Super Dual Auroral Radar Network observations of fluctuations in the spectral distribution of near range meteor echoes in the upper mesosphere and lower thermosphere

The Doppler shifts of meteor echoes measured by the SuperDARN HF radar netw ork have been used in several studies to observe neutral winds in the upper mesosphere and lower thermosphere region. In the absence of accurate heigh t information for individual meteors, it has been necessary to assume a sta tistical mean meteor layer where the variations in altitude were not correl ated to changes in the horizontal winds. Observations of spectral width dis tribution variations made by the radars allow an independent determination of the systematic error in the height. We have investigated the dependence of this distribution on a number of factors including the radar geometry, d iurnal and seasonal cycles, variations in solar UV irradiance and geomagnet ic activity. Changes in the altitude of the mean meteor layer observed at different rada r ranges provide us with some insight into the structure of the upper mesos phere and the lower thermosphere within which the meteors are being ablated . An examination of the spectral widths, as measured by the CUTLASS Finland radar, in the days preceding and following a Storm Sudden Commencement in April 1997, illustrates how the spectral properties of the observed region can be affected. The variations in the widths were consistent with model ca lculations of the changes to the temperature profile over this interval. Fu rther refinements in the determination of the spectral width are outlined f or future experiments.