Long duration meteor echoes characterized by Doppler spectrum bifurcation

Abstract

[1] 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. Citation: Bourdillon, A., C. Haldoupis, C. Hanuise, Y. Le Roux, and J. Menard (2005), Long duration meteor echoes characterized by Doppler spectrum bifurcation, Geophys. Res. Lett., 32, L05805,

Interferometry observations of meteor trail irregularity using the Sanya VHF radar

“…It was found that the drifts of trail irregularity have an obvious shear from 50 ms -1 to -50 ms -1 . The shear structure of the winds observed in this study reminds us of suggestions made by Bourdillon et al [7] that the structured wind shear drives parts of the trail plasma in a way similar to the process responsible for the formation of sporadic E layer, and sustains the trail for a long time. In this context, the cause responsible for the structural evolution of trail irregularity from the initially formed strip-type to layer-type could be linked with the neutral wind effect.…”

“…On the other hand, it is known that the durations of meteor trail at different altitudes are dictated primarily by the external electric field/wind, diffusion rate and the polarization of meter trail as a function of altitude [6]. In particular, on the basis of the Doppler spectral bifurcation or Doppler velocity shear characteristics associated with long duration RSTEs, the neutral wind shear has been suggested to be a possible candidate to account for the observed long duration RSTEs at a specific altitude [7,10]. By taking a similar way as Li et al [5], the drifts of trail irregularity associated with the long duration RSTEs were obtained.…”

“…But for long lasting meteor echoes with lifetimes ranging from tens of seconds to several minutes, such as the trails observed by VHF/HF radars at middle latitudes, the generation mechanism is still not well understood. From the obvious change of RSTE Doppler velocity at different range bins, some investigators [7,10] invoked the neutral wind shear driven instability as a possible candidate to account for the observed long duration RSTE events. Later, Dyrud et al [6] demonstrated that the long duration meteor trails, which shown as a triangular shaped profile in radar range-time-intensity maps, are a natural consequence of the meteoroid ablation and plasma instability under the effect of strong wind or electric field.…”

Thermospheric Neutral Wind Measurement Using the LWA1 and the AFRL Digisonde

“…However, because the signal strength depends on the inverse-cube of the distance to the meteor, there should be many very bright nearby RFI sources on the horizon but these are not observed. This pattern is also inconsistent with non-specular reflections, which are preferentially located in a relatively small region of the sky that satisfies the requirement that the pointing vector is perpendicular to the geomagnetic field (Bourdillon et al 2005;Close et al 2011). For these reasons it seems unlikely that forward scattering is responsible for the signals detected from fireballs.…”

“…The observed transients are also inconsistent with light curves from non-specular echoes, which vary greatly from one to the next, following no particular pattern. More importantly, however, non-specular echoes are weaker and more rare than over-dense specular echoes (Bourdillon et al 2005;Close et al 2011). Therefore if the LWA1 were seeing non-specular reflections it should also see many more bright specular reflections scattering from the same transmitters.…”