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

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

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. CO 2 and NO) Airglow

Solar radiation energy deposition Solar Heat Chemical Potential O, e,… Quantum Internal CO 2 ( 2 ), O 2 ( 1  )… UV, Vis., IR Loss AirglowCooling After Mlynczak et al.

Solar UV Energy Deposition Courtesy Stan Solomon

Hartley continuum Schumann-Runge continuum S-R Bands

Global average heating rates From [Roble, 1995]

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

Chemical heating through exothermic reactions H + O 3  OH + O 2 (k4) O + O 2 + M  O 3 + M (k2) O + O + M  O 2 + M (k1) O + O 3 +  O 2 + O 2 (k3) OH + O  H + O 2 (k5) HO 2 + O  OH + O 2 (k6) H + O 2 + M  HO 2 + M (k7) K/day

Global average cooling rates From [Roble, 1995]

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 CO 2 15 µm (LTE and non-LTE) important below 120km IR emission by ozone and water vapor in the middle atmosphere

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

Airglow O2O2 O3O3 O2O2 O Q4Q4 Q2Q2 Q1Q1 Q3Q3 A nm A nm JHJH J SRC, Ly-a JHJH 3P3P 1D1D 1∑1∑ A µm 1∆1∆ 3∑3∑ g 762 nm After Mlynczak et al. [1993] O 2 ( 1 ∑) emission Burrage et al. [1994]

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

Gravity waves 1. 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 After Holton & Alexander [2000]

Mean zonal wind at solstice UARS reference atmosphere project E W SummerWinter

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 After Holton & Alexander [2000] LATITUDE EQ ALTITUDE SummerWinter F x > 0 F x < 0

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

O 2, N 2 O3O3 H2OH2O EQ NoonSRSS HEATING LOCAL TIME LATITUDE HEIGHT Schematic representation of solar heating After Forbes [1987]

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

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

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

GSWM-98 migrating semi-diurnal tide

Hagan et al. [2001] Migrating thermospheric tides

Many waves are always present Simulated winds at the equator Migrating diurnal componentCombined field

Composition

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

Total density height variation Ideal gas law Hydrostatic balance where

Which leads to: In the “homosphere,” where eddy diffusion tends to mix the atmosphere, the mean molecular weight is almost constant ( m ~ 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 ( H i = kT/m i g ) as the mean free path becomes longer. This is the “heterosphere.” The i th 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.

Diffusive separation From Richmond [1983] Turbopause

To recap… Above the turbopause (~105km), molecular diffusion causes constituents to drop off according to their mass. Below, the atmosphere is fully mixed: 78%N 2, 21%O 2, <1% Ar, <0.1% CO 2 Density decreases with a mean scale height: H = kT/mg ~7km The lower thermosphere is also the transition from a molecular to atomic atmosphere.

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 D i is the diffusion coefficient: Recall : What about chemistry?

From Richmond [1983]

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 i th constituent:

The distribution of ozone

Chapman chemistry Zonal mean Ox loss rates 2.5ºN

Catalytic cycles Mesosphere [GSFC, NASA] Stratosphere

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

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

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

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

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).

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

[NASA LWS report]

Solar Proton Ionization Rates

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

The End