Neutral Upper Atmosphere

1. CSI 662 / ASTR 769 Lect. 10 Spring 2007 April 10, 2007 Neutral Upper Atmosphere • References: • Prolss: Chap. 2, P11-77; Chap. 3.2-3.5, P103 – 159 (main) • Gombosi: Chap. 8, P125 – P156; Chap. 9.1-9.4, P158 – 169 (supplement) • Tascione: Chap. 6, P. 79 – 88 (supplement)

2. Fast and Slow Wind Neutral Upper Atmosphere • Atmospheric layers • Barospheric density distribution • Exospheric density distribution • Solar radiation energy • Radiation absorption processes • Temperature distribution • Thermospheric winds • Atmospheric waves

3. Atmospheric Layers • Classified by temperature • Troposphere • 0  10 km • ~300 K  200 K • Stratosphere • 10 50 km • ~200 K 250 K • Mesosphere • 50 km  80 km • ~250 K  160 K • Thermosphere • > 80 km (~10000) • 160 K  ~1000 K

4. Fast and Slow Wind Atmos. Layers • Classified by Gravitational binding • Barosphere • 0 km 600 km • binding • Exosphere • > 600 km • Escaping or evaporation • Classified by Composition • Homosphere • 0 km 100 km • Homogeneous • Heterosphere • 100 km  ~2000 km • Inhomogeneous • Hydrogensphere (Geocorona) • > ~2000 km • Dominated by hydrogen

5. Basic Parameters • Chemical composition (ni/n): • Height = 0 km, 78% N2, 21% O2, 1% others (trace gases) • Height = 300 km, 78% O, 21% N2, 1% O2 • Pressure: • Height = 0 km, P = 105 pa • Height = 300 km, P=10-5 pa • H He N O N2 O2 • Atomic Number 1 2 7 8 • Mass Number 1 4 14 16 28 32 • f (Degree of freedom) 3 3 3 3 5 5 • 3 translation + 2 rotation T: temperature Uf: internal energy per freedom k: Boltzmann constant

6. Some Kinetic Parameters • Flow and random velocity • Actual gas particle velocity v • Flow (bulk, wind) velocity u • Random (thermal) velocity c • Test particle: type 1; Gas particle: type 2 • Cross section σ1,2 : probability of interaction (m2) • Ideal collision (billiard ball collision) • Collision frequency: • Mean free length

7. Barospheric Density Distribution Hydrostatic equilibrium or aerostatic equations Pressure Scale Height Barometric Law isothermal

8. Barospheric Density Distribution In upper thermosphere (~ >200 km) Isothermal, thermopause temperature Dominated by atomic oxygen In lower thermosphere (120 km -- 200 km) Bates temperature profile h0=120 km, T(h0)=350 K, and s=0.021 km-1

9. Isothermal Scale Heights Hi = kT/(mig) for g(200 km) HN2 = 0.032* T HO2 = 0.028* T HO = 0.0567* T Altitude interval where density decreases by 10: Barospheric Density Distribution N2 O O2

10. Homosphere to Heterosphere In hydrostatic state, particle distribution is determined by gravitational setting and molecular diffusion Density scale height depends on the molecular mass This would create a gravitationally-separated atmosphere Molecular diffusion flux Molecular diffusion coefficient

11. Homosphere to Heterosphere In hydrostatic state, particle distribution is determined by gravitational setting and molecular diffusion Density scale height depends on the molecular mass. Heavy gas has small scale height, and decreases rapidly with height This would create a gravitationally-separated atmosphere Molecular diffusion flux Molecular diffusion coefficient

12. Homosphere to Heterosphere • However, the atmosphere remains homogeneous up to 100 km. • This mixing is caused by eddy diffusion or turbulence • Turbopause or Homopause • Where eddy and molecular diffusion rates are equal • Typically ~ 105 km • But transition is smooth over some altitude interval • Below is called the homosphere • Above is the heterosphere

13. Turbopause Heterosphere Turbopause Homosphere Gombosi, Fig 8.3

14. Fast and Slow Wind Exosphere • Exosphere: where the density is so small that direct escape (evaporation) of gas particles is possible • The lower boundary of the exosphere is called the exobase • Exobase height: above which, a radially outward moving particle will suffer less than one collision Number of collisions Column density (above h)

15. Fast and Slow Wind Exosphere • Exobase height: • E.g., ~ 420 km, for σH,O=2 X 1019 m2, T=1000 K, H0=60 km, n0(250 km) =1.5X1015 m-3 • Escape flux: • Depends on how many particles at the exobase have an escape velocity? • Ves ≈ 11 km/s • At T=1000 K, CO=1.25 km/s, CHe=2.5 km/s, CH=5 km/s Particle Maxwellian distribution function • Hydrogen: φes=1012 m-2 S-1, τes=70000 years • Other elements are well gravitationally-bound

16. Fast and Slow Wind Solar Radiation Energy The temperature structure of the Earth’s upper atmosphere is determined by the properties of solar radiation, the primary source of energy

17. SOLAR - TERRESTRIAL ENERGY SOURCES Source Energy Solar Cycle Deposition (Wm-2) Change (Wm-2) Altitude • Solar Radiation • total 1366 1.2 surface • UV 200-300 nm 15.4 0.17 10-80 km • VUV 0-200 nm 0.15 0.15 50-500 km • Particles • electron aurora III 0.06 90-120 km • solar protons 0.002 30-90 km • galactic cosmic rays 0.0000007 0-90 km • Peak Joule Heating (strong storm) • E=180 mVm-1 0.4 90-200 km Solar Wind0.0006 above 500 km

18. TOTAL IRRADIANCE VARIABILITY SPECTRUM VARIABILITY

19. Solar Energy Deposition Atmospheric Structure SPACE WEATHER GLOBAL CHANGE EUV FUV MUV RADIATION

20. Atmospheric Absorption Processes • Ionization • O2 + h  O2+ + e*, … • Dissociation • N2 + h  N + N, … • Excitation • O + h  O* • O* O + h ’ radiation • O* + X  O + X quenching or deactivation • Dissociative ionization – excitation • N2 + h  N+* + N + e, …

21. Energy Thresholds for Processes* * From Heubner et al., Astrophys. Space Sci., 195, 1-294, 1992 Useful relationship: E(eV) x  (Å) = 12397

22. Radiative Absorption F Change in photon flux n = # absorbers/cc • = absorption cross section Integrating, dz’ F+dF

23. Optical Depth • Definition • For several species • i = N2, O2, O • Altitude of unit optical depth: F(z)= F() e-1 • Solve (z) = 1 for z • Where solar radiation is effectively extinct

24. Optical Depth • If  const., • Vertical Column Density  number of atoms/molecules in a 1 cm2 column above altitude z

25. Assume plane parallel atmosphere H << earth radius Away from terminator s = z / cos  and (s) = (z) / cos    solar zenith angle What happens at the terminator ( = 90o)? Slant path optical depth F() 0  (s) s  (z) z

26. Large solar zenith angles • Allowing for earth curvature and isothermal atmosphere, then: (s) = (z) Ch(x, ) • X = (Re + z)/H • Re = earth radius • H = scale height • Ch(x, ) = Chapman function where +   > 90o and -   < 90o • Must do numerical integration along slant path from Sun: • If not isothermal • If accounting for oblate spheroid shape of Earth * * From Rishbeth and Garriott, Intro. To Iono. Phys.

27. Fast and Slow Wind Energy Deposition from Radiation Eph=hpc/λ : energy of single photon Φph(h): radiation flux at height h The deposition rate at height h depends on the combination of the radiation flux and gas density at h.

28. Energy Deposition from Radiation • Change composition through photo-dissociation • Change the upper atmosphere into a conducting medium through photo-ionization • Heat is also generated, e.g., hot photoelectron through ionization • Buoyancy oscillation O + photon (λ = 30.4 nm)  O+ + e + excess energy 41 ev 14 ev 27 ev • Hot photoelectron first transfer kinetic energy to ambient thermal electron, leading to heating of electron gas • Hot electron gas heats up the cooler neutral gas

29. Heat Loss • Radiation cooling. Important for trace gases like NO and CO2, whose vibration and rotational transitions are effective excited at thermospheric temperature • Molecular heating conduction.

30. Temperature Profile • Temperature profile is determined by the heat balance equation (or energy equation) qW: heat production by radiation absorption lW Heat loss through radiation cooling dW Heat gain/loss through conduction

31. MSIS Class Empirical Atmospheric Models • MSIS-class Models Are the Community Standard • Inputs: Day, Time (UT, Apparent Solar Local Time), Location, Solar EUV Flux Proxy (F10.7 ,F10.781 day ave), Magnetic Activity (ap, Ap) • Outputs: Composition (N2, O2, O, N, He, Ar, H, Oa), Total Mass Density, and Temperature, 0 - 1000 km • Empirical, Analytic, Assimilative • Spherical Harmonics + Bates-Walker Altitude Profile • Interpolates Among Or Extrapolates Numerous Data Sets To User-Specified Inputs • Nominal 1s Error 15-25 % (vs Altitude/Latitude) • NRLMSIS 2000E supercedes MSISE-90: Automated/Web Distribution

32. MSIS Accessibility • Description and downloading: • http://uap-www.nrl.navy.mil/models_web/msis/msis_home.htm • http://modelweb.gsfc.nasa.gov/models/msis.html • MSIS 90 (earlier version) • Reference to NRLMSIS2000 • Picone, J. M., A. E. Hedin, D. P. Drob, and A. C. Aikin, J. Geophys. Res., 107(A12), 1468 (2002) • MSIS solar-geophysical inputs can be found at: • ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/INDICES/KP_AP/ • http://www.sec.noaa.gov/today.html

33. NRLMSIS Example: March 21F10.7 = 150, Ap = 4 , Altitude = 300 km

34. Fast and Slow Wind Thermospheric Winds • Thermosphere is not statis • It corotates with the Earth • Uco=Ωrcosφ • At h=300 km, Uco=500 m/s • It has global circulation winds • It has atmospheric waves

35. Fast and Slow Wind Diurnal Wind Circulation • Caused by considerable temperature and density differences exist in the thermosphere between the day and night • High pressure in the afternoon sector (~ 3 PM) • Low pressure shifts by 12 hours (~ 3 AM) • Pressure difference is 4 μPa • Produce airflow as high as 200 m/s (700 km/hr) • This airflow is also called tidal wind because of its 24 hour periodicity

36. Fast and Slow Wind Diurnal Wind Circulation • Pressure gradient force • Viscosity force • Gravity force • Ion drag force • Coriolis force

38. Altitude-Latitude Variation of Thermospheric Circulation from GCM Model Geomagnetic Activity Quiet Average Storm

39. Fast and Slow Wind Atmospheric Waves • Acoustic Wave • ~ 340 m/s at h=0 • ~860 m/s at h=300 km, T= 1000K • Buoyancy oscillation • At h=300 km, τg=2π/ωg ~13 min • Gravity wave • a combination of acoustic wave and buoyancy effect • Responsible for quasi-periodic fluctuation of electron density and layer height

40. The End