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[1] The midlatitude sporadic E layers form when metallic ions of meteoric origin in the lower thermosphere are converged vertically in a wind shear. The occurrence and strength of sporadic E follow a pronounced seasonal dependence marked by a conspicuous summer maximum. Although this is known since the early years of ionosonde studies, its cause has remained a mystery as it cannot be accounted for by the windshear theory of E s formation. We show here that the marked seasonal dependence of sporadic E correlates well with the annual variation of sporadic meteor deposition in the upper atmosphere. The later has been established recently from long-term measurements using meteor radar interferometers in the Northern and Southern Hemispheres. Knowing that the occurrence and strength of sporadic E layers depends directly on the metal ion content, which apparently is determined primarily by the meteoric deposition, the present study offers a cause-and-effect explanation for the long-going mystery of sporadic E layer seasonal dependence.

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Horizontal wind fields in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the 09. atmospheric band. The validity of the derived winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind-induced Doppler shift. Relative changes in these positions are readily identified in the routine measurements of onboard calibration lines. The determination of the absolute values relies on the comparison of HRDI observations with those obtained by MF radars and rockets.In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effects of these parameters both on the amplitude of the HRDI-derived diurnal tidal amplitude and on the variance of the wind differences with correlative measurements. This paper describes these validation procedures and presents comparisons with correlative data. The main discrepancy appears to be in the relative magnitudes measured by HRDI and by the MF radar technique. Specifically, HRDI generally observes larger winds than the MF radars, but the size of the discrepancy varies significantly between different stations. HRDI wind magnitudes are found to be somewhat more consistent with measurements obtained by the rocket launched falling sphere technique and are in very good agreement with the wind imaging interferometer (WINDID, also flown on UARS. 1. Introduction The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) is designed to measure horizontal winds in the mesosphere and lower thermosphere (50-115 kin) and in the stratosphere (10-40 kin), as part of a coordinated mission which is aimed at an improved understanding of global atmospheric change [Reber et al., 1993]. One of the most important objectives of the HRDI project is to develop a comprehensive global climatology of the winds in the atmosphere from 50 to 115 kin. This will serve as a reference point for future investigations and will provide comparison for global models. Prior to UARS the understanding of the global circulation in the Paper number 95JD01700. 0148-0227/96/95JD-01700505.00 upper mesosphere was based primarily on a collection of localized (ground-based or in situ) observations. Many studies of this region have employed the view of the circulation based on the COSPAR International Reference Atmosphere (CIRA). The widely used CIRA-72 model was derived from data obtained with meteorological rockets prior to 1970. The more recent CIRA-86 contains global gradient winds derived from Nimbus 5 and 6 satellite radiance data in the altitude range 20-80 km and MSIS-83 [Hedin, 1983] satellite and ground-based data in the altitude range 80-120 kin. Wind measurements of the mesosphere and lower thermosphere (MLT) obtained from a network of MF and meteor detection radars show features not present in the simplified CIRA view o...