The characteristics of meteor trails observed at a frequency of 300 Mc/s

Abstract

Some observations of radar echoes from meteor trails at frequencies of 300 Mc/s and 32 Mc/s, carried out using transmitters installed in the 25 m radio telescopes at the Royal Radar Establishment, Malvern, are described. The 32 Mc/s observations have been used to assist in the interpretation of echoes occurring simultaneously at the higher frequency, and it is shown that the intensities of the echoes observed at 300 Mc/s are of the order of 80 dB below the values predicted by simple theory. The theoretical limiting sensitivity of the 300 Mc/s equipment was such that a meteor trail of electron line density α = 2 × 10 8 cm −1 would give a detectable echo. In fact all the observed echoes were from trails with α > 2 × 10 12 cm −1, giving rise to “long duration” echoes at 32 Mc/s. Doppler measurements at 300 Mc/s show that the majority of trails still behave as specular reflectors to within ± 0.6° at this frequency, showing that the effective scattering length is usually about 50 m. In order to explain the low echo intensities from trails with such high values of α it is necessary to postulate that the ionised columns are formed initially with radii of the order of 1.5 m, so that the volume electron density is always less than the critical density at 300 Mc/s. It is deduced that the overall length of the ionised trail for meteors with 2 × 10 12 < α < 10 13 cm −1 is only about 3 km.