Long duration meteor echoes characterized by Doppler spectrum bifurcation A. Bourdillon(1), C. Haldoupis(2), C. Hanuise(3), Y. Le Roux(4), J. M

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Long duration meteor echoes characterized by
Doppler spectrum bifurcation A. Bourdillon(1),
C. Haldoupis(2), C. Hanuise(3), Y. Le Roux(4),
J. Ménard(4)

• (1) University of Rennes / CNRS, France(2)
  University of Heraklion, Crete, Greece(3) LPCE /
  CNRS, France(4) ENST Bretagne, France

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Experiment in Puerto Rico

• HF portable radar uses the SCIPION sounder and a
  receiving linear array made with 16 magnetic
  loops
• A Yagi antenna was used to transmit at 14.5 MHz
• A 2 kW transmitter

Fig. 1 The HF receiving antenna
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• Introduction In addition to the short-lasting
  and fairly-well understood underdense /overdense
  meteors, there are occasional reports on long
  lasting meteor echoes with lifetimes ranging from
  several seconds to many minutes. Such long
  enduring meteor events are rather exceptional and
  constitute a mystery. On the other hand, a more
  common category of meteor echoes with lifetimes
  ranging from a few seconds to a few minutes, has
  been identified with VHF coherent backscatter
  radars at middle, low and equatorial magnetic
  latitudes. These are magnetic aspect sensitive
  echoes which are spread in range and have broad
  Doppler spectra similar to those obtained when
  the E region plasma becomes strongly unstable to
  the gradient drift and the modified two stream,
  or Farley, instabilities. Therefore, these echoes
  were attributed to meteor-induced backscatter
  (MIB) from the lower Eregion caused by Bragg
  scattering from meter-scale plasma
  irregularities.

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• Figure 2a (left) shows a Doppler spectrum with
  several discrete lines from -70 to 100 m/s.
• Figure 2b (right) shows 2 discrete lines with
  different duration.

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Fig. 3 Meteor echo with Doppler bifurcation
observed at 50 MHz by SESCAT in Crete
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• Figure 4a (left) Meteor echo with 2 spectral
  lines.
• Figure 4b (right) Meteor echo with 3 spectral
  lines and large Doppler spread.

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• Figure 5a (left) Echo with a diffuse type
  spectrum.
• Figure 5b (right) Echo with 2 spectral lines and
  large Doppler spread.

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Discussion

• Here we report on rare events of long lasting
  meteor echoes having a lifetime ranging from
  several seconds to a few minutes which have been
  observed at HF (15 MHz) and VHF (50 MHz). This
  type of meteor returns have a distinct Doppler
  spectral signature characterized by a bifurcation
  and strong narrow Doppler striations often of
  opposite polarity. The spectral properties imply
  that Bragg scattering cannot be the generation
  mechanism behind the meteor echoes here,
  therefore these do not associate with long living
  meteor-induced backscatter from the lower E
  region.

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• The available experimental evidence is not
  sufficient to substantiate a reasonable
  interpretation, thus the options discussed below
  are speculative. The spectral evidence here is
  suggestive of specular reflections, either
  underdense and/or overdense, coming from a
  distorted meteor trail with parts undergoing
  discretely different motions. Suggestions have
  been made in the past, e.g., see McKinley 1961,
  that vertical shears in the mesosphere-lower
  thermosphere winds distort the ionization trail
  such that portions of it become perpendicular to
  the radar beam and thus act as specular
  reflectors. These reflecting trail fragments
  could account for the observed discrete spectral
  structuring and opposite Doppler shifts.

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• But why this fragmented meteor trails last so
  long? Unfortunately, there is not an easy answer
  since long duration meteors remain a mystery for
  many years. Given that the Doppler structures can
  be caused by the fragmentation of the meteor
  trail by a vertical wind shear, we postulate that
  the ionization could be maintained against
  diffusion by ion convergence in the same way a
  sporadic E layer forms in the lower E region
  under the action of a vertical wind shear in the
  zonal wind.