Transverse reflection

Here the trajectory of the meteor is orthogonal to the (mono-static) radar. The scattering of radio waves by the ionization created by the meteor can be analysed in terms of Fresnel diffraction and the analysis has a convenient analogue in optical diffraction at a straight edge. For meteor scattering the Fresnel zone length is about 1 km for HF radars and as ionization is progressively deposited more Fresnel zones contribute with different phases and in summation most of the reflected energy is produced from a region on the meteor train of length ~ 1 km centred at the geometrically orthogonal point. The instant in time when the meteoroid reaches that orthogonal position is termed the t0 point and the received radar signal is termed the 'body echo'.

The ionization column (cylindrical in the absence of an external magnetic field) is created with a finite diameter: additionally ambipolar diffusion of the plasma will lead to an increasing column diameter with time: if the column size is comparable to the operating wavelength phase differences in the scattering from individual electrons in a train cross-section will result in destructive interference and a reduction in the reflected energy.

The time-history of the reflected energy to produce a radar echo can be conveniently analysed with the aid of the Cornu spiral (depicting phase behaviour) with the presence of ambipolar diffusion (leading to an exponential decay of the meteor echo) introducing a modification of the classical behaviour. In the absence of meteoroid fragmentation or irregular plasma the frequency of post t0 amplitude oscillations give a measure of the meteor's scalar speed. Conversely the post t0 phase oscillations are too small (< 30°) to be useful speed indicators whereas the large pre-io phase changes are valuable for meteoroid speed measurements. Radars with phase capability can employ the pre-io rapid phase changes to secure accurate speed measurements because the ionization train in its initial formation has no adverse effects arising from train diffusion, no ionization irregularities and no disruption by grain fragmentation and for small times atmospheric wind shear has not sufficient time to operate. Good examples of echo behaviour are well illustrated in Elford [1] Figures 1 and 2. A contributing factor to the suppression of post to amplitude oscillations is the presence of continuous fragmentation along its trajectory of the ablating grain. If, on plasma train creation, the orthogonality condition does not hold so that the central Fresnel interval is outside the main radiation pattern of the radar, then the classical meteor echo is not formed so that the rapid leading edge is absent: however, the phase changes are still present and speed measurements can be made on such echoes (see Elford [1] Figure 3.)

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