A longer version of the department seminar. More introductory material about the radar and tidal source mechanisms.

1. The 8-hour Tide in the Atmosphere
Charlotte Beldon
Centre for Space, Atmospheric and Oceanic Science,
Department of Electronic and Electrical Engineering
5th
December 2005

2. Outline
•The Mesosphere and Lower Thermosphere
•What causes a meteor
•The meteor radar technique
•Distribution of meteor echoes
•Tides in the atmosphere
•The 8-hour tide

3. Atmospheric Temperature Profile
0
20
40
60
80
100
120
TEMPERATURE (DEGREES)
HEIGHT(km)
-120 -80 -40 0 40
Meteors
Everest (8848m)
Planes
Weather Balloons
Satellites

4. Meteors
Meteor seen by eye
•‘Shooting Stars’
• Particle enters the atmosphere
•Heating and ablation
•Column of ionisation
•De-excitation emits light

5. Tx Antenna
Five-Antenna Rx Array
~ 90 km
Radio
Pulses
Mesosphere / lower-
thermosphere region
Meteor Radar

6. Tx Antenna
Five-Antenna Rx Array
~ 90 km
Radio
Pulses
Mesosphere / lower-
thermosphere region
Meteor Radar

7. Radar
Meteor trail
Reflection point
Meteor Radar

8. TIME (ms)
AMPLITUDE
Meteors seen by radar
TIME (ms)
AMPLITUDE
Meteors
•Overdense
(>1014
e-
m-1
)
•Underdense
(<1014
e-
m-1
)

9. Parameters from a Meteor Radar
Tx Antenna
Five-Antenna Rx Array
Radio
Pulses
• Date and Time
• Range
• Height
• Radial Velocity
• Elevation
• Azimuth
• Meteor Brightness
• Meteor Decay Time
• Meteor Entry Speed
• Signal to Noise Ratio

10. Calculating Horizontal Winds
•Meteor trails act as tracers
•Assume :
-Atmosphere moves as a slab
above the radar
-Vertical winds are small
compared with horizontal
winds
•Plot velocity of trails against angle
around the radar
•Sine curve amplitude = speed
phase = direction

11. Meteor Radar in the UK
•January 1988 – 2005
•Two Beam System (~30o
)
•No height determination
•3000 – 4000 meteors per day
Castle Eaton, UK (52.6 N, 2.2
W)
Frequency: 25 MHz
Peak Transmission Power: 20 kW
Pulse Repetition Frequency: 300 Hz
Pulse Length: 30 μsec

12. Meteor Radar in the Arctic
•October 1999 – 2005
•SKiYMET – All sky
•5 Receiving Antennas,
1 Transmission Antenna
•~4000 meteors per day
Esrange, Sweden (67.9 N, 21.1 E)
Frequency: 32.5 MHz
Peak Transmission Power: 6 kW
Pulse Repetition Frequency: 2144 Hz
Pulse Length: 13 μsec

13. Distribution of Meteor Echoes
2nd
November 2005 5402 Meteors found
100
200
300
400
NORTH
SOUTH
WEST EAST
Norway Sweden Finland
Russian
Federation

14. Distribution of Meteor Echoes
Distribution in Range
METEORCOUNT(x105
)
100 200 300 400 500
RANGE (km)
0
1
2
3
4
6
5
•Number of echoes decreases
with range
•Power decays as a function of
range

15. Distribution of Meteor Echoes
METEORCOUNT
HOUR
0 5 10 15 200 5 10 15 20
0
50
100
150
200
250
300
350
METEORCOUNT
HOUR (UT)
0
50
100
150
200
250
300
350
Distribution in Time
•Change in number of echoes
over one day
•Peaks in early morning
•More meteors on the leading
hemisphere of the earth

16. Distribution of Meteor Echoes
0 50 100 150 200 250 300 350 400 450 500
70
75
80
85
90
95
100
105
110
METEOR COUNT
HEIGHT(km)
Distribution in Height
HEIGHT(km)
METEOR COUNT
0 100 200 300 400 500
70
75
80
85
90
10
0
95
10
5
11
0 •Detect echoes ~70 to ~110 km
•3 km height resolution
•Strongly peaked at ~90 km

17. Horizontal Winds Observed by the radar

18. East-West Horizontal Winds
•Wind blows alternately east and
west
•Regular 12-hour oscillation

19. Solar Atmospheric Tides
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
0
1
2
3
4
5
6
7
FREQUENCY(CYCLES PER DAY)
AMPLITUDE(m/s)
FREQUENCY (CYCLES PER DAY)
AMPLITUDE(m/s)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.50
1
2
3
4
5
6
7
12-hour tide
24-hour tide
8-hour tide
6-hour tide
•Winds dominated by solar
tides
•Analogous to ocean tides
•12-hour tide is largest over the
UK
•Source of the 8-hour tide is
uncertain

20. The 8-hour Tide
Band passed zonal winds, October 2000
DAY OF YEAR
WINDSPEED(m/s)
24-hour tide
12-hour tide
8-hour tide
•High day-to-day variability
•8-hour tide can match 12-
hour tide on occasion
•Variability from changes in
excitation and environment

21. Sources of the 8-hour Tide – Solar Heating
Solar Heating
HEIGHT(km)
TEMPERATURE
(DEGREES)
Troposphere
Thermosphere
Mesosphere
Stratosphere
O2
H2O
O3
0
20
40
60
80
100
120
-120 -80 -40 0 40
•Solar heating - periods are integer sub-
harmonics of a solar day
•24-, 12-, 8- and 6-hour tides are generated

22. Sources of the 8-hour Tide – Nonlinear coupling
24-hour tide 12-hour tide
Nonlinear interaction
Family of secondary waves :
SumDifference
•Sum and difference waves
•Related frequencies and wave
numbers
•Vertical wavelength :
•Testable predictions
λ8 = (λ24 * λ12)
(λ24 + λ12)

23. Observations of the 8-hour Tide - UK
Amplitudes of the 8-hour tide
•UK radar has no height finding
•Clear seasonal behaviour
•Maximum amplitudes in autumn
•Elevated values in winter

24. Observations of the 8-hour Tide - Esrange
8 5
9 0
9 5
HEIGHT(km)
0
1
2
3
4
5
6
7
8
9
1 0
J F M A M J J A S O N D
AMPLITUDE(ms-1
)
•Height resolution
•Tide grows with height
•Large tide in autumn
•Small tide in winter – different to UK
Amplitudes of the 8-hour tide

25. Conclusions
•Meteor radar used to measure horizontal winds between 80-100 km in
the atmosphere
•Atmospheric tides dominate motion in the mesosphere and lower
thermosphere
•The 8-hour tide has two possible sources
•There is a clear seasonal cycle that changes with latitude

26. Thank you
Charlotte Beldon
Centre for Space, Atmosphere and Oceanic Science,
Dept. Electronic and Electrical Engineering