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The Neutral Atmosphere

The Neutral Atmosphere. Dan Marsh ACD/NCAR. Overview. Thermal structure Heating and cooling Dynamics Temperature Gravity waves Mean winds and tides Composition Primary constituents Continuity equation Minor constituents - ozone, NO Storm impacts. Thermal structure.

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The Neutral Atmosphere

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  1. The Neutral Atmosphere Dan Marsh ACD/NCAR

  2. Overview • Thermal structure • Heating and cooling • Dynamics • Temperature • Gravity waves • Mean winds and tides • Composition • Primary constituents • Continuity equation • Minor constituents - ozone, NO • Storm impacts

  3. Thermal structure

  4. Thermodynamic equation + … Heat advection Diabatic heating/cooling Adiabatic heating

  5. Sources of diabatic heating/cooling • Absorption of solar radiation and energetic particles (e.g. ozone) • Chemical heating through exothermic reactions (A + B -> AB + E) • Collisions between ions and neutrals (Joule heating) • IR cooling (e.g. CO2 and NO) • Airglow

  6. Solar radiation energy deposition Solar Quantum Internal CO2(2), O2(1)… Chemical Potential O, e,… Heat UV, Vis., IR Loss Cooling Airglow After Mlynczak et al.

  7. Solar UV Energy Deposition Courtesy Stan Solomon

  8. Schumann-Runge continuum S-R Bands Hartley continuum

  9. Global average heating rates From [Roble, 1995]

  10. Joule Heating (K/day) Heating from collisions between ions and neutrals

  11. Chemical heating through exothermic reactions H + O3 OH + O2 (k4) O + O2 + M  O3 + M (k2) O + O + M  O2 + M (k1) O + O3 +  O2 + O2 (k3) OH + O  H + O2 (k5) HO2 + O  OH + O2 (k6) H + O2 + M  HO2 + M (k7) K/day

  12. Global average cooling rates From [Roble, 1995]

  13. Radiative cooling • IR atomic oxygen emission (63 µm) in the upper thermosphere • Non-LTE IR emission of NO (5.3 µm) 120 to 200 km • CO2 15 µm (LTE and non-LTE) important below 120km • IR emission by ozone and water vapor in the middle atmosphere

  14. TIMED/SABER observations of NO cooling Mlynczak et al., Geophys. Res. Lett., 30(21), 2003. Response to the solar storm during April, 2002

  15. Airglow O2(1∑) emission O2 O3 JH JSRC, Ly-a JH 1D Q4 1∑ Q2 A1 630 nm 1∆ Q1 Burrage et al. [1994] g 762 nm A2 762 nm Q3 A3 1.27 µm 3P 3∑ O O2 After Mlynczak et al. [1993]

  16. Vertical temperature structure (solstice) WACCM simulations - solar max. conditions Why is the mesopause not at its radiative equilibrium temperature? 130 K 230 K Summer Winter

  17. Gravity waves 1. 80 • Gravity waves are small scale waves mainly generated in the troposphere by mechanisms such as topography, wind shear, and convection. • Gravity wave amplitudes increase as they propagate upwards (conservation of momentum). ALTITUDE 65 50 After Holton & Alexander [2000]

  18. Mean zonal wind at solstice UARS reference atmosphere project W E Summer Winter

  19. Summer Winter 80 ALTITUDE 70 60 -90 EQ +90 Fx> 0 Fx< 0 • Gravity wave momentum deposition drives a meridional circulation from summer to winter hemisphere • Mass continuity leads to vertical motion and so adiabatic heating in the winter and cooling in the summer. Observed temperatures are 90K warmer in the winter and 60K cooler in summer than radiative equilibrium temperatures LATITUDE After Holton & Alexander [2000]

  20. Transport affects constituent distributions UARS reference atmosphere project Marsh & Roble, 2002

  21. Schematic representation of solar heating HEATING O2, N2 -90 EQ +90 HEIGHT LATITUDE O3 HEATING H2O HEATING SR Noon SS After Forbes [1987] LOCAL TIME

  22. Atmospheric Tides • Atmospheric solar tides are global-scale waves in winds, temperatures, and pressure with periods that are harmonics of a 24-hour day. • Migrating tides propagate westward with the apparent motion of the sun • Migrating tides are thermally driven by the periodic absorption of solar radiation throughout the atmosphere (UV absorption by stratospheric ozone and IR absorption by water vapor in the troposphere. • Non-migrating tides are also present in the upper atmosphere and can be caused by latent heat release from deep tropical convection or the interaction of tides and gravity waves

  23. Meriodinal wind at noon local time observed by UARS McLandress et al. [1996]

  24. GSWM-98 migrating diurnal tide (Equinox) Hagan et al. [1999]

  25. GSWM-98 migrating semi-diurnal tide

  26. Migrating thermospheric tides Hagan et al. [2001]

  27. Many waves are always present Migrating diurnal component Combined field Simulated winds at the equator

  28. Composition

  29. Primary constituent Hydrogen 2,500 km Helium 700 km Oxygen 200 km Nitrogen 0 km

  30. Total density height variation Ideal gas law Hydrostatic balance where

  31. Which leads to: In the “homosphere,” where eddy diffusion tends to mix the atmosphere, the mean molecular weight is almost constant (m ~ 28.96 amu), and the density will decrease with a mean scale height of ~ 7km. Above about 90km, constituents tend to diffuse with their own scale height (Hi = kT/mig) as the mean free path becomes longer. This is the “heterosphere.” The ith constituent (assuming no significant sources or sinks) will have the following gradient: Constituents with low mass will fall off less rapidly with height, leading to diffusive separation.

  32. Diffusive separation Turbopause From Richmond [1983]

  33. To recap… • Above the turbopause (~105km), molecular diffusion causes constituents to drop off according to their mass. • Below, the atmosphere is fully mixed: • 78%N2, 21%O2, <1% Ar, <0.1% CO2 • Density decreases with a mean scale height: H = kT/mg ~7km • The lower thermosphere is also the transition from a molecular to atomic atmosphere.

  34. What about chemistry? Recall: If there’s chemical production or loss of a minor constituent then this equality will not hold and a diffusive flux occurs. Above the turbopause this will be: Where Di is the diffusion coefficient:

  35. From Richmond [1983]

  36. Continuity equation The total diffusive flux will include both molecular and eddy diffusion terms: So, neglecting transport, we now have a continuity equation for the ith constituent:

  37. The distribution of ozone

  38. Chapman chemistry Zonal mean Ox loss rates 2.5ºN

  39. Catalytic cycles Mesosphere Stratosphere [GSFC, NASA]

  40. Nitric Oxide in the lower-thermosphere Equatorial NO (Barth et al., 2003)

  41. Produced by (1-10 keV) precipitating electrons and solar soft X-rays (2-7 nm)

  42. Thermosphere: SNOE Nitric Oxide Obs. 3 EOFs = 80% of var.

  43. Solar forcing of the neutral atmosphere • UV/EUV • Precipitating particles in auroral regions • Solar proton events (SPEs) • Highly-relativistic electrons (HREs) >1MeV • Galactic cosmic rays

  44. upper panel: WACCM temperature, ozone, and water vapor for July solar minimum conditions. lower panel: Solar min/max percentage differences. Data only plotted where differences are statistically significant (95% confidence level).

  45. Mesosphere/Stratosphere response MLS ozone (30S-30N) vs. 200-205 nm solar flux DeLand et al., 2003 Hood & Zhou, 1998

  46. [NASA LWS report]

  47. Solar Proton Ionization Rates

  48. Coupling processes • Downward transport of thermospheric nitric oxide by ~1keV electrons • NOy production in lower mesosphere and upper stratosphere via energetic electron precipitation (4-1000 keV) • Both processes lead to stratospheric ozone destruction POAMII ozone SH 30km (2xAp) Callis et al. 1998 Randall et al. 1998

  49. The End

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