[1] An all-sky VHF meteor radar at Esrange (68°N, 21°E) near Kiruna in Northern Sweden has been used to investigate the 8-hour tide in the Arctic mesosphere and lower thermosphere. We present a climatology of the 8-hour tide over the period October 1999 to April 2001. The tide appears to be a persistent feature of the Arctic atmosphere, although a large day-to-day variability of the tidal amplitude is observed. At times the 8-hour tide reaches amplitudes over 30 m s À1 . The amplitude of the tide increases with height across the observed height range of $80-100 km. Monthly mean tidal amplitudes range from <2 m sÀ1 to values as large as 10 m s À1 . A clear seasonal behavior is apparent with maximum amplitudes observed in the autumn. Vertical wavelengths are shortest in winter and spring (25-35 km) and longest in summer and autumn (50-90 km). At least on some occasions the vertical wave number relationships between the 8-, 12-, and 24-hour tides suggest that the 8-hour tide is being generated by nonlinear interaction between the 12-and 24-hour tides.

Abstract.Some preliminary results about the planetary wave characteristics observed during the first seven months (October 2001-April 2002 of observations over Ascension Island (7.9 • S, 14.4 • W) are reported in this study. The zonal wind is dominated by the 3-7-day waves, while the meridional component -by the quasi-2-day wave. Two wave events in the zonal wind are studied in detail: a 3-4-day wave observed in the end of October/November and the 3-6-day wave in January/February. The moderate 3-and 3.2-day waves are interpreted as an ultra-fast Kelvin wave, while for the strong 4-day wave we are not able to make a firm decision. The 6-day wave is interpreted as a Doppler-shifted 5-day normal mode, due to its very large vertical wavelength (79 km). The quasi-2-day wave seems to be present almost continuously in the meridional wind, but the strongest bursts are observed mainly in December and January. The observed period range is large, from 34 to 68 h, with some clustering around 43-44 and 50 h. The estimated vertical wavelengths indicate shorter lengths during the equinoxes, in the range of 25-30 km, and longer ones, ∼ 40-50 km, in January/February, when the 48-h wave is strongest.

Abstract.Results are presented of a study of the temporal and spatial variability in meteor count rate observations from three VHF meteor radars. These radar are located in the Arctic (at Esrange, 68 • N), in the Antarctic (at Rothera, 68 • S) and near the Equator (on Ascension Island, 8 • S). It is found that for all three locations there is a strong diurnal cycle in observed hourly meteor counts and the time of maxima and minima in these counts depends on the month of the year. In addition, at high latitude there is a strong annual cycle in observed monthly-mean meteor counts, whereas for the radar at low latitude there is a semi-annual cycle. At high latitude there is also an annual cycle in the mean height at which meteors are observed. However, no such annual cycle is found in observed meteor count rates from the low latitude radar. The meteor count data from all the radars are combined to investigate the sporadic radiant distribution (i.e. the distribution of direction of arrival on the celestial sphere of sporadic meteors). This combined radiant distribution shows that there are six main source regions for meteors. The latitudinal and temporal dependence in observed meteor count rates appears to result from a combination of the sporadic radiant distribution, annual fluctuations in atmospheric density, the sensitivity of the radar to meteors from different source directions and the temporal and spatial variability in meteor fluxes.

Analyses of intra‐seasonal oscillations (i.e., oscillations with periods of ∼30–100 days) in the MLT region made by meteor radars at middle (UK, five years of data) and high latitudes (Esrange, three years of data) are presented in this work. There is clear evidence of ∼75‐day variability in the mean winds. This intra‐seasonal oscillation appears to be a prominent feature of the mean zonal circulation and was observed quite regularly in the time intervals December 1996–September 2001 over UK and October 1999–December 2002 over Esrange. The intra‐seasonal oscillations observed over Esrange are coherent in both amplitude and phase over the height range of ∼80–100 km covered by the radar. They are especially strong in the range 90–97 km, weaker near 87–88 km and stronger again near 81 km. At these lower heights the intra‐seasonal oscillations are usually of longer period (∼80 days) than, and are out of phase with, the oscillations at the upper heights. We also note that this oscillation is a common feature in the meridional winds over the UK. It is present, but intermittent, in the meridional winds over Esrange.

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