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Heterosphere ➡ Deviations from Hydrostatic Balance: Diffusive and Chemical Time Constants

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Heterosphere ➡ Deviations from Hydrostatic Balance: Diffusive and Chemical Time Constants 高层大气环境 完全电离 （无碰撞） 逃逸层/磁层 热层/电离层 部分电离 中间层 中性气体 平流层 （碰撞） 对流层 Ionosphere-Thermosphere Basics - I Neutral Atmosphere Vertical Structure Topics • Thermal Structure and Thermal Balance ➡ Energy Sources and Sinks ➡ Molecular Conduction and Thermal Gradient • Hydrostatic Balance ➡ Thermosphere Composition ➡ Mean Molecular Weight vs. Height ➡ Homosphere-Heterosphere ➡ Deviations from Hydrostatic Balance: Diffusive and Chemical Time Constants 2 Ionosphere-Thermosphere Basics - I Neutral Atmosphere Vertical Structure 3 Ionosphere-Thermosphere Basics - I Neutral Atmosphere Vertical Structure 4 Global Mean Temperature Structure Tropo (Greek: tropos); “change” Lots of weather Strato (Latin: stratum); Layered Meso (Greek: messos); Middle Thermo (Greek: thermes); Heat Exo (greek: exo); outside 5 Discussion for Temperature Structure ØWhy do most of aircrafts fly in the Stratosphere? ØWhy is the Stratosphere is more stable than the Troposphere? ØCan we use a conventional thermometer to measure temperature in the upper atmosphere? Alternately, why do astronauts in space shuttle feel cold in ionosphere/thermosphere even if the upper thermosphere temperature is so high? Satellite Bias and Temperature Yue et al., 2011; Lin et al., 2015 Satellite Bias and Temperature Zhong et al., 2015 Global Mean Temperature Structure It is important to distinguish between three types of velocities: • the actual particle velocity • the flow velocity • and random (thermal) velocity of a gas particle Wind! Global Mean Temperature Structure It is important to distinguish between three types of velocities: • the actual particle velocity • the flow velocity • and random (thermal) velocity of a gas particle ? Global Mean Temperature Structure How about the mean random velocity ? For the earth’s atmosphere, Reduce temperature different species of gas have the same temperature! Global Mean Temperature Structure Question （ homework）: At what altitudes different species of neutral gas do not have the same temperature? Global Mean Temperature Structure Tropo (Greek: tropos); “change” Lots of weather Strato (Latin: stratum); Layered Meso (Greek: messos); Middle Thermo (Greek: thermes); Heat Exo (greek: exo); outside 13 Energy source of the upper atmosphere (1)Absorption of solar ultra-violet and X-ray radiation, causing photodissociation, ionization and consequent reactions that liberate heat. (2)Energetic charged particles entering the upper atmosphere from the magnetosphere . (3)Joule heating by ionospheric electric currents (4)Dissipation of tidal motions and gravity waves by turbulence and molecular viscosity. High latitude region especially during stormtime Energy source of the upper atmosphere Energy source of the upper atmosphere Deposition of solar EUV energy in the thermosphere as a function of wavelength and altitude Qian and Solomon, JGR, 2005 Altitude of Maximum Solar Radiation Absorption 18 Solar Radiation Absorption in the Thermosphere Absorbing gas is thin here; therefore little absorption. Q F Three factors determine the rate of solar radiation absorption, Q: N ≈ 120 - • No. of photons (solar flux, F) 160 km • No. of absorbing molecules, N • Efficiency of absorption (cross-section) Few photons left here; Heating per volume! therefore little absorption. QI()() e−τλ(,h00 , χ ) ( hv )×ρ EUV = ∑ ∞ λσ λλλ ε λ heating per mass 19 Global Mean Temperature Structure Tropo (Greek: tropos); “change” Lots of weather Strato (Latin: stratum); Layered Meso (Greek: messos); Middle Thermo (Greek: thermes); Heat Exo (greek: exo); outside 20 Energy source of the upper atmosphere (1)Absorption of solar ultra-violet and X-ray radiation, causing photodissociation, ionization and consequent reactions that liberate heat. (2)Energetic charged particles entering the upper atmosphere from the magnetosphere . (3)Joule heating by ionospheric electric currents (4)Dissipation of tidal motions and gravity waves by turbulence and molecular viscosity. Energy sink of the upper atmosphere? Energy sink of the upper atmosphere (1) Radiation, particularly in the infra-red; the mesosphere is cooled by radiation from CO2 at 15 μm and from ozone at 9.6μm (2) Heat transport • Radiation • Molecular conduction • Eddy diffusion, or convection • Transport by large-scale winds • Chemical transport of heat Energy sink of the upper atmosphere Molecular conduction (2) • more efficient in the thermosphere (above 150 km); here Heat transport the thermal conductivity is large because of the low pressure and the pressure of free electrons • Radiation • Molecular conduction • Eddy diffusion, or convection • Transport by large-scale winds • Chemical transport of heat Radiation • the most efficient process at the lowest levels. Eddy diffusion • The atmosphere is in radiative equilibrium between 30 and • more efficient than conduction below the turbopause. 90 km Thus, below about 100 km eddy diffusion carries heat from the thermosphere into the mesosphere. •This is a major loss of heat from the thermosphere but a minor source for the mesosphere Global Mean Energy Sources and Sinks Thermosphere: • Energy sources: • Absorption of EUV (200-1000Å; photoionizing O, O2, N2) and UV (1200-2000 Å), photodissociating O2), leading to chemical reactions and particle collisions, liberating energy • Joule heating by auroral electrical currents • Particle precipitation from the magnetosphere • Dissipation of upward propagating waves (tides, planetary waves, gravity waves) • Energy sinks: • Thermal conduction into the mesosphere, where energy is radiated by CO2, OH and H2O • IR cooling by CO2 NO, O Mesosphere: • Energy sources: Troposphere: • Some UV absorption by O3 (lower heights) • Energy sources: • Heat transport down from thermosphere • Planetary surface absorption (IR, visible), (minor, upper heights only) convection & conduction to atmosphere • Chemical heating • Atmospheric absorption of terrestrial • Energy sinks: and solar IR • IR radiation by CO2 H2O, OH • Latent heat release by H2O • Energy sinks: Stratosphere: • IR radiation • Energy sources: • Evaporation of H2O • Strong absorption of UV by ozone (causing stratopause temperature peak) • Energy sinks: • IR radiation by O3 , CO2 , H2O 26 Thermodynamic energy equation heat conduction Advection z ∂T ge ∂ ⎧K ∂T 2 ⎛ g 1 ∂T ⎞⎫ ! ⎛ ∂T RT ⎞ T K H C V T w = ⎨ + E P ρ⎜ + ⎟⎬ − ⋅∇ − ⎜ + ⎟ ∂t P0C p ∂z ⎩ H ∂z ⎝ CP H ∂z ⎠⎭ ⎝ ∂z CP m ⎠ Q − L + C p adiabatic heating or Heating and cooling cooling where T is neutral temperature, t is time, g is gravity, Cp is specific heat per unit mass, KT is the molecular thermal conductivity coefficient, H is the pressure scale height, KE is the eddy diffusion coefficient which is assumed to be equal to the eddy thermal conductivity, rho is the neutral mass density, z is the model vertical coordinate, P is pressure and a reference pressure (5 x 10-4 mb) at z = 0; V is the horizontal velocity vector, w is the vertical velocity defined by , R is the universal gas constant, and m is the mean molecular mass of neutral gas. 27 Heating rate profiles of the thermal budget • QT total neutral heating rate • SRB and SRC O2 absorption heating • e-i collision heating between thermal electrons, ions and neutrals • Qic ion-neutral chemistry heating • Qnc neutral-neutral chemistry heating • QJ Joule heating • O(3P) fine structure cooling by O • O3 heating • heating from quenching of O(1D) Roble, 1995 28 Radiative Cooling by CO2, NO, O is also important to the thermal budget • QT total neutral heating rate • Km molecular thermal conduction • Ke eddy thermal conduction • NO cooling at 5.3 mm • CO2 total CO2 rate • O(3P) fine structure cooling by O • IR total of O, NO, CO2 • NO cooling rate can increase by two orders of magnitude during a storm. Roble, 1995 29 Molecular Conduction Determines Thermosphere Temperature Profile Shape Thermal conduction (molecular and turbulent) removes heat from the thermosphere to the mesosphere (here collision frequencies are high enough that polyatomic molecules CO2, O3, H2O can radiate energy away in infrared). dT Let Φ = heat flux due to conduction = k dh As a first approximation, heat input is balanced by loss due to conduction: dΦ Q ≈ dh ⇒ Φ ≈ ∫ Qdh 30 € € € € dT 1 ∞ Therefore ≈ ∫ Qdh dh z k z dT must always be sufficiently large to conduct away dh heat deposited at higher levels. Therefore • dT above 200 km → 0 dh • also dT is maximum around dh 120 - 130 km 31 € € € € Diurnal variation and heating sources 32 Diurnal variation and heating sources 中性温度 离子温度 地方时 Midnight temperature enhancement Altitude of Maximum Solar Radiation Absorption 34 Solar Radiation and Thermosphere Variability Radiative Cooling by CO2, NO, O is also important to the thermal budget • QT total neutral heating rate • Km molecular thermal conduction • Ke eddy thermal conduction • NO cooling at 5.3 mm • CO2 total CO2 rate • O(3P) fine structure cooling by O • IR total of O, NO, CO2 • NO cooling rate can increase by two orders of magnitude during a storm. Roble, 1995 36 “Greenhouse Cooling ” Roble and Dickinson [1989] first pointed out the effects of CO 2×CO2 2 enhancement in the troposphere on the upper atmosphere. Cicerone [1990] “Roble and Dickinson’s report opens our eyes to further disturbances to the global atmosphere that are primarily due to human activities.” 36 Roble and Dickinson [1989] 0.5×CO2 2×CO2 [Roble and Dickinson, Geophys. Res. Lett., 16, 1441-1444, 1989] 37 38 Long-Term Decrease in Thermosphere Density - CO2 Cooling Effect? Average behavior for 5 satellites near 400 km Model consistently overestimates density during solar minimum, so this trend
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