The decay of radar echoes from meteors with particular reference to their use in the determination of temperature fluctuations near the mesopause

Abstract. The rate of decay of a radar echo from an ionised meteor train will be governed by the diffusion coefficient of the plasma and this in turn will depend on the temperature. Very recently the temperature fluctuations near the mesopause have been monitored by this means, by the recording of the decay times of underdense trains. The usual derivation of the precise expression relating the underdense echo decay time to the temperature contains two important assumptions, (i) that the train is created with a Gaussian ionisation profile, and (ii) that kinetic theory may be applied to calculate the diffusion coefficient. We investigate the effect of these assumptions, showing that the first assumption is unnecessary, an underdense backscatter echo decaying exponentially with a decay time equal to λ²/(32π²D), where λ is the wavelength and D the diffusion coefficient, independently of the initial distribution. However, the second assumption is shown to be incorrect, and whereas according to kinetic theory D∝T^1/2/ρ, where T and ρ are the atmospheric temperature and density, the correct result is D∝Tρ. This leads to an appreciable correction to the results for the temperature fluctuations.