Radial reflection

An ablating meteoroid not only deposits ionization along its path (and that ionization quickly attains dynamic equilibrium with the ambient atmosphere so is stationary unless transported by the atmospheric neutral wind) but also creates a plasma spheroid surrounding the meteoroid itself. This plasma ball shares the meteoroid's motion. The scattering from such a plasma ball produces what is termed a 'head echo': the scattering cross-section depends on the radar wavelength but the reflection coefficient is very small compared to that for transverse reflection (the body echo) so that the echo is not not discernible for orthogonal geometry. However, if the geometry is radial so that the meteoroid is moving in the line-of-sight then the body echo is absent and the head echo dominates. The radar-approaching plasma ball acts as a moving target that directly represents the meteoroid atmospheric speed: the echo will rapidly decrease in range traversing successive range bins and also with a phase-sensitive radar system rapid phase changes will occur. Notice that for radar sampling pulse rates even as high as 1 kHz the plasma target will move through several wavelengths between samples and results in phase aliassing: however, the range shift and phase changes can be combined to produce an accurate (uncertainty ~ 0.3%) radial speed.

With a single station radar the trajectory aspect angle is unknown so that there is an uncertainty in the radial speed and direction. For accurate results therefore, a narrow pencil beam ~ 1° is required and provision for measuring the across-beam angle. Using such, both the meteor trajectory (the upstream direction of which is termed the 'radiant') and speed can be deduced and hence, after appropriate transformations and corrections, the heliocentric orbit.

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