comprehensive
than presented previously.
- -------------------------------------------------------------------
1. Introduction
The highest ZHR Leonid storms of the 19th and 20th centuries, as well
as many other sharp outbursts, have occurred when the Earth encountered
a young dust trail within the Leonid stream. Such a trail is generated
every 33 years or so when Comet 55P/Tempel-Tuttle returns to
perihelion. Each trail progressively lengthens while remaining narrow
and dense (density dilution being due to lengthening alone, not
broadening) until it is scattered into the Leonid background after a
few centuries. Readers should refer to [1] for further details.
The predictive power of the dust trail theory is demonstrated by the
following facts.
Storms and outbursts over the past 200 years that correspond to the
Earth's encounter with a given young dust trail have the calculated peak
times (i.e., based on the centre of the Earth reaching the calculated
nodal longitude of the trail) and the observed peak times matching to
10 minutes or less [1], in cases where the observed peak is known to
better than that accuracy.
A topocentric correction improves the match even further [2]. Data from
the past 200 years now indicate that close encounters are predictable with
an uncertainty of 5 minutes.
The peak time of the 1998 Draconid outburst [3] was predicted equally
accurately by Reznikov [4] using the same form of dust trail
calculations.
Moreover, Leonid timings relating to what we term young trail encounters
have been independently confirmed by the Russian group that includes
Reznikov [5,6] and by Lyytinen [7]. See [6] for references by the same
authors describing work on other streams.
2. Update
In [1], a desire to avoid excessive effort led us to make our
calculations comprehensive (covering the past 200 years) only for
trails 6 or less revolutions old; three encounters with slightly older
trails were considered as special cases. Also an error in the
calculations caused very small inaccuracies (less than 0.0001 AU)
in the determination of trails' nodal distances.
Now we provide an updated list of encounters (covering the next few
years only, these being of greatest interest for meteor observers) for
trails up to 9 revolutions old. Changes to the ZHR fit due to the error
are not substantial but a new fit is done.
Table 1 shows the data for past trail encounters. The only corrections
to the entries in Tables 2 and 3 of [1] are in r_E-r_D. Reference [1]
can be consulted for more details, but in summary, the strength of an
outburst is affected by Delta a_0 (which effectively corresponds to the
ejection velocity, this in turn being related to the mass
distribution), r_E-r_D (the miss distance of the Earth from the trail
node) and f_M (which measures the change in density due to trail
lengthening). Table 2 is for encounters over the next few years (cf.
Table 5 of [1]). Roughly, f_M is expected to be inversely related to
the age of the trail, but for example, the value for the part of the
9-rev trail that is encountered in 2001 shows that gravitational
perturbations can cause deviations from this simplistic relationship,
after a few revolutions.
Table 1 - Past trail encounters
Observed Calculated
Year Trail Node Delta a_0 r_E-r_D f_M ZHR/f_M ZHR/f_M
(J2000) (AU) (AU)
1966 2-rev 235.158 +0.168 -0.00013 0.52 170,000 100,000
1833 1-rev 233.184 +0.174 -0.00021 0.95 63,000 76,000
1866 4-rev 233.333 +0.059 -0.00029 0.37 22,000 22,000
1867 1-rev 233.420 +0.373 -0.00014 1.00 4,500 4,600
1869 3-rev 233.536 +0.320 -0.00047 0.44 2,300 2,200
1969 1-rev 235.272 +0.934 -0.00004 0.95 - -
Table 2 - Future trail encounters
Year Trail Node Delta a_0 r_E-r_D f_M
(J2000) (AU) (AU)
1999 3-rev 235.291 +0.138 -0.00066 0.38
2000 8-rev 236.103 +0.064 +0.00076 0.27
2000 4-rev 236.276 +0..114 +0.00077 0.13
2001 7-rev 236.114 +0.081 -0.00043 ~0.14
2001 9-rev 236.429 +0.041 +0.00015 0.43
2001 4-rev 236.463 +0.142 +0.00022 0.13
2002 7-rev 236.610 +0.113 -0.00015 0.13
2002 4-rev 236.888 +0.172 -0.00005 0.15
2006 2-rev 236.615 +0.961 -0.00009 0.53
A model in which ZHR/f_M (ZHR being the observed peak ZHR in past
encounters) is fitted as a function of Delta a_0 and r_E-r_D, as
described in [1], is done, and applied to the future years. The five
storm years in Table 1 (1969 excluded) are used in the fit, to
interpolate ZHR estimates for 1999-2002. It is inappropriate to apply
exactly the same model to very different values of Delta a_0 and so
1969 alone is used to predict 2006 alone. The predictions are in Table
3 (cf. Table 6 of [1]). Only the fit centred at r_D (cf. Tables 4 and
9 of [1]) is given, the formal uncertainty in the fit being 20%.
Whilst the overall fit is reasonable, there is now a discrepancy
between the calculated ZHR values for 1833 and 1966. For 1966 the
calculated ZHR is 53,000. Despite the uncertainty in the observed ZHR
in 1966, it is probably one of the more reliable data points used in
the fit and 1833 the least reliable. In 1999, the formal ZHR prediction
is 500 and this appears fairly robust, regardless of whether the 1833,
1966 or both are used in the fit, but values 200 < ZHR < 2000 give a
reasonable fit.
Table 3 - Predictions
Time (UT) Trail Estimated Moon Visible from
ZHR age
1999 Nov 18, 02:08 3-rev 500 10 Africa, Europe
2000 Nov 18, 03:44 8-rev 30? 22 W. Africa, W. Europe, NE S. America
2000 Nov 18, 07:51 4-rev 20? 22 N. America, Central America &
NW S. America
2001 Nov 18, 10:01 7-rev 1,500? 3 N. & Central America
2001 Nov 18, 17:31 9-rev 15,000 3 Australia, E. Asia
2001 Nov 18, 18:19 4-rev 15,000 3 W. Australia, E., SE & Central Asia
2002 Nov 19, 04:00 7-rev 15,000 15 W. Africa, W. Europe, N. Canada,
NE S. America
2002 Nov 19, 10:36 4-rev 25,000 15 N. America
2006 Nov 19, 04:45 2-rev 100 28 W. Europe, W. Africa
Figures 1 and 2 are the visibility maps that are not in [8]. Note that
in these figures, the Moon's phase is displayed as seen from the
southern hemisphere.
Figure 1
(See http://www.atnf.csiro.au/asa_www/images/2001csl.gif [15Kb])
Figure 2
(See http://www.atnf.csiro.au/asa_www/images/2002bsl.gif [15Kb])
3. Discussion
Revised values of the nodal distance required a reassessment of the ZHR
predictions from the dust trail density model. Overall, the rates have
not changed substantially, although the uncertainty in the fit has
increased. For 1999, the predicted ZHR for the 3-rev dust trail
encounter is probably of the order of 500. This value requires some
elaboration. Activity from this dust trail will be additional to
background activity, which could itself have a ZHR in the hundreds.
Thus, the observed ZHR at the time of the peak will probably lie in the
range 500-1000, if the dust trail contributes a ZHR of 500. This value
is entirely consistent with past data, given that there are
uncertainties in the past peak ZHRs used in the fit, and different fits
can be done (cf. [1]) centred on slightly different values of r_D.
Given that some older trails have been demonstrated to be capable of
delivering high rates (e.g. the 9-rev trail of high f_M in 2001), it
will be necessary to check that for the years of the storm data used in
the ZHR fit, no additional old dust trails were contaminating the ZHR.
In future analyses, we shall also remove the background component from
the peak ZHRs, to more truly represent the contribution of the dust
trails alone.
One interesting change to the ZHR fit, is that the predicted ZHR in
1801 from the 2-rev trail is now 300. This is much smaller than our
original predictions that suggested a minor storm had occurred. Thus,
this potential anomaly of an unobserved storm in the last 200 years
over western Europe, is no longer a problem. A short lived peak ZHR of
around 300 would probably not have attracted much attention in those
years. However, we are aware of no data that refute the possibility of
a storm in that year.
There are no additional encounters with trails up to 19 revolutions old
in 1999; therefore, unpredicted high activity is unlikely. There
appear to be no other encounters of significance in the following years
up to this age, although in 2001, an encounter with a disrupted 10-rev
trail of uncertain density could produce a peak ZHR of around 1,000 on
Nov 18, 18:01 UT. Although the disrupted nature of this section of the
10-rev trail makes this time unreliable (pending more detailed
simulations), it appears to be during the 48 minute gap between the
stronger encounters. These three encounters in close succession, with
no interference from the Moon, suggest the highest observable rates
will occur in 2001, although the ZHR in 2002 is likely to be higher.
For the 3-rev trail encounter in 1999, the time of maximum is predicted
to be at Nov 18 02:08 UT in the Mediterranean region, with an
uncertainty of around 5 minutes. The time of maximum is dependent on
location [2], with the peak predicted at 01:58 in South Africa and
02:14 in northern Scandanavia. The dust trail model does not make any
prediction about the time or intensity of the background activity
maximum, but in the past 200 years the highest Leonid rates outside
young dust trail encounters, can approach a ZHR of 500.
Further information is available in [9].
References
[1] R.H. McNaught, D.J. Asher, `Leonid dust trails and meteor storms.' WGN
27, 1999, pp. 85-102.
[2] R.H. McNaught, D.J. Asher, `Variation of Leonid maximum times with
location of observer.' Meteorit. Planet. Sci. 34, 1999, pp. 975-978.
[3] R. Arlt, `Bulletin 13 of the International Leonid Watch: The 1998 Leonid
meteor shower.' WGN 26, 1998, pp. 239-248.
[4] E.A. Reznikov, `The Giacobini-Zinner Comet and Giacobinid meteor
stream.' Trudy Kazan. Gor. Astron. Obs. 53, 1993, pp. 80-101 (in
Russian). See also IMO-News mailing list, 1998 Sept 9.
[5] E.D. Kondrat'eva, E.A. Reznikov, `Comet Tempel-Tuttle and the Leonid
meteor swarm.' Sol. Syst. Res. 19, 1985, pp. 96-101.
[6] E.D. Kondrat'eva, I.N. Murav'eva, E.A. Reznikov, `On the forthcoming
return of the Leonid meteoric swarm.' Sol. Syst. Res. 31, 1997,
pp. 489-492.
[7] E. Lyytinen, `Leonid predictions for the years 1999-2007 with the
satellite model of comets.' Meta Res. Bull. 8, 1999, pp. 33-40.
[8] R.H. McNaught, `Visibility of Leonid showers in 1999-2006 and 2034.'
WGN 27, 1999, pp. 164-171.
[9] Armagh Observatory Leonid WWW pages are http://www.arm.acdot uk/leonid/
and general notes for the public are at
http://www.atnf.csiro.au/asa_www/leonids.html
Acknowledgments
DJA thanks Esko Lyytinen for extremely valuable discussions on this work.
Authors' addresses
Robert H. McNaught, Siding Spring Observatory, Coonabarabran,
NSW 2357, Australia (rmn@aaocbn.aaodot gov.au)
David Asher, Armagh Observatory, College Hill,
Armagh, BT61 9DG, N. Ireland, UK (dja@star.arm.acdot uk)
===================
(3) A REVIEW OF THEORIES FOR LEONID STORM PREDICTION
>From Rob McNaught <rmn@aaocbn.aaodot gov.au>
Benny,
Several hours ago I sent a badly drafted version of the following to
the IMO-News and Meteorobs lists, thinking I would have no time to
get back to it before flying out in several hours. However a bit of
time became available, and the following notes are really just an
update of the misplaced words and bad phrasing. There is still a lot
to do with it, but I really must go now. Thought it might be of
interest for CCNet.
Cheers, Rob
The following notes were started as preparation for a lecture.
However I felt that they might be of general interest, given that
many of the works cited below often go unmentioned, or are quoted
without analysis. I am aware that I have not mentioned some works
(e.g. Kresak's 1993 study), but will include these at a later date.
As I'm leaving for overseas in several hours, my attempts to knock
this into better shape has come to an end. Despite many helpful
comments by David Asher on a much earlier draft, about half of what
follows has not been submitted to anyone for comment. It would thus
be inappropriate to quote anything from what follows as if it were
from a refereed journal. It is my intention to work on this much more
after my return from the Leonids trip and submit it for publication.
A Review of Theories for Leonid Storm Prediction
R. H. McNaught
Last edit 99Nov09b
Introduction
There has been little critical evaluation of the various Leonid storm
predictions, either in the professional literature or in the popular
astronomy media. This has resulted in speculative methods with no
theoretical basis or historical validation, being presented side by
side with theoretically rigorous approaches that have been carefully
validated against the historical record. I shall discuss some of
these methods here, so that a clearer assessment can be made about
the various prediction methods.
Studies using the comet node.
Yeomans (1981) demonstrated an obvious correlation between the timing
of storms and the time and position of the Earth in relation to the
comet node. In terms of predictive power this model fails, with years
of high and low activity intermixed. It is also only an
approximation, as the nodal distance of the comet is only of physical
relevance when the comet is actually at the node. So, should one
choose the osculating orbit at the epoch of the comet being at the
node, or the time of observation?
Differential perturbations between the comet and the ejected dust,
lead to the dust having a different nodal distance and longitude than
the comet. For storms in the last 200 years, using the node of the
comet gives an approximation to the peak time of storms to a few
hours, using nodal values for either epoch This is discussed more
fully in McNaught (1999).
That the storm years do indeed cluster in one quadrant of the
Yeomans' plot indicates that it does have some predictive value, but
there are both false positives and false negatives. Although both
axes of the plot are qualitatively reasonable, only the time axis is
also quantitative. The effect of solar radiation pressure is to push
particles into longer period orbits, and therefore they return after
the comet. The density of the dust decreases with age since ejection
but this is not accommodated in the diagram. The radial distance axis
is problematic, as noted above. David Asher comments "as for the
inside/outside distance being a factor, while the idea is
qualitatively correct (i.e. radiation pressure does tend to cause
particles to be on marginally bigger orbits in space) it's
qualitatively irrelevant, compared to the effect on the nodal
distance of gravitational perturbations, for visual meteor size
particles."
Cooke (1997) looks at the Yeomans' diagram through a statistician's
eye. He tries to derive probabilities of storm conditions in various
years. To some extent this must be seen as a failure of this general
approach using the comet node. No amount of math can compensate for
not undertaking a rigorous dynamical analysis of the ejected dust.
To understand Leonid storms, or any physical phenomenon for that
matter, one needs both maths and a physical understanding of the
phenomenon involved.
Ferrin (1999) uses a similar form of analysis as Yeomans, but gives
the Yeomans diagram an additional dimension of ZHR intensity at
maximum. Whilst one can argue about the values of ZHR used in the
diagram (e.g. the almost certainly spurious storm values for 1900 and
1901), and the way individual values were selected from the available
data (e.g. 1866 and 1867) the idea is initially reasonable, given the
limitations noted for the Yeomans (1981) paper above. The intensity
of the Leonid activity of the last 200 years has isolines of shower
intensity empirically fitted. A "ridge" of uniform high intensity
(ZHR = 150,000) is identified crossing the diagram in a curve from
the comet.
Given the small amount of data for high intensity storms (ZHR >
10,000), it is notable that one of these lies significantly away from
the ridge and is too high by a factor of 10 over the fit. Given that
the fit is completely empirical, this is a major problem for such
sparse data. Probably the most unusual thing about the fit is the
assumption that the ridge of high intensity is of uniform intensity.
This is clearly false in the close vicinity of the comet. The ZHR
immediately beside the comet would be enormous, such dust hardly
having time to dissipate during that specific perihelion passage.
However there is a big difference between the dust density near the
comet and that a year or so behind. It is well known that solar
radiation pressure causes particles, to orbit more slowly. Thus an
initially uniform ejection of dust will, one revolution later, result
in a mass separation with most "visual meteoroids" being concentrated
away from the comet. Thus, even with the probably spurious storm
level values for 1900 and 1901, these facts immediately suggests to
the eye a series of closed loops off-centered from the comet. The
consequence of this would be that the rates during the current epoch
would be considerably lower than the values Ferrin suggests, from his
unjustified empirical fit. Whilst a number of theoretical
considerations are made, there is no attempt to look at the actual
spatial distribution of dust through rigorous orbital integrations.
Brown (1999) has analysed the available historical observations of the
Leonids, deriving the time and ZHR of maximum and the width of Leonid
activity. This represents a major achievement and all Leonid storm
prediction methods should be demonstrated to be consistent with this
historical data. Utilising this data Brown uses the same idea as
Ferrin, but allows a contour plotting program to contour the ZHR
data. For the limitations presented above, and the reasons given
below, the use of the comet node cannot succeed. The fundamental
reason is that the dust behaves independently of the comet and
detailed dynamical studies of the ejected dust must be used.
Dynamical studies of ejected dust
Wu and Williams (1996) present an analysis of the orbits of dust
ejected from comet 55P/Tempel-Tuttle. They apply rigorous
corrections for planetary perturbations. Part of their argument is
that high ejections velocities of several hundred metres/sec are
necessary to produce the orbits of observed Leonid meteors in
1965-66. These orbits remain stable over the past 100 years and do
not converge to a common origin. This is in stark contrast to their
later modelling where they assume the activity in 1933, 1966, and
predictions for 1998-99, can be based solely on dust ejected from the
comet on the previous two revolutions.
Using the high velocities of ejection derived from the meteor orbits,
they believe particles can be ejected into orbits as short as 17
years or as long as 120 years. This provides pathways for particles
to make one, two or three revolutions in 66 years. However, if
Leonid activity is dominated by recently ejected particles, then the
meteor orbits should converge to the comet orbit at either of the
previous two returns. That they do not indicates that either
a) the orbits are too uncertain to be useful in this analysis and/or
b) the assumption that activity is dominated by the most recent returns
of the comet, is false.
If we assume for the moment that these high ejection velocities are
possible, it is reasonable to assume that the extreme orbits of both
shorter and longer period are likely to be significantly less
populated than those closer to the orbital period of the comet. They
specifically make this point in section 4. This is most important
when they come to assess the number of test particles that pass close
to the Earth. They take 20 test particles from one revolution of the
comet earlier, with a 33 year period and 60 from 2 revolutions of the
comet earlier, 20 each from particle periods of 22, 33 and 66 years.
Simple summing of these 80 particles has no validity. It is probable
that there will be many more particles with periods of 33 years than
22 or 66 years.
The orbits they integrate have starting orbital periods that make an
integral number of revolutions during the time taken for one or two
comet orbits. However, despite a claim that they did, there is no
evidence in their work that they have iterated these orbital periods
to correct for changes due to planetary perturbations resulting in
the particles not arriving at the node at the same time as the Earth.
The nodal distance is irrelevant if the particle orbit cannot produce
a close approach to the Earth. Looking at their Fig 7, the last two
bars for each year give the relative number of particles within a
nodal distance of 0.002 AU and within a distance from the Earth of
0.005 AU. If the correct orbital period is chosen, then the closest
approach will always be (slightly) inside the nodal distance, so the
bar giving passage within 0.005 AU of the Earth must always be equal
to, or greater than, the bar showing particles within 0.002 AU nodal
distance. In two of the four cases they are less, one substantially.
Thus most test particles do not in fact have the correct period to
have a close encounter with the Earth, and the integrated particles
are irrelevant in determining the approach distances and relative
numbers. It was found by McNaught and Asher (1999) (see below)
that the density of dust trails can vary substantially on scales of
the order of an Earth diameter, so the bin sizes used are
substantially too coarse to be useful indicators of storm activity.
Any conclusions based on Fig 7 are necessarily invalid.
Even assuming the Figure is valid, the comparison of these relative
numbers of particles for various years shows 1933 coming in at a
little under 10% of 1966, in the important quantities (number of
particles with nodal distance near Earth, and number with small
distance of closest approach to the Earth). However, the ZHR in 1933
was around 3 orders of magnitude smaller than in 1966 (Brown (1999)),
so their statement that these figures "roughly mirror the
observations" really has little meaning.
Overall, the assumptions behind this work are reasonable, but in
restricting the calculations to only the previous two orbits, and not
choosing the precise orbital period to make a close encounter, the
work has no validity as a predictive tool. Also they do not attempt
to derive the time of storms from the nodal longitudes of the dust
orbits. This is a necessary test of any theory, as it would have
available some of the best data for comparison.
Kondrat'eva and Reznikov (1985) were the first group to determine
meteoroid orbits that had the precise orbital period to arrive at
their descending node at the same time as the Earth. Their work has
been largely overlooked. The idea is extremely simple. The only
meteoroids we can experience as meteors, are ones that have an
orbital path from the comet at or near perihelion, to the Earth in
some specific November. The application of rigorous planetary
perturbations and the consideration of solar radiation pressure, give
a nominal orbital solution from which the nodal longitude and
distance is derived. Meteoroids with any other orbital period don't
pass the node at the same time as the Earth and thus could not become
meteors. It is the component of the ejection velocity along the
comet's velocity vector that causes the change in orbital period.
The spread of the meteoroids about this nominal solution are a result
of other components of the ejection velocity, that are orthogonal
to the comet's velocity vector, and of solar radiation pressure.
Their work shows a great consistency with the historical data for the
years presented. Their predicted time for 1966 is exact, to the
resolution of their prediction, which is 0.01 day. In 1993, Reznikov
predicted the time of Giacobinid activity as 1998 Oct. 08.550 UT.
This was confirmed within observational error! Clearly the group had
the ability to make predictions with high time resolution.
Kondrat'eva, Murav'eva and Reznikov (1997) update this work by
considering dust ejected at earlier passages of 55P/Tempel-Tuttle
through perihelion and derive the nodal longitudes and distances for
the dust during the period 1760-2002. Curiously, they only give the
predictions of the time of maximum activity to one decimal of a day
(+/- 1.2 hours). There is an exceptionally strong correlation between
the close approaches to dust "swarms" with moderate ejection
velocities (<40 m/s), and years with observed storms. All their
derived times for the storm years agree with the observed times
derived by Brown (1999) to within +/- 1 hour. This was clearly a
major advance in Leonid storm prediction.
Asher (1999) was unaware of the Kondrat'eva et al. studies when he
basically replicated their early work with his own similar technique.
However, he did this with higher precision in nodal longitude than
the later Russian study. This led to the realisation that the derived
times from the "dust trail" nodal longitude were almost identical to
the times of Leonid storm maxima derived by Brown (1999). This was
initially discussed by McNaught (1999).
McNaught and Asher (1999a) extended the Asher (1999) results by
looking at dust trails up to 6 revolutions old (plus some older
trails identified by Kondrat'eva et al. (1997)). This indicated that
the times of maxima were consistent to within +/-10 minutes for all
storms and short duration outbursts that had well defined times of
maximum (1866, 1867, 1869, 1966 and 1969. Additionally, they derived
a density model based on the ejection velocity (change in semi-major
axis) required to produce passage close to the Earth and the nodal
distance of the dust trail. This approach also took into account the
mass distribution of the ejected dust encountered in a specific year
(which is correlated with ejection velocity) and the dilution of the
trail density with age. It was demonstrated from simulations of dust
ejected isotropically from the comet, that the resulting trail width
remains essentially constant over several revolutions, dilution of
the trail density being by stretching alone. Using this model of
trail density, they were able to show a remarkable consistency (+/-
20%) between the calculated relative density and the observed ZHR for
the storm data of 1833, 1866, 1867, 1869 and 1966. Earlier storm
years were not included due to poor data quality and contamination
from additional dust trails. The fit to the data was by a double
Gaussian. This will limit the predictive value, as it is believed
that the dust trails are not symmetrical in radial distance mostly
due to the action of solar radiation pressure. Until a theoretically
derived dust trail profile in radial distance is developed, the data
is too sparse to suggest what improvement may be achieved.
McNaught and Asher (1999b) derived a topocentric correction for the
observer being offset from the center of the Earth which had been
used in the earlier calculations. This indicated that the times
calculated from the dust trails could be improved from +/-10 minutes
to +/-5 minutes against the observed times calculated by Brown
(1999).
Lyytinen (1999), unaware of the Kondrat'eva et al. and Asher and
McNaught studies, came up with the same results in the time of dust
trail encounters, but from a very different starting point. Using
van Flandern's satellite model of comets, he derived the times of
closest encounter with dust trails through to 2007. Despite that
radically different initial assumption, the dynamical analysis was
done rigorously and the results of the time of maximum agree within
minutes of the results of the earlier studies. Lyytinen himself did
not do any rigorous historical validation, but his results were
clearly very consistent with the historical data.
All three groups (Kondrat'eva et al., Asher and McNaught, and
Lyytinen) found a small number of dust trail encounters missed by
others. These were mostly of older trails. Calculations by other
groups confirmed these.
This "dust trail" approach to predicting Leonid storms is clearly
very powerful and has demonstrated a very close correspondence to the
time of storms (+/-5 minutes) and to their ZHR (20% error in the fit
to 5 storm ZHRs).
Background activity.
Although one work of Brown (1999) was mentioned above, he and
collegues including Jones, have continued with their studies of the
Leonid stream as a whole. The above dynamical studies only addressed
the storm peak, whereas Brown et al. are not considering storms in
isolation. As I have not seen their latest work, I can only comment
on what I believe is their current approach. By ejecting meteoroids
(using an ejection model they derived) over a period around
perihelion and for many revolutions of the comet into the past, they
try to derive the overall activity of the Leonid shower. This
requires substantial computing power, but is probably the
only way to approach the overall structure. The limitation in this
method may be that it lacks adequate temporal and spatial resolution.
One could liken this approach to a general geological survey of an
area where sampling at coarse intervals can miss narrow dense veins.
It may however be the case that the resolution is adequate to
identify the dust trails, although the results presented by Brown el
al. earlier this year do not confirm many of the dust trail
predictions for the coming years. They do however quote the same time
of maximum as the dust trail theory in 1999, although the nature of
this prediction is unknown to me.
Conclusion.
Leonid storms are predictable from dust trail calculations based on
the orbit of the parent comet 55P/Tempel-Tuttle. The ZHR predictions
are limited by the lack of storm ZHR data, but the dust trail density
model of McNaught and Asher (1999) is very consistent with data
available.
Asher, D.J. (1999) "The Leonid meteor storms of 1833 and 1966.",
MNRAS, 307, 919-924
Brown, P. (1999) "The Leonid meteor shower: historical visual observations.",
Icarus, 138, 287-308
Cooke, W. (1997) "Estimation of Meteoroid Flux for the Upcoming Leonid
Stroms.", http://see.msfc.nasadot gov/see/mod/leonids.html
Ferrin, I. (1999) "Meteor storm forcasting: Leonids 1999-2001.",
Astron. Astrophys., 348, 295-299
Kondrat'eva, E.D. & Reznikov, E.A. (1985), "Comet Tempel-Tuttle and the
Leonid meteor swarm.", Solar System Research, 199, 96-100
Kondrat'eva, E.D., Murav'eva, I.N. and Reznikov, E.A. (1997) "On the
forthcoming return of the Leonid meteoric swarm.",
Solar System Research, 31, 489-492
Lyytinen, E. (1999) "Leonid Predictions for the Years 1999-2007 with the
Satellite Model of Comets.", Meta Research Bulletin, 31, 489-492
McNaught, R.H. (1999) "On predicting the time of Leonid storms.",
The Astronomer, 35, 279-283
McNaught, R.H. & Asher, D.J. (1999a) "Leonid dust trails and meteor storms.",
WGN, 27, 85-102
McNaught, R.H. & Asher, D.J. (1999b) "Variation of Leonid maximum times
with location of observer.", Meteorit. Planet. Sci. (in press)
Wu, Z. and Williams, I. P. (1996) "Leonid meteor storms",
MNRAS, 280, 1210-1218
Yeomans, D.K. (1981) "Comet Tempel-Tuttle and the Leonid meteors",
Icarus, 47, 492-499
================
(4) THE AEROSPACE CORPORATION TO PROVIDE REPORTS DURING
LEONID STORM
>From Ron Baalke <baalke@ssd.jpl.nasadot gov>
The Aerospace Corporation
The Aerospace Corporation to Provide Reports During Leonid Storm
El Segundo, Calif., 11/8/99 -- When the 1999 Leonid meteoroid storm
occurs Nov. 17-18, The Aerospace Corporation will be operating a near
real-time Web site, where the latest observations on the storm's
intensity will be posted.
The company plans to provide updated information every 15 minutes from
observers around the world. This information can be accessed at
www.leonidstorm.com. Included at this site is information on how
observers who wish to participate can register.
William Ailor, Ph.D., principal director of the company's Center for
Orbital and Reentry Debris Studies, said it is hoped that the
information will help satellite owners and operators determine when
it is prudent to switch from defensive to normal operations.
During peak activity some operators plan to take measures to
safeguard their satellites. Options include turning off sensitive
components, turning solar panels away from the stream of meteoroids,
orienting satellites to minimize exposure and other measures.
Scientists predict the Leonids event will reach storm level this year
with peak activity of 1,000 meteors or more per hour predicted.
Meteors are flashes of light created when meteoroids burn up as they
slice through the Earth's atmosphere.
Last year some 300 meteors per hour were observed at peak levels. A
number of satellite owners and operators who attended the Leonids
Storm and Satellite Threat Conference sponsored by The Aerospace
Corporation and the American Institute of Aeronautics and
Astronautics in May 1999 reported that sensors aboard their
satellites detected impact with the tiny meteoroids, but no satellite
was reported seriously damaged.
The Leonids occur with intensity every 33 years or so when the Earth
passes through the most dense part of the debris stream created by
the comet Tempel-Tuttle.
For further information on the Leonid meteoroid storm, please visit:
"Understanding the Leonid Meteor Storms",
http://www.aero.org/leonid/index.html
===================
(5) TELESCOPIC VIDEO OF LEONIDS & LINEARIDS SOON
>From Joan and David Dunham <dunham@erols.com>
Everyone interested in the sky will be interested in the third item
below, a possible intense meteor shower during the next couple of nights
from Comet LINEAR, if you don't already know about it. But this is
being distributed to all observers on my occultation observer
list, mainly to encourage those with video capability for
recording occultations telescopically to use their equipment for
recording certain aspects of the Leonids that can not be recorded,
or recorded as well, with the mainly wide-field systems of
dedicated meteor observers. This is an opportunity for occultation
observers to make some unique and valuable observations in another
field of astronomy. The major topics are given below.
1. Telescopic video observations of Leonid meteors.
2. Leonids hitting the dark side of the Moon - especially the Americas.
3. Meteors from Comet LINEAR possible November 10-12 - Leonid preview?
________________________________________________________________________
1. Telescopic video observations of Leonids are sought, with and
without image-intensified systems. For example, observation with the
sensitive PC23C camera with any telescope are sought. Although an
audio record of shortwave time signals is nice if available, be sure
to give some visual cue to the time since when successful observation
tapes will be scanned, there is no access to the audio track. A
time/date stamp generator is ideal but if that is not possible,
passing a white card across the field of view (or briefly shining a
flashlight into it) at a known (recorded/written down) time works as
well. Timings to the nearest second are preferred.
Use the widest field of view (FOV) that your telescope/video
combination can achieve (use a focal reducer lens if you have one).
Observations are sought from 0h UT November 17 to 0h UT November 19.
Peter Gural, who is coordinating a worldwide effort to record
Leonids with wide FOV (20 deg. and more) intensified video systems, writes
about this:
"I had a thought yesterday that you may be interested in trying but
unfortunately I do not have time to coordinate another data collection
effort and will be out of the country for the Leonids. This has to do
with the narrow field of view imaging systems that your group uses. Dr.
Bob Hawkes of Mount Allison University (the video meteor guru) claimed
to have seen "jets" protruding perpendicular to several Leonid tracks in
last year's video. The physical phenomena is unknown at this time.
Conjectures are ejected material (explosively), discharges (lightning)
in the E-layer, or just pixel noise in the intensifiers. An interesting
project may be to record narrow field of view (telescopic video) to try
and resolve the question of the jets' reality. The drawback is that
with a narrow field of view, the chances of capturing a meteor are low.
Having a storm helps but there is no guarantee a storm will occur.
Alternatively, by having many telescopic video cameras operating, the
odds are greatly improved of capturing several meteors on tape. [In a
given area where several telescopes with video are available,] one could
set up a bank of several non-overlapping telescopic fields of view and
record away all night. Time stamps are nice but not required. Setting
the scopes pointing in the same region of sky (but non-overlapping)
would enable the possibility of a meteor traversing several cameras.
Best pointing would be 60 degrees elevation and 60 degrees from the
radiant. Above the north celestial pole is a possibility. All the
systems could be in a common site but again that is not required
[certainly, observations at any location in the world could potentially
catch some Leonids]. You would have to do the coordination of observers
and equipment [I will attempt that in the Maryland-DC-n. Virginia area;
others can coordinate other areas, as for grazes or asteroidal
occultations]. I can process the tapes after I get back. We will have
15 intensified cameras operating at various sites around the world but
all will have fairly wide 20-40 degree fields of view."
Peter S. Gural
Science Applications International Corp. (SAIC)
4001 N. Fairfax Drive, Suite 400
Arlington, VA 22203
Phone +1-703-816-5954
If you obtain a useful tape, send it (or a VHS copy) to Peter Gural.
Specify your observing location, preferably including longitude,
latitude, and height above sealevel (don't need to be as precise as for
occultations, although for some observations accurate locations might be
useful, especially if two separated cameras happen to catch the same
meteor, a very unlikely occurrence). Also, specify where the telescope
was pointed, at least approximately (alt. and azimuth, or RA and Dec,
and whether driven or not). If you know that you caught one or more
meteors, it would be helpful to give the approximate time, but that is
not necessary; tell whether you have viewed the tape or not (viewing is
not ncessary). Specify whether the telescope was clock-driven or not
(doesn't need to be), and the start and stop times of the recording.
Include your address, telephone number(s), and e-mail address (if you
have one) in the report that you enclose with the tape.
Since accurate pointing is not necessary, the telescope and equipment
can be left mainly unattended while the video records; you just need to
be there to note the end time of a recording and change tapes. For
observation periods like this that are much longer than for
occultations, you need to be sure adequate power and blank tapes are
available. Also, you need to try to prevent dewing using dew caps
and/or heating elements (and then, more power needed).
________________________________________________________________________
2. It has been suggested that observers watch the dark side of the Moon
for large Leonid impacts (see, for example,
http://www.earthchangestv.com/comets/). Last year, there were some
Leonids as bright as the full Moon, which I figure at the 4000 times
greater distance might show up around 4th mag. on the dark side of the
Moon, although the dynamics of a lunar impact would certainly be
different than one in the Earth's atmosphere. Maybe a lunar impact,
being quicker, might even be brighter, so that events on Earth of -2 to
- -4 mag. might even show up on the Moon around 7th or 8th mag., which
would be within reach of many occultation video systems. So maybe in
this way, we could detect the Leonid peak from the Americas, since the
expected peak is around 2h U.T. of Nov. 18 U.T., during the evening of
the 17th local time when the Moon will be well-positioned for observing
from the Americas and 67% sunlit (or 1/3rd of its disk dark). This
would mean observing ALL night, first the Moon, then after midnight the
sky, for meteors. But the good Leonid opportunities only come every 33
years, so I think it is worth it.
All you need to do is image the dark side of the Moon, excluding all,
or as much as possible, of the sunlit side as you can, very similar to
recording an occultation. To avoid duplication and observational bias,
if your system can not cover the entire dark side (few can), then you
should image the northern third (or sector) of the dark side (cusp angle
0N to 60N) if your family name starts with A to H, the central (or
equatorial) third of it (cusp angle 60N to 60S) if your family name
starts with I to P, and the southern third (cusp angle 60S to 0S) if
your family name starts with Q to Z. If your system FOV is
significantly less than 10' so you can't image a whole third of the dark
side, try to observe a part of the sector closer to the north side of
the sector if your last name starts with a letter closer to the lower
end of the alphabetic range for your sector. As far as I know, a meteor
striking the Moon has never been recorded before. If you make a
recording of the dark side of the Moon, send me a message giving the
location from which you observed, the start and end times of your tape,
the approximate FOV, its location (approximate cusp angles covered), and
an estimate of your limiting magnitude on the dark side (the occultation
stars mentioned below might help), and if you noticed any lunar meteor
impacts or not. If so, give their approximate brightness, time, and, if
possible, an estimate of their location (you may need to image a tiny
bit of the terminator to be able to tell the location, since most
systems will not be able to image features on the dark side illuminated
by feeble Earthshine; if you image the terminator, you might do that
only briefly every 5 to 10 minutes by moving the telescope smoothly in
R.A., just to establish your viewing area). If positive observations
are made, we will want to collect the tapes so that the impact locations
can be measured and correlated with others, and we'll ask others to
examine their tapes at the appropriate times. But we'll work out the
proceedures for that ONLY IF we positive observations are obtained.
See the remarks about observing at the top of item 1. above, since they
will apply if there is success and we want to scan your tape.
Of course, as long as you are imaging the dark side, you may as well
try to record some occultations of stars. Perhaps the brightest to be
occulted will be 4th-mag. psi1 Aquarii (= ZC 3419) before and after 4h
U.T. of the 18th U.T. visible from most of the western and north-central
USA and western Canada. For example, in Los Angeles, the star will
disappear at 3:57 U.T. (7:57 pm PST) at a cusp angle of 36S. Times for
many dozen other cities will soon be posted on IOTA's Web page at
http://www.lunar-occultations.com/iota
A good grazing occultation will occur along the southern limit of the
occultation, passing just north of Yuma and south of Flagstaff, Arizona;
south of Colorado Springs; just south of Sioux Falls, Iowa and
Minneapolis; and very close to Sault Ste. Marie. This limit is shown as
line 203 on p. 153 of the RASC Observer's Handbook, that map also being
found at the IOTA Web site given above. Another southern-limit graze
path, of 7.0-mag. ZC 3409 around 2h U.T. of the 18th U.T., passes near
Fresno, Calif.; Brigham City, Utah; and Winnipeg, Manitoba; it is path
202 on the RASC Handbook map. Also, some 7th and 8th-mag. stars will be
occulted; times for two locations are given below. Many of you have
your own predictions for your station. C.A. is cusp angle. Those with
U.T. 23h are Nov. 17th U.T.; other hours are of Nov. 18th, and all
events are during the evening of the 17th local time:
Washington, D.C. Los Angeles, Calif.
Star mag. U.T. C.A. Star mag. U.T. C.A.
h m h m
SAO 146590* 7.1 23:13 14N SAO 146570* 7.8 2:51 25N
SAO 146518 8.3 23:29 78N SAO 146578* 8.2 2:51 56N
SAO 146570* 7.8 3:35 81N SAO 146577* 7.6 2:53 55N
SAO 146577* 7.6 3:55 79S psi1 Agr. 4.5 3:57 36S
*star probably double, expect disappearance in steps.
________________________________________________________________________
3. The Earth will pass only 0.0115 A.U. from the orbit of Comet LINEAR
(C/1999 J3) at 19:41 U.T. of November 11th, U.T., 40 days after the
comet itself was in that area. The geometry is rather similar to that
for Comet Giacobini-Zinner when it produced Giacobinid storms in 1933
and 1985. But Comet G-Z is a short-period (6 years) comet, while Comet
LINEAR is expected to be relatively fresh with a period of about 63,000
years. So it is not obvious that there will be any "Linearids", but it
is certainly worth looking for them. The radiant is in the bowl of the
Big Dipper, near Phecda, at R.A. 11h 40m, Dec. +53 deg. Asia is favored
for seeing LINEAR meteors at the time of closest approach, but the
International Meteor Organization is calling for a watch during the 48
hours surrounding the time of closest approach given above (that is,
from 19:41 UT of Nov. 10 to 19:41 UT Nov. 12). More information is at
http://science.nasadot gov/newhome/headlines/ast05nov99_1.htm
If there is a significant shower of Linearids, we may as well use them
to practice for the Leonids. The Moon will be near new, so it won't be
possible to see Linearids striking the Moon.
David Dunham, IOTA, 1999 November 9
Joan and David Dunham
7006 Megan Lane
Greenbelt, MD 20770
(301) 474-4722
dunham@erols.com
- ----------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
- ----------------------------------------
The CCNet is a scholarly electronic network. To subscribe/unsubscribe,
please contact the moderator Benny J Peiser <b.j.peiser@livjm.acdot uk>.
Information circulated on this network is for scholarly and
educational use only. The attached information may not be copied or
reproduced for any other purposes without prior permission of the
copyright holders. The fully indexed archive of the CCNet, from
February 1997 on, can be found at http://abob.libs.ugadot edu/bobk/cccmenu.html
------- End of Forwarded Message
To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html