Hr Middleton

The 8‐hour tide in the Arctic mesosphere and lower thermosphere

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 -1 . 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 s -1 to values as large as 10 m s -1 . 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.

Evidence for the tongue of ionization under northward interplanetary magnetic field conditions

[1] The activities of the International Ionospheric Tomography Community open up new possibilities of simultaneously imaging the large-scale spatial structure of the ionosphere in different longitude sectors. In the study, tomography receiver chains in Scandinavia and Greenland were used to provide a wide view of the plasma density structure in the winter, magnetic postnoon sector under conditions of stable, positive interplanetary magnetic field Bz component. The spatial distributions of the plasma are discussed in light of a high-latitude plasma convection pattern pertinent to the conditions, which is supported by DMSP flow measurements. The observations are consistent with a tongue of dayside photoionization being drawn antisunward by the convection pattern to form an arc of enhanced plasma density around the periphery of the polar cap.