Abstract
Some spectral echoes with short-living asymmetric ion-acoustic peaks, observed with the EISCAT UHF radar, are interpreted to be of meteoric origin. Under certain conditions meteor trails can scatter radio waves incoherently. Already a meteor of about 3 mm size causes enough ionisation that a diffusing trail after tens of seconds still can have enhanced ionisation. By this time the expansion rate of the trail radius per experiment integration time is small compared to the total radius. The enhanced plasma inside the trail approaches thermal equilibrium and can sustain ion-acoustic waves. A qualitative model for incoherent scatter from meteor trails is proposed, and a few examples of trail echoes are presented. They show strongly asymmetric incoherent ion-line spectra, which we assume to result from the Doppler shift caused by heavy meteoric ions in the drifting trail. The relation to other observations of enhanced ion-acoustic waves in connection to auroral displays is discussed. The applicability of the proposed model to the data is evaluated.
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