The document discusses the 8-hour tide in the mesosphere and lower thermosphere over the UK, particularly the role of atmospheric tides and their variability. It highlights the use of meteor radar to measure these tides, noting that the 8-hour tide exhibits significant seasonal and inter-annual variability, peaking in autumn and winter. Comparisons with other latitudes reveal different tidal behaviors, suggesting further research into higher frequency tides and tidal excitation mechanisms.

1. The 8-hour Tide in the Mesosphere and Lower Thermosphere over the UK Charlotte Beldon Centre for Space, Atmospheric & Oceanic Science University of Bath [email_address] 12/9/5/05

2. Outline Introduction to mesosphere and lower thermosphere Atmospheric tides The UK meteor radar The 8-hour tide over the UK Comparisons with other latitudes Conclusions

3. The mesosphere and lower thermosphere Inaccessible to direct measurement but includes the meteor region Plays a crucial role in linking the upper and lower atmosphere Sensitive to climate change

4. Atmospheric tides Some of the largest waves in the MLT are solar tides. The ‘square wave’ shape of tidal heating results in tides with periods of 24 -, 12 -, 8 - and 6 -hours

5. Tidal excitation Solar heating of: Atomic oxygen by EUV at ~ 150 km Molecular oxygen by NUV at 100 – 150km Ozone by UV at 30 – 70 km Water vapour in the troposphere by IR For the 8-hour tide - Non-linear interactions between the 12- and 24- hour tides Source of the 8-hour tide is uncertain Water vapour Ozone Molecular oxygen

6. The UK meteor radar Based in Castle Eaton (52.6 ºN, 2.2 ºW ). Operating since 1988 – very long dataset Bounces a radio pulse off ionised meteor trails which drift with the atmosphere Measures horizontal wind velocities representative of ~ 90 – 95 km with a resolution of 1 hour

7. The 8-hour tide All tides are highly variable Over the UK the 12-hour tide is largest but the 8-hour tide can sometimes be as large for short periods

8. The 8-hour tide over the UK Largest amplitudes during autumn & winter-time Later phases during summer Phase quadrature (2 hour phase difference)

9. Inter-annual variability High degree of inter-annual variability Seasonal pattern similar from year-to-year Amplitudes can change by ~6 ms -1 from year to year e.g., successive Octobers Variability caused by: - changes in the strength of excitation - changes in the propagation environment

10. Comparisons with the other latitudes At 90 – 95 km autumnal maximum, but with no winter maximum UK: autumnal maximum and large winter amplitudes Lower middle latitudes : large winter amplitudes and no autumnal maximum Arctic: Measured by meteor radar in Esrange, Sweden (69 ºN, 21 ºE )

11. Conclusions The 8-hour tide is a persistent feature of the MLT over the UK Climatology: amplitudes largest in autumn, smallest in summer High inter-annual variability - although general seasonal structure remains the same Seasonal behaviour changes from UK to Arctic latitudes Future work : vertical wavelengths as diagnostic of tidal excitation Higher frequency tides e.g., the 6-hour tide

12. Charlotte Beldon Centre for Space, Atmosphere and Oceanic Science, Dept. Electronic and Electrical Engineering University of Bath Thank you [email_address]