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(meteorobs) Excerpts from "CCNet DIGEST, 21 April 1999"
A good issue for meteors. By the way, has anyone had any contact with the Kevin
Conod of the Newark (NJ, US) Museum, quoted in the Lyrids write-up below?
Lew
------- Forwarded Message
From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST, 21 April 1999
Date: Wed, 21 Apr 1999 11:15:13 -0400 (EDT)
CCNet DIGEST, 21 April 1999
---------------------------
(1) A SPACE CHARGE MODEL FOR ELECTROPHONIC BURSTERS
Luigi Foschini <L.Foschini@isao.bo.cnrdot it>
(2) ON PREDICTING THE TIME OF LEONID STORMS
http://wwwdot demon.codot uk/astronomer/leonid_maxima.html
[...]
(4) VIEWER'S GUIDE TO THIS WEEK'S LYRID METEOR SHOWER
EXPLOREZONE, 20 April 1999
[...]
==============
(1) A SPACE CHARGE MODEL FOR ELECTROPHONIC BURSTERS
>From Luigi Foschini <L.Foschini@isao.bo.cnrdot it>
Dear Benny,
I would like to inform you about a new theory on electrophonic sounds,
that was published yesterday.
"A space charge model for electrophonic bursters"
by Martin Beech and Luigi Foschini
ASTRONOMY AND ASTROPHYSICS 345, (1999), L27-L31.
ABSTRACT: The sounds accompanying electrophonic burster meteors are
characteristically described as being akin to short duration "pops"
and staccato-like "clicks". As a phenomenon distinct from the enduring
electrophonic sounds that occasionally accompany the passage and
ablation of large meteoroids in the Earth's lower atmosphere, the
bursters have proved stubbornly difficult to explain. A straightforward
calculation demonstrates that in contradistinction to the enduring
electrophonic sounds, the electrophonic bursters are not generated as a
consequence of interactions between the meteoroid ablation plasma and
the Earth's geomagnetic field. Here we present a novel and hitherto
unrecorded model for the generation of short-duration pulses in an
observer's local electrostatic field. Our model is developed according
to the generation of a strong electric field across a shock wave
propagating in a plasma. In this sense, the electrophonic bursters are
associated with the catastrophic disruption of large meteoroids in the
Earth's atmosphere. We develop an equation for the description of the
electric field strength in terms of the electron temperature and the
electron volume density. Also, by linking the electron line density to
a meteor's absolute visual magnitude, we obtain a lower limit to the
visual magnitude of electrophonic burster meteors of M_{v}~ -6.6, in
good agreement with the available observations.
Greetings,
Luigi
Dr. Luigi Foschini
CNR - ISAO (formerly FISBAT and IMGA)
Via Gobetti 101, I-40129 Bologna (Italy)
Tel. +39 0516399620; Fax +39 0516399654
E-mail: L.Foschini@isao.bo.cnrdot it
URL: http://www.isao.bo.cnrdot it/~dinamica/
============
(2) ON PREDICTING THE TIME OF LEONID STORMS
>From http://wwwdot demon.codot uk/astronomer/leonid_maxima.html
On Predicting the Time of Leonid Storms
R. H. McNaught.
First published in The Astronomer, 1999 March.
The use of the orbit of 55P/Tempel-Tuttle in predicting Leonid storms
is discussed and rejected in favour of dust trail calculations. Asher
(1999) has calculated the perturbations to the individual dust trails
from ejection from the comet for three revolutions or more prior to any
potential encounter. These calculations confirm the encounter
circumstances given by Kondrat'eva et al. (1997), but with ten times
better resolution in the node of the dust trails. All observed storms
correlate closely with the Earth's passage within about 0.001 AU from a
dust trail. It is shown that the time of encounter with Asher's dust
trails very closely represent the observed times of maxima given in
Brown (1999) for all six storms (ZHR>1000) of the last 200 years.
Asher's dust trails indicate that the Leonid maxima in the next two
years will occur at:
1999 Nov. 18 02:08 UT
2000 Nov. 18 07:50 UT
with an uncertainty of +/- 90 minutes and possibly as small as +/- 10
minutes. The uncertainty is based on the close correspondence between
the time of encounter of the dust trails and the time of
observed storms in the last 200 years.
===============
(4) VIEWER'S GUIDE TO THIS WEEK'S LYRID METEOR SHOWER
>From EXPLOREZONE, 20 April 1999
http://explorezone.com/archives/99_04/20_lyrid_meteors.htm
By Robert Roy Britt, explorezone.com
04/20/99: Though the Lyrids are not high-profile as far as meteor
showers go, the annual event should still provide a good opportunity to
see ancient cosmic dust stream through Earth's atmosphere this week.
The Lyrids are named for Lyra, the constellation from which the
meteors, often called "shooting stars," seem to come. Provided skies
are clear, viewers may be treated to 15-20 meteors an hour late
Wednesday night and early Thursday morning, astronomers said.
How to see them
The best viewing will be in the wee hours of Thursday after the Moon,
now in its first quarter, has set.
"Lyra doesn't get very high in the sky until after midnight, so the
best time to look will be the early morning hours of Thursday, April
22," said Kevin Conod, astronomer at the Newark Museum's Dreyfuss
Planetarium in New Jersey.
Conod told explorezone.com that unlike some meteor showers, no hunting
is required to find the Lyrids.
"Although Lyra will be rising in the East, you don't necessarily have
to face in that direction," Conod said. "Meteors will appear all over
the sky. It is more important to find a spot where there are no lights
shining in your eyes and you can face the darkest, clearest part of the
sky."
Where the debris comes from
The Lyrid meteors are caused when Earth passes through a trail of
debris left behind by Comet Thatcher, which was discovered in 1861,
Conod said.
Comets are thought to be ancient storehouses of relatively pristine
cosmic material leftover from the early days of our solar system's
formation, 4 to 6 billion years ago. As a comet nears the Sun during
its often lengthy orbit, gas and dust burn off and are left behind.
When these grains of dust, which scientists then call meteoroids, hit
the atmosphere, they're name changes to meteors. They rub against air
particles and create friction, heating up to more than 3000 degrees
Fahrenheit. The heat vaporizes most meteors, creating what we call
shooting stars (most become visible at around 60 miles up).
Comet Thatcher takes 415 years to make its elongated trip around the
Sun, Conod said, and it won't pass through the inner solar system again
until the year 2276. The annual Lyrids, therefore, are created by the
debris left back in 1861.
What to expect this year
Though some scientists have noted a periodic increase in the Lyrids,
they haven't figured out why the activity fluctuates. Conod said this
year will probably be a typical year.
"It is very hard, if not impossible, to predict how many meteors we
will see in a shower," he said. "The Lyrids have occasionally produced
a very nice shower; in 1982 about 100 per hour were seen in the U.S."
Conod said that while the Perseids and Geminids are stronger, the
Lyrids are still worth a look, probably even for city dwellers.
"Given that it usually produces 15-20 meteors per hour, I'd give it "8"
or "9" on a Top-10 list of best showers," he said. ez
Copyright 1999, Explorezone
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