Long duration meteor echoes characterized by doppler sprectrum bifurcation
Résumé
We report on a new category of long lasting meteor echoes observed occasionally with HF and VHF radars. These meteoric returns, which have lifetimes from many seconds to a few minutes, are characterized by a dis-tinct Doppler spectral signature showing a pronounced Doppler bifurcation which includes narrow bands of dis-crete Doppler velocities, often of opposite polarity. The large signal to noise ratios and the narrowness of the spec-tra imply that Bragg scattering cannot be the generation mechanism, therefore these echoes do not associate with the long living meteor-induced backscatter (MIB) from the unstable lower E region. A reasonable interpretation needs to explain both the Doppler spectrum bifurcation and the long echo duration. As such, we propose the idea of a structured vertical wind shear in the lower E region which traps different fragments of a meteor trail plasma in the same way that sporadic E layers form. These trail parts inside the shear-related wind profile may act as relatively long-lasting meteoric reflectors moving with different Doppler velocities, also of opposite polarity.