[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.

[1] A westward propagating zonal wave number 1 wave with a period near 6.5 days was a prominent feature in the mesosphere and lower thermosphere (MLT) during the 1994 equinoxes. The meridional structure of the wave in the upper stratosphere and the MLT is consistent with the 5-day wave structure predicted by normal mode theory. However, the amplitude increases sharply above 80 km, where the wave exhibits a highly organized baroclinic circulation. The eddy fluxes and the background state suggest that the wave is amplified by instability of the mesospheric winds.

Abstract. Radar echoes from ranges less than 500 km are routinely observed by the Super Dual Auroral Radar Network (SuperDARN) on most days. Many of these echoes have properties which are markedly different from what one would expect from E or F region irregularities. We show that these unusual short-range HF echoes are due to scattering off meteor trails. This explains why, among other things, the Doppler shift from the short-range echoes taken from the SuperDARN Saskatoon antenna are consistent with the mesospheric winds observed by the Saskatoon MF radar. This means that the SuperDARN radars can be used to study neutral winds at meteor heights, a result which is especially interesting since it opens up the capability for a global coverage of mesospheric winds using the worldwide distribution of SuperDARN radars.

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Abstract. The "Scandinavian Triangle" is a unique trio of radars within the DATAR Project (Dynamics and Temperatures from the Arctic MLT (60-97 km) region): Andenes MF radar (69 • N, 16 • E); Tromsø MF radar (70 • N, 19 • E) and Esrange "Meteor" radar (68 • N, 21 • E). The radar-spacings range from 125-270 km, making it unique for studies of wind variability associated with small-scale waves, comparisons of large-scale waves measured over small spacings, and for comparisons of winds from different radar systems. As such it complements results from arrays having spacings of 25 km and 500 km that have been located near Saskatoon. Correlation analysis is used to demonstrate a speed bias (MF smaller than the Meteor) between the radar types, which varies with season and altitude. Annual climatologies for the year 2000 of mean winds, solar tides, planetary and gravity waves are presented, and show indications of significant spatial variability across the Triangle and of differences in wave characteristics from middle latitudes.

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A new methodology of ionosonde height–time–intensity (HTI) analysis is introduced which allows the investigation of sporadic E layer (Es) vertical motion and variability. This technique, which is useful in measuring descent rates and tidal periodicities of Es, is applied on ionogram recordings made during a summer period from solstice to equinox on the island of Milos (36.71N; 24.51E). On the average, the ionogram HTI analysis revealed a pronounced semidiurnal periodicity in layer descent and occurrence. It is characterized by a daytime layer starting at 120km near 06 h local time (LT) and moving downward to altitudes below 100km by about 18 h LT when a nighttime layer appears above at_125 km. The latter moves also downward but at higher descent rates (1.6–2.2 km/h) than the daytime layer (0.8–1.5 km/h). The nighttime Es is weaker in terms of critical sporadic E frequencies (foEs), has a shorter duration, and tends to occur less during times close to solstice. Here, a diurnal periodicity in Es becomes dominant. The HTI plots often show the daytime and nighttime Es connecting with weak traces in the upper E region which occur with a semidiurnal, and at times terdiurnal, periodicity. These, which are identified as upper E region descending intermediate layers (DIL), play an important role in initiating and reinforcing the sporadic E layers below 120–125 km. The observations are interpreted by considering the downward propagation of wind shear convergent nodes that associate with the S2,3 semidiurnal tide in the upper E region and the S1,1 diurnal tide in the lower E region

[1] A large amplitude, 7-day period westward propagating S = 1 planetary wave (PW) of global response has been reported from ground radar and satellite wind measurements in the mesosphere-lower thermosphere (MLT) during the second half of August and well into September 1993. Following recent suggestions that PW might play a role in the formation of midlatitude sporadic E layers (E s ), Haldoupis and Pancheva [2002] found a strong 7-day periodicity present in all stations concurrently with the 7-day planetary wave reported elsewhere, by analyzing sporadic E critical frequency (foEs) time series from eight midlatitude ionosonde stations covering a longitudinal zone from about 58°E to 157°W. This study provided the first direct proof in favor of a PW role on E s formation. In the present paper we further investigate this role by considering the same PW event and correlating the 7-day periodicity in foEs directly with concurrent variations in the mesospheric neutral wind measured with atmospheric radars in Saskatoon, Canada, and in Sheffield, United Kingdom. Although our analysis cannot exclude a direct PW role on E s formation, it shows clearly that E s is affected indirectly by the PW through the action of the diurnal and semidiurnal tides which are strongly modulated by the same PW, apparently through a nonlinear interaction process at altitudes below 100 km. This 7-day PW modulation was found to be clearly present simultaneously in the amplitude of the zonal 12-hour tidal wind, the meridional 24-hour tidal wind, and in both, the 12-hour and 24-hour periodicities which existed in the foEs time series. The results here provide a new physical explanation for the observed relation between sporadic E layers and planetary waves.