Radar studies of the mesosphere & lower thermosphere
This project used ground-based meteor radars, satellites and models to study the Earth's mesosphere and lower thermosphere (MLT). These are the regions of the atmosphere at heights from about 50-110 km above the ground. The radars measure the winds of the MLT-region by detecting the drifting of meteor trails as they are carried by the winds at these heights. The radars used in the study are sited at Esrange in the Swedish Arctic, at Castle Eaton in the UK, on Ascension Island in the equatorial Atlantic and at Rothera, the British Antarctic Survey base in the Antarctic.
The scientific focus of the work was to understand the role of winds, waves and tides in coupling together the lower, middle and upper atmosphere and to investigate the Arctic and Antarctic MLT regions to determine the nature of any differences between to two polar regions of the Earth. A wide range of studies were undertaken. Some of the major results are listed below.
Observations revealed that there are significant differences in the winds and atmospheric tides of the Arctic and Antarctic. The equatorward flow of the summertime circulation was found to be both stronger and at a lower height over Esrange than over Rothera. Since this circulation is driven by gravity waves launched from the lower atmosphere, these differences suggest that there are systematic differences in the strength of these waves between the Arctic and Antarctic. The atmospheric tides were also found to be very different, with those of the Arctic being of much larger amplitude (factor ~ 2) than their Southern counterparts. This suggests that there are significant differences in the strength of excitation and the propagation of the tides between the two hemispheres.
Data from the NASA TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) satellite were used to examine the underlying mechanisms that cause variability in the tides of the MLT. Surprisingly, it was found that the activity of planetary waves in the Antarctic stratosphere appears to greatly influence the variability of the tides of the Arctic - indicating a planetary-scale control of tidal variability.
A mysterious 2-day wave observed in the Arctic MLT during winter was shown to be an entirely different phenomenon to the 2-day wave regularly observed in the summer MLT. Data from the AURA satellite revealed it to be a planetary wave that originates in the lower stratosphere and then ascends to the MLT, rather than being generated in situ.
Other studies revealed that the waves and tides of the MLT appear to influence the Earth's ionosphere, imprinting wave-like signals into its variability. Finally, observations revealed that high-frequency tides of periods 6 and 8 hours and lunar gravitational tides can all reach significant amplitudes in this part of the atmosphere and must be accounted for if it is to be successfully modelled.
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