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[Programme]
Session 7: "Observations of Meteors
Using Large Aperture Radars"
Date: Wednesday
14.30-17.15
The Role of Large-Aperture V/UHF Radar Meteor
Observations in Meteor Science
J. D. Mathews (1), D. Janches (2,1), D.
D. Meisel (3,1), Q.-H. Zhou (4), S. Close (5) and
A. Pellinen-Wannberg (2)
1) Communications and Space Sciences Laboratory,
The Pennsylvania State University, University Park,
PA 16802-2707 USA; 2) Swedish Institute of Space
Physics, Box 812, S-981-28, Kiruna Sweden; 3) Dept.
of Physics & Astronomy, SUNY-Geneseo, Geneseo,
New York, 14454-1401 USA; 4) Arecibo Observatory,
Box 995, Arecibo, Puerto Rico 00613; 5) MIT Lincoln
Laboratory, Lexington, MA 02420-9108 USA
Meteor science based solely on "classical"
HF/VHF meteor radar observations was characterized
by a number of long-standing unresolved issues that
have been solved or refined with the advent of
radar meteor observations made using high-power,
large-aperture V/UHF radars. These radars include
those located at Arecibo Observatory, the Jicamarca
Radio Observatory, EISCAT, and the MU and ALTAIR
radars. Radio science issues successfully addressed
with the new observations include the origins of
"head-echoes" and anomalous trail-echos. Meteor
trails have been found to rapidly B-field align
throughout the 80-120 km altitude meteor-zone
giving rise to FAI (field-aligned irregularity)
scattering. Doppler observations have resolved
issues related to the speed-distribution of at
least micrometeoroids and micrometeoroid mass
fluxes have been found. Additionally, it is
becoming clear that assumptions regarding
ionization mechanisms for the smallest meteoroids
must be reexamined. Finally, we note the planetary
astronomy role of these radars in providing vast
numbers of micrometeoroid instantaneous orbits.
7.2
The High Power Large Aperture Radar Method
for Meteor Observations
Asta Pellinen-Wannberg (Swedish Institute of
Space Physics, Box 812, SE-981 28 Kiruna,
Sweden)
The high power large aperture radar meteor
method will be presented. It is compared to the
classical meteor radar method in terms of working
frequency ranges, beam widths, power and radar
energy fluxes at meteor altitudes to show their
crucial differences. The classical meteor radars
are sensitive to perpendicular meteor trails and
are thus perfectly suited for shower meteor
observations when orienting the radar beam
perpendicular to the radiant direction. By
contrast, the large aperture radars, due to their
working frequencies and high power, observe the
meteoroid-atmosphere interaction at all
look-angles. Tristatic measurements from the EISCAT
UHF system show that the scattering is isotropic up
to angles of 110°. Due to the high sensitivity
and narrow beams of these radars they mainly
observe the numerous populations of the very small
sporadic background particles. Hardly any
shower-related increase in fluxes has been observed
in the EISCAT Geminid, Perseid and Leonid
observations and in the Arecibo Leonid
observations. Simultaneously with meteor modes the
radars can operate in their usual incoherent
scatter modes to observe the electron density
variations in the background ionosphere. Thus
evolution of sporadic E layers, their average ion
composition and their relation to meteor activity
can be monitored. 7.1
A Problem of a Meteor Head Echo
A.Yu. Ol'khovatov (Radio Instrument Industry
Research Institute, Moscow, Russia)
An unresolved problem of meteor physics is the
meteor head echo, i.e. a radar target moving with a
meteor velocity. About a decade ago the author has
proposed that a head echo is caused by generation
of plasma waves in surrounding ionospheric plasma
and in meteor's ablation products. The ion beam
instability could be a one of sources of the plasma
waves, as at the head echo heights a density of
ions "sprayed and repelled" by a meteoroid is
rather high, and moreover, the ions are weakly
trapped by geomagnetic field. Probably these
processes at high altitudes were videotaped during
the 1998 Leonids campaign. Also maybe coupling with
some types of ionospheric waves is important also.
This interpretation of a head echo predicts some
shift between a meteoroid velocity calculated from
its trajectory, and from its Doppler radar return,
due to the plasma waves. And it seems that the
prediction are being confirmed. Recent radar data
indicate some difference between these two
velocities. PSB-9
Observations of Field-aligned Irregularities
in Meteor Trails Using the MU Radar
Qihou H. Zhou (1), Takuji Nakamura (2)
and John D. Mathews (3)
1) Arecibo Observatory, National Astronomy and
Ionosphere Center, Arecibo, Puerto Rico; 2) Radio
Science Center for Space and Atmosphere, Kyoto
University, Kyoto, Japan; 3) Communications and
Space Sciences Laboratory, Pennsylvania State
University, University Park, Pennsylvania
A large number of range-spread trail echoes
(RSTE) have been observed using the Kyoto
University Middle and Upper (MU) Atmosphere 46.5
MHz Radar. In fact, essentially all the head echoes
displaying an along-the-beam velocity component
were followed by range spread echoes in the
perpendicular-to-B pointing geometry. RSTE's are
typically observed a couple of hundred milliseconds
after the passage of the head echo. This indicates
the occurrence of plasma instability in meteor
trails. In addition, all the spectra are limited
within a bandwidth corresponding to a Doppler shift
of 320 m/s, suggesting that the two stream
instability is absent most of the time. The MU
observations show that the model of a smooth meteor
trail does not exist in reality. We will present
the characteristics of RSTE's and discuss the
implications of the MU observations on meteor
science and aeronomy applications. 7.3
Astronomical and Physical data for
Micrometeoroids Recorded by the ALTAIR
Radar
P. Brown (Los Alamos National Laboratory,
Los Alamos, New Mexico USA, and Department of
Physics and Astronomy, University of Western
Ontario, London, Ontario, CANADA), S. Hunt and S.
Close (MIT Lincoln Labs)
We present preliminary results of orbital and
physical measurements of a selection of meteoroids
observed at multiple frequencies by the ALTAIR
radar on Kwajalein island in November, 1998. The
head echoes observed by ALTAIR allow precise
determination of velocities and decelerations, from
which both ballistic parameters for individual head
echoes and orbits have been measured and will be
presented. The ALTAIR radar observes several
thousand head-echoes per hour and each head echo
has a known trail orientation relative to the beam.
Examination of the trail orientations produces an
estimate of the effective beam collecting area.
This collecting area when taken in conjunction with
the observed rate information, allows an
independent estimate of the limiting observed mass
of meteoroids observed by ALTAIR through comparison
with the known sporadic meteoroid flux. The
principle function of ALTAIR is as a contributing
sensor to the US Space Command satellite-tracking
network. ALTAIR is a high-power (5 Mw peak at both
frequencies), narrow beam (3° at VHF,
1.2° UHF), 43-m diameter mechanically steered
dish. ALTAIR transmits right circular polarized
energy and records left circular with a range
resolution of 15 m VHF and 7.5 m UHF meters.
Azimuth and elevation difference channel data are
also measured, which contribute to the accurate
determination of target position in three
dimensions. The aforementioned characteristics
allow ALTAIR to reliably detect a -62 dBsm target
in VHF and a 81 dBsm target at UHF at a range of
100 km. Examples of the head echoes observed by
ALTAIR and some indications of the likely origin
for the population observed by ALTAIR during this
campaign will also be discussed. PSB-8
Two-frequency Meteor Observations Using
ALTAIR
Stephen Hunt and Sigrid Close (MIT
Lincoln Laboratory, 244 Wood Street, Lexington MA,
02173, USA)
We present a sample of radar meteors detected
during the November 1998 Leonid shower that were
collected using the ARPA Long Range Tracking and
Instrumentation Radar (ALTAIR). A total of 29
minutes of VHF data were collected near the peak of
the shower, which produced over 900 head echoes; 17
minutes of simultaneous UHF data were collected,
which resulted in over 500 head echoes. These data
were analyzed to determine frequency-dependent
radar cross section and shape characteristics. In
addition, the azimuth and elevation data were used
to compute the true velocity and deceleration of
the head echoes, which produced the result that a
meteoroid's deceleration is not constant. Finally,
the first head echo that was detected using three
frequencies (160 MHz, 422 MHz, 1320 MHz) is
discussed.
ALTAIR is a highly sensitive, two-frequency
radar that is uniquely suited for detecting meteor
head echoes. ALTAIR transmits right circular
polarized energy and records both left circular and
right circular data, as well as azimuth and
elevation difference channel data. These data allow
the accurate determination of a target's position,
velocity and deceleration. ALTAIR's system
sensitivity allows the reliable detection of a -55
dBsm target in VHF and a -75 dBsm target at UHF at
a range of 100 km. 7.4
Meteor Head Echo Observations Using the
Millstone Hill/MIDAS-W UHF Incoherent Scatter Radar
System
Philip J. Erickson and Frank D. Lind
(Atmospheric Sciences Group, MIT Haystack
Observatory)
UHF incoherent scatter radars can study meteor
influx into the upper atmosphere through recording
so-called meteor head echoes, which result from
scattering of the incident wave from structures
surrounding an inbound meteor as it ablates in the
80 - 120 km altitude range. The Millstone Hill 440
MHz incoherent scatter radar, with its associated
68 m zenith and 46 m steerable antennas and 2.5 MW
peak power transmitter, was used as early as 1962
in observations leading to a seminal series of
papers by John Evans describing UHF meteor
scattering characteristics. Recently, we have
employed the Millstone system, using a prototype
wideband data acquisition system (MIDAS-W) and a
Barker code transmission scheme similar to work
done at EISCAT, in a series of head echo detection
experiments during the Leonid meteor showers of
November 1999 and November 2000. We describe the
capabilities of the MIDAS-W system for meteor
research, and present selected results of these
Leonid observations along with future plans.
7.5
Meteor Trail Evolution: Comparison between
ALTAIR Radar Observations and Plasma
Simulations
Lars Dyrud, Meers Oppenheim, Sigrid
Close, Stephen Hunt, Axel vom Endt and Kelley Mc
Millon (Center for Space Physics, Boston
University, 725 Commonwealth Ave, Boston, MA 02215,
USA)
We present the first direct comparisons between
plasma simulations and radar observations to
explain the the most salient features of
non-specular meteor trail echos. The radar
observations were obtained during Leonids 1998 with
the highly sensitive ALTAIR radar in the Kwajelein
Atoll. Plasma simulations demonstrate that meteor
trails are unstable to growth of gradient-drift
Farley-Buneman (GDFB) waves that become turbulent
and generate large B-field aligned irregularities
(FAI). These results indicate that the non-specular
echos, that can extend between 5-10 km in altitude
range, are reflections from plasma instability
generated FAI. The simulation results can explain a
number of characteristics of these non-specular
observations. The observed altitudinal extent of
the trail echos. Trail diffusion in the plane
perpendicular to B that can be up to an order of
magnitude larger than expected from ambipolar
diffusion. echos. Additionally, meteor and
atmospheric properties including neutral
temperature, neutral-ion collision frequency, and
meteoric ion mass may be inferred with greater
accuracy then previously possible. 7.6
Tristatic measurements of meteors using the
930 MHz EISCAT radar system
Diego Janches (1, 2), Asta
Pellinen-Wannberg (1), Gudmund Wannberg (3), Assar
Westman (3) and Ingemar Häggström (3)
1) Swedish Institute of Space Physics, Box 812,
SE-981-28, Kiruna, Sweden; 2) Communication and
Space Sciences Lab, Penn State University,
University Park, PA, 16802, USA; 3) EISCAT
Scientific Association, Box 164, SE-981 23, Kiruna,
Sweden
We report results from the first tristatic
measurements of radar meteors obtained during
November 17-18, 1997 and 1998, using the UHF (930
MHz) EISCAT radar system. This system consists of
three antennas located in Tromso, Norway (69.6 N;
19.2 E); Kiruna, Sweden (67.9 N; 20.4 E) and
Sodankyla, Finland (67.4 N; 26.6 E). Since EISCAT
observes mostly head-echoes, a general
characteristic of high power/large aperture radars,
very precise Doppler velocity determination are
possible. In addition using the technique reported
here, absolute geocentric meteor velocity and
precise radiant information are deduced for those
meteors that are detected simultaneously by all
three receivers. An overview of the methodology and
a summary of the results obtained so far are
reported in this work. To the best of our
knowledge, these observations represent the first
of their kind and prove EISCAT to be a crucial
instrument for the study of extraterrestrial
particles entering the earth's atmosphere, in
particular at very high geocentric latitudes.
7.7
Sondrestrom ISR Meteor Measurements
C.J. Heinselman and J. Jørgensen
(SRI International, 333 Ravenswood Ave, Menlo Park,
CA 94025, USA)
Measurements of the meteoric phenomena with the
Sondrestrom ISR at 1290 MHz have previously been
limited to studies of sporadic E layers and their
relationship to infrequent, strong meteor echoes.
Recent enhancements to the Sondrestrom data
acquisition system have enabled the development of
data collection codes better suited to probing the
meteor head echo phenomena at this frequency.
Initial results will be presented of measurements
from recent meteor showers. Future plans will also
be discussed for making more routine meteor
measurements with the Sondrestrom system. 7.8
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