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

T

t (d T

z)w vT Q

c p

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. CO2 and NO)• Airglow

Solar radiation energy deposition

Solar

HeatChemicalPotentialO, e,…

QuantumInternal

CO2(2), O2(1)…

UV, Vis.,IR Loss

AirglowCooling

After Mlynczak et al.

Solar UV Energy Deposition

Courtesy Stan Solomon

Hartley continuum

Schumann-Rungecontinuum

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 + 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

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

• CO2 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

Mly

ncz

ak

et

al.,

Geop

hys.

Res.

Lett

., 3

0(2

1),

20

03

.

Response to the solar storm during April, 2002

Airglow

O2 O3

O2O

Q4

Q2

Q1

Q3

A1

630 nmA2

762 nm

JHJSRC, Ly-a

JH

3P

1D

1∑

A3

1.27 µm

1∆

3∑

g

762 nm

After Mlynczak et al. [1993]

O2 (1∑) emission

Burrage et al. [1994]

Vertical temperature structure (solstice)

Why is the mesopause not at its radiative equilibrium temperature?

Summer Winter

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).

ALT

ITU

DE

65

80

50

After Holton & Alexander [2000]

Mean zonal wind at solstice

UARS reference atmosphere project

E W

Summer Winter

• 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-90 +90

ALT

ITU

DE

Summer Winter

70

80

60

Fx> 0 Fx< 0

Transport affects constituent distributions

Marsh & Roble, 2002UARS reference atmosphere project

O2, N2

O3

H2O

EQ-90 +90

NoonSR SS

HE

AT

ING

HEATING

HE

AT

ING

LOCAL TIME

LATITUDEHE

IGH

TSchematic 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

QuickTime™ and aGIF decompressor

are needed to see this picture.

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 component Combined 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 ~ 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.

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%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.

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:

Recall:

What about chemistry?

From Richmond [1983]

Continuity equationThe total diffusive flux will include both molecular and eddy diffusion terms:

So, neglecting transport, we now have a continuity equation for the ith constituent:

The distribution of ozone

Chapman chemistry

O2 h O O

O O2 M O3 M

O3 h O2 O

O O M O2 M

O O3 2O2

Zonal mean Ox loss rates 2.5ºN

Catalytic cycles

O OH O2 H

H O2 M HO2 M

O HO2 O2 OH

net : 2O O2

O OH O2 H

H O3 O2 OH

net : O O3 2O2

Mesosphere

[GSFC, NASA]

Stratosphere

Nitric Oxide in the lower-thermosphere

Production:

N(2D) + O2 NO + O (fast)

N(4 S) + O2 NO + O (slow)

Loss:

NO + h N(4 S) + O

N(4 S) +NO N2 + O

[a "canabalistic" reaction]

Equatorial NO

(Barth et al., 2003)

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

N(2D) production mechanisms

electron impact :

N2 e* N(2D)+N

(e * secondary/photo - electron)

dissociative recombination :

e.g. NO+ + e N(2D)+ O

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.

200-205 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 (4-1000 keV)

• Both processes lead to stratospheric ozone destruction

Callis et al. 1998

POAMII ozone SH 30km

Randall et al. 1998

(2xAp)

The End