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

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

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.

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.

Fig. 3 Meteor echo with Doppler bifurcation observed at 50 MHz by SESCAT in Crete

Figure 4a (left) Meteor echo with 2 spectral lines. Figure 4b (right) Meteor echo with 3 spectral lines and large Doppler spread.

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

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.

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.

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.