Long duration meteor echoes characterized by Doppler spectrum bifurcation

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 distinct Doppler spectral signature showing a pronounced Doppler bifurcation which includes narrow bands of discrete Doppler velocities, often of opposite polarity. The large signal to noise ratios and the narrowness of the spectra imply that coherent or Bragg scattering is not of relevance here, therefore these echoes do not associate with the long living meteor- induced backscatter ( MIB) from the 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.