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(meteorobs) Excerpts from "CCNet 52/2001 - 4 April 2001"
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From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet, 4 April 2001
Date: Wed, 4 Apr 2001 10:00:41 +0100
CCNet 52/2001 - 4 April 2001
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[...]
(9) EM EFFECTS CAUSED BY IMPACTS OF LARGE METEOROIDS
Ivan V. Nemtchinov <ivvan@idg1.chph.ras.ru>
[...]
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* LETTERS TO THE MODERATOR *
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(9) EM EFFECTS CAUSED BY IMPACTS OF LARGE METEOROIDS
>From Ivan V. Nemtchinov <ivvan@idg1.chph.ras.ru>
Dear Benny,
In CCNet on March 22 James Perry wrote: "I am interested in the assertion
that asteroid impacts generate Electromagnetic Pulse. I have never heard it
before. I thought EMP was generated only by man-made sources (e.g. nuclear
explosions)". On March 22, 2001 Luigi Foschini and Collin Keay commented on
electrophonic bursts, which were detected during the atmospheric entry of
large meteoroids.
Duncan Steel draws attention to the fact that British Antarctic Expedition
observed a strong aurora in the long Arctic winter, near the time of the
Tunguska event, but several hours earlier. It stimulates the thought that if
the Tunguska object was actually cometary in nature, the Earth would have
been within cometary the ion tail as it approached from the sunward side of
our planet.
Gerrit Verschnur in his reply mentioned that Shoemaker-Levy 9 comet
fragments smashed into the Jupiter's southern hemisphere but they triggered
auroras at the mirror points in its Northern hemisphere.
Please note that SL-9 fragments were about 200-400 meters in size. We should
keep in mind that the Jupiter's magnetic field strength is by an order of
magnitude larger than at the Earth, Jupiter's size is by an order of
magnitude larger than that of the Earth, so the total energy of the
Jupiter's magnetosphere is by five order of magnitude larger than that of
the Earth. So magnetic disturbances caused by impacts onto the Earth with
the same kinetic energy will be much larger than in the case of Jupiter.
We should add some additional consideration.
EM EFFECTS CAUSED BY IMPACTS OF LARGE METEOROIDS
Estimates of electromagnetic (EM) effects caused by the impacts of large
meteoroids were given by Adushkin and Nemtchinov (1994). 2D numerical
simulations of gasdynamic processes associated with impacts were conducted
for Tunguska-class and larger bodies. For stony and icy bodies with sizes of
200 m and larger the impactors reach the ground without substantial
deceleration and create a vapor plume which rises upward with the maximum
velocity approximately equal to a half of the impact velocity. We note that
the energy of a 200-m icy body with the velocity of 50 km/s is about 1000
Mt.
The conical jet moving upwards produces a shock wave. The air heated up to
1-3 eV is strongly ionized and acts like a conducting piston, expulsing the
Earth's magnetic field. But even for lower velocities and temperatures of
the air expulsion of the magnetic field occurs as the air at high altitudes
is already ionized. Large-scale deformation of the magnetosphere and
disruption of the Van Allan radiation belts will be the result.
It was suggested in Adushkin and Nemtchinov (1994) that substantial
ionospheric heating, large amplitude oscillations of the ionospheric and
magnetospheric plasma, precipitation of trapped particles from the radiation
belts may be the result of impacts of smaller meteoroids.
Numerical simulations of atmospheric entry processes and long-term
disturbances of ionosphere caused by an icy body with radius of 30 m and
velocity of 30 m were conducted recently by Shuvalov and Artemieva (2001)
and Shuvalov (2001). They take into account fragmentation and evaporation.
If was shown that such a body completelly evaporates at an altitude of about
10 km but the debris reach the minimum height of about 4 km (close to the
estimated altitude of the Tunguska explosion).
The most part of hot air and vapor accelerates upward. The density of a
plume is higher than the ambient air density. Up to about 400-500 km the
hydrodynamic approximation is valid for the shock wave.
The jet continues to move upwards ballistically and expands radically by
inertia and at the moment of about 500 s reaches altitudes of about 1500 km,
having a radius of 1000 km, and falls back due to gravity. Meeting more
dense layers of atmosphere, it compresses and heats air in a shock wave. At
the moment of 700 s a thin dense and hot (several times higher than the
normal temperature of the air at the altitudes under consideration) disk is
formed. It begins to expand upwards and radially (at the moment of 900 s up
to 500 km while radius reached 1500 km). The lower boundary of the
disturbance remains at an altitude of about 100 km, where the boundary
between the strongly stratified and weakly stratified atmosphere is located.
The acoustic-gravity wave propagating from the zone of reentry impact will
further increase the size of the zone of ionospheric disturbances. Large
amplitude oscillations of the ionosphere in large area will obviously cause
disruption of short radiowave communications in the very large region of the
Earth as the altitude of radio wave reflection will change in time and will
be corrugated.
Such effects have been observed in Northern Europe in 1961 after the nuclear
test at Novaya Zanlya with the yield of 58 Mt at the altitude of 3.5 km.
This energy and altitude are close to the values estimated for the Tunguska
event.
The Tunguska event of 1908 occurred at the eve of the radiowave era and no
short-period EM signals were detected. But observatory at Irkutsk (900 km
from Vanavara) detected magnetic disturbances several minutes after the air
blast. An attempt to interpret these signals was made by Nemtchinov et al.
(1999). Oscillations of the atmosphere were analyzed. Simulations show that
they lasted for at least 1500 sec. These oscillations finally decay, but
rise the temperature at the altitudes of 100-150 km to about 1000-15000 K.
This decreases density of the atmosphere in that region at high altitudes.
According to Boslough and Crawford (1997) for the oblique impact of the
Tunguska-class object the plume mainly moves along the wake, i.e. there is
the horizontal advection in the direction opposite to the impact velocity,
i.e. towards Baikal, and that may explain earlier start of the geomagnetic
signal in Irkutsk.
The local increase of conductivity changes the system of ionospheric
currents and creates the magnetic disturbances. The calculated magnetic
field values are close to the observed ones in Irkutsk.
The theoretical consideration does not contradict observations. We
anticipate that global disturbances of the ionosphere and magnetosphere for
the meteoroids of the size of about 100 m will become global and could not
be neglected in estimates of hazards in our information age.
All the estimates given above are of a preliminary character. To make more
detailed analysis and prediction of the impact consequence, including
magnetohydrodynamic and EM signals, we proposed to start a Research program
of the Tunguska-class impactors. It is submitted to the International
Science and Technology Center in Moscow (Project 1814). Its text can be
found in the ISTC Web Site (www.istc.ru). We hope that the ISTC board to be
held in July will accept this program.
We note that the nuclear explosions disturbances substantially differ from
these caused by impacts, in the latter case we do not have intense
ionization by the penetrating radiation (neutrons, gamma quanta and X-rays).
Nevertheless to use all the experience attained in the course of the nuclear
tests analysis, the scientists from the Russian Federal Nuclear Center
Arzamas-16 were incorporated into the team.
It is also suggested to use the results of the laboratory experiments and
geophysical experiments with jets created by cumulative generators using
high-explosives developed in IDG and detonated at altitudes of 150-300 km
(see e.g. Gavrilov et al., 1999). These generators produce high-velocity
jets (with velocities up to 40-50 km/s, well in the impact velocity range of
meteoroids).
We have established collaboration with the US National Laboratories (Los
Alamos and Sandia), and DLR Institute of Planetary Exploration in Berlin. We
are searching other possible collaborators.
References
Adushkin V.V., and Nemchinov I.V. 1994. Consequences of impacts of cosmic
bodies on the surface of the Earth. Hazards due to Comets and Asteroids (Ed.
T.Gehrels). Univ. Arizona Press, Tucson and London, 721-778.
Boslough M.B., and Crawford D.A. 1997. Shoemaker-Levy 9 and plume forming
collision on Earth. Near-Earth Objects (Ed. J.L.Remo). Annals of the New
York Academy of Sciences V.822. New York, 236-282.
Gavrilov B.G., Podgorny A.I., Podgorny I.M., Sobyanin D.B., Zetzer J.I.,
Erlandson R.E., Meng C.-I., Stoyanov B.J. 1999. Diamagnetic effect produced
by the Fluxus-1 and 2 artificial plasma jet. Geophys. Res. Lett. 26 (11),
1549-1552.
Nemtchinov I.V., Losseva T.V., Merkin V.G. 1999. Estimate of geomagnetic
effect caused by Tunguska meteoroid impact. Physical processes in
geospheres: its manifestations and interaction. Institute for Dynamics of
Geospheres RAS, Moscow, 324-335 (in Russian).
Shuvalov V.V 2001. Atmospheric entry of Tunguska-like meteoroids: 2D
numerical model. Lunar and Planet. Sci. (Houston) XXXII, # 1124.
Shuvalov V.V., and Artemieva N.A. 2001. Long-term disturbances of ionosphere
caused by Tunguska-like impacts. Lunar and Planet. Sci. (Houston) XXXII, #
1123.
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